Search

Abstract: In most applications, with seawater conductivity, temperature, and pressure data measured in situ by various observation instruments e.g., Conductivity‐Temperature‐Depth instruments (CTD), the density which has strong ties to ocean dynamics and so on is computed according to equations of state for seawater. This paper, based on density computational formulae in the Thermodynamic Equation of Seawater 2010 (TEOS‐10), follows the Guide of the expression of Uncertainty in Measurement (GUM) and assesses the main sources of uncertainties. By virtue of climatological decades‐average temperature/Practical Salinity/pressure data sets in the global ocean provided by the National Oceanic and Atmospheric Administration (NOAA), correlation coefficients between uncertainty sources are determined and the combined standard uncertainties u c ( ρ ) in seawater density calculations are evaluated. For grid points in the world ocean with 0.25° resolution, the standard deviations of u c ( ρ ) in vertical profiles cover the magnitude order of 10−4 kg m−3. The u c ( ρ ) means in vertical profiles of the Baltic Sea are about 0.028kg m−3 due to the larger scatter of Absolute Salinity anomaly. The distribution of the u c ( ρ ) means in vertical profiles of the world ocean except for the Baltic Sea, which covers the range of ( 0.004 , 0.01 ) kg m−3, is related to the correlation coefficient r ( S A , p ) between Absolute Salinity S A and pressure p. The results in the paper are based on sensors' measuring uncertainties of high accuracy CTD. Larger uncertainties in density calculations may arise if connected with lower sensors' specifications. This work may provide valuable uncertainty information required for reliability considerations of ocean circulation and global climate models. [ABSTRACT FROM AUTHOR]

The Kuroshio intrusion has a profound influence on the marine dynamic and ecological environment in the East China Sea (ECS) shelf. In most previous studies, the intrusion has been evaluated by the volume transport across the 200 m isobath. However, this approximate substitution leads to an underestimation of the Kuroshio intruding flux as it fails to separate the onshore and offshore transport of Kuroshio and shelf water. In this paper, a whole‐region passive‐tracing modeling method is proposed to quantitatively explore the bidirectional exchange of Kuroshio and shelf water, by introducing the Kuroshio water concentration as a passive tracer into the flux calculations. In terms of the climatological annual mean, the Kuroshio intrusion onto the ECS shelf is as high as 1.85 Sv (1 Sv = 106 m³ s−1), with a shelf water outflow rate of 0.42 Sv. The real Kuroshio intruding flux is about 30% higher than the net volume transport across the 200 m isobath (1.43 Sv), which indicates that the influence of the Kuroshio intrusion on the ECS may be far greater than believed before. The volume exchange on the inner shelf of the ECS is also discussed. Analyses of the momentum balances indicate that the exchanges on both the shelf break and the inner shelf are generally dominated by the geostrophic balance, but the exchange on the shelf break is more susceptible to nonlinear effects. The results of a set of sensitivity experiments show that variation of the Kuroshio intrusion is mainly regulated by the combined effect of the Kuroshio transport, the Taiwan Strait current, and the local wind stress. In contrast, the local wind stress has a predominant effect on the outflow of shelf water. Plain Language Summary: In this paper, the intruding flux of the Kuroshio water onto the East China Sea shelf is separated from the total cross‐shelf break transport and calculated with a whole‐region passive‐tracing method, revealing a quantity that is approximately 31% higher than those of the traditional net cross‐shelf break transport methods. The results of sensitivity experiments show that the variation of the Kuroshio intrusion is mainly regulated by the combined effect of the Kuroshio transport, the Taiwan Strait current, and the local wind stress. In contrast, the local wind stress has a predominant effect on the outflow of shelf water. [ABSTRACT FROM AUTHOR]

We present a first examination of Arctic sea ice snow depth estimates from differencing satellite lidar (ICESat‐2) and radar (CryoSat‐2) freeboards. These estimates cover the period between 14 October 2018 and the end of April 2019. Snow depth is related to freeboard differences by the refractive index/bulk density of the snow layer—the only free parameter in the approach. Area‐averaged snow depth ranges from 9 cm (on first‐year ice: 5 cm, multiyear ice: 14 cm) in late October to 19 cm (first‐year ice: 17 cm, multiyear ice: 27 cm) in April; on average, this snow is thinner over FYI. Spatial patterns and gradients of snow depth estimates compare well with reconstructions using snowfall from ERA‐Interim and ERA5, although snowfall from ERA5 is systematically higher. For all months, the results suggest that ~50% of the total freeboard is comprised of snow. Retrievals are within a few centimeters of snow depth data acquired by Operation IceBridge in April 2019. Sources of uncertainties associated with this freeboard‐differencing approach are discussed. Further, sea ice thicknesses calculated using the retrieved snow depth and a modified climatology are contrasted. Comparatively, the snow depth and calculated ice thickness using a modified climatology are higher by ~5 cm and 0.33 m, although these differences are not uniform throughout the season. Snow accumulation was slower between October and December but increased between December and January, unlike the modified climatology, which exhibited a monotonic accumulation for all months. Future opportunities for assessment and improvement of these estimates are discussed. Plain Language Summary: The snow layer atop Arctic sea ice is an important component of the climate system. In winter, the insulating effects of snow slow the rate of ice growth. In spring, the onset of ice surface melt is delayed until the highly reflective snow layer disappears. During summer, meltwater from snow collects in depressions to form melt ponds, which enhances the absorption of solar radiation leading to more rapid surface warming. Presently, there are no routine measurements of snow depth suitable for assessing the impact of climate changes on the precipitation and accumulation of snow on sea ice. Our current understanding of snow depth is limited and has been based on field measurements conducted in the middle of last century and from airborne surveys conducted over the last decade. In this paper, we combine the measurements from two altimetry missions (ICESat‐2 and CryoSat‐2) to calculate snow depth over the entire Arctic Ocean. ICESat‐2 (a lidar) and CryoSat‐2 (a radar) measure the heights to the top and bottom of the snow layer, respectively. Differencing the two heights provides an estimate of the thickness of the snow layer. This paper describes this novel approach and an assessment of the snow depth estimates. Key Points: Our current understanding of snow depth is based largely on climatology developed during last century and from recent airborne surveysWe present a first examination of Arctic sea ice snow depth estimates from differencing satellite ICESat‐2 and CryoSat‐2 freeboardsSea ice thickness can now be calculated with snow loading from satellite retrievals without resorting to climatology or reconstructions [ABSTRACT FROM AUTHOR]

One of the foci of the Forum for Artic Modeling and Observational Synthesis (FAMOS) project is improving Arctic regional ice‐ocean models and understanding of physical processes regulating variability of Arctic environmental conditions based on synthesis of observations and model results. The Beaufort Gyre, centered in the Canada Basin of the Arctic Ocean, is an ideal phenomenon and natural laboratory for application of FAMOS modeling capabilities to resolve numerous scientific questions related to the origin and variability of this climatologic freshwater reservoir and flywheel of the Arctic Ocean. The unprecedented volume of data collected in this region is nearly optimal to describe the state and changes in the Beaufort Gyre environmental system at synoptic, seasonal, and interannual time scales. The in situ and remote sensing data characterizing ocean hydrography, sea surface heights, ice drift, concentration and thickness, ocean circulation, and biogeochemistry have been used for model calibration and validation or assimilated for historic reconstructions and establishing initial conditions for numerical predictions. This special collection of studies contributes time series of the Beaufort Gyre data; new methodologies in observing, modeling, and analysis; interpretation of measurements and model output; and discussions and findings that shed light on the mechanisms regulating Beaufort Gyre dynamics as it transitions to a new state under different climate forcing. Key Points: Introduction to the FAMOS‐2 special collection with papers mainly on studies in the region of the Arctic Ocean's Beaufort GyrePhysical, geochemical, biological, and ecosystem components of the Beaufort Gyre system are investigatedNew observations, ideas, and hypotheses revealing mechanisms influencing the Beaufort Gyre are discussed [ABSTRACT FROM AUTHOR]

Tidal motion of oceanic salt water through the ambient geomagnetic field induces periodic electromagnetic field signals. Amplitudes of the induced signals are sensitive to variations in electrical seawater conductivity and, consequently, to changes in oceanic temperature and salinity. In this paper, we computed and analyzed time series of global ocean tide‐induced magnetic field amplitudes. For this purpose, we combined data of global in situ observations of oceanic temperature and salinity fields from 1990–2016 with data of oceanic tidal flow, the geomagnetic field, mantle conductivity, and sediment conductance to derive ocean tide‐induced magnetic field amplitudes. The results were used to compare present day developments in the oceanic climate with two existing climate model scenarios, namely, global oceanic warming and Greenland glacial melting. Model fits of linear and quadratic long‐term trends of the derived magnetic field amplitudes show indications for both scenarios. Also, we find that magnetic field amplitude anomalies caused by oceanic seasonal variability and oceanic climate variations are 10 times larger in shallow ocean regions than in the open ocean. Consequently, changes in the oceanic and therefore the Earth's climate system will be observed first in shelf regions. In other words, climate variations of ocean tide‐induced magnetic field amplitudes are best observed in shallow ocean regions using targeted monitoring techniques. Key Points: Magnetic signals estimated from in situ temperature and salinity observations correlate well with oceanic heat content variationsThe derived magnetic field amplitude and ocean heat content trends match the expectations of previous studiesVariations in oceanic tidal magnetic fields are largest in shelf regions, which allow a targeted monitoring of oceanic climate there [ABSTRACT FROM AUTHOR]

The Southern Ocean has complex spatial variability, characterized by sharp fronts, steeply tilted isopycnals, and deep seasonal mixed layers. Methods of defining Southern Ocean spatial structures traditionally rely on somewhat ad hoc combinations of physical, chemical, and dynamic properties. As a step toward an alternative approach for describing spatial variability in temperature, here we apply an unsupervised classification technique (i.e., Gaussian mixture modeling or GMM) to Southern Ocean Argo float temperature profiles. GMM, without using any latitude or longitude information, automatically identifies several spatially coherent circumpolar classes influenced by the Antarctic Circumpolar Current. In addition, GMM identifies classes that bear the imprint of mode/intermediate water formation and export, large‐scale gyre circulation, and the Agulhas Current, among others. Because GMM is robust, standardized, and automated, it can potentially be used to identify structures (such as fronts) in both observational and model data sets, possibly making it a useful complement to existing classification techniques. Plain Language Summary: The Southern Ocean is an important part of the climate system, in part because it absorbs a large fraction of the heat and carbon that is added to the atmosphere/ocean system by human‐driven fossil fuel burning. In this work, we use a machine learning technique to automatically sort Southern Ocean temperature measurements into groups based on how those temperature measurements change with depth. Different groups have the fingerprints of different large‐scale circulation patterns, such as the powerful Antarctic Circumpolar Current that flows around Antarctica. The groups that we identify are consistent with our understanding of the Southern Ocean, which gives us confidence that our machine learning technique may be useful for automatically grouping measurements and computer model data in the future. This matters because the climate science community needs a new set of tools, possibly including the machine learning technique that we use in this paper, to deal with a very large, ever‐increasing volume of observational and computer model data. Key Points: We apply Gaussian mixture modeling (GMM) to Southern Ocean temperature dataGMM identifies spatially coherent profile types without using latitude or longitude informationGMM offers a complementary approach for objectively classifying temperature profiles [ABSTRACT FROM AUTHOR]

The summertime eastward jet (SEJ) in the South China Sea (SCS) can be regarded as the boundary between the southern and northern upper circulation of the SCS in climatology. The dynamic process of SEJ migration in response to the variation of SCS summer monsoon (SCSSM) strength on the interannual scale is investigated, based on the Simple Ocean Data Assimilation (SODA) dataset and by a 1.5‐layer reduced gravity model. Analysis shows that SEJ position is closely related to SCSSM strength, with southward/northward migration of SEJ coinciding with stronger/weaker SCSSM. Besides migration of wind stress curl (WSC) zero‐line and north‐south difference of WSC variation, the north‐south difference of the SCS upper layer thickness (ULT) in summer, that is, thick in the southern SCS and thin in the northern SCS, also plays a significant role in the dynamic process of SEJ migration. It is the joint effect of wind strength and ULT distribution. With the variation of SCSSM strength, the thinner ULT in the northern SCS corresponds to the larger magnitude of potential vorticity (PV) variation inputted by wind stress, which leads to more pronounced strengthening/weakening of the western boundary current (WBC) in the northern SCS than that in the southern SCS. Then, the north and south WBCs compete with each other until the upper layer circulation reaches a new equilibrium state, and SEJ migrates southward/northward accordingly. Plain Language Summary: The summertime eastward jet (SEJ) in the South China Sea (SCS) plays a significant role in the regional climate and ecosystem by transporting the coastal cold water with high chlorophyll‐a concentration into the interior sea, and thus its north‐south migration from year to year is an important issue. Analysis shows that with enhanced/weakened summer monsoon in the SCS, the SEJ migrates southward/northward. Our study aims to understand their relationship better and highlight the role of the summer upper layer thickness difference between the southern and northern basins. With the variation of summer monsoon strength, the circulation of the thinner water column responds more significantly than that of the thicker water column, and then SEJ migrates southward/northward. This is another important perspective to understand the migration of the summertime eastward jet, besides the migration of zero‐line of wind stress curl and the difference of monsoon variation over the southern and northern basins. Key Points: The summertime eastward jet migrates southward/northward with enhanced/weakened summer monsoon in the South China SeaIts migration is caused by different variations of potential vorticity input between southern and northern basinsBesides the effect of wind stress curl, the joint effect of monsoon strength and upper layer thickness distribution also plays a role [ABSTRACT FROM AUTHOR]

The Strait of Belle Isle (SBI) is an important pathway for water mass exchanges between the Labrador Shelf and the Gulf of St. Lawrence (GSL), particularly for transport toward the GSL. Seasonal and inter‐annual variability of transport in the SBI are examined using 15 years of moored acoustic Doppler current profiler data. Tidal currents are largely along strait and homogeneous with depth. Transports are toward the GSL on average, lowest (−1.0 ± 0.8 dSv) from April to July, and highest (−4.0 ± 1.1 dSv) from September to January. Averaged seasonal transports are usually within one standard deviation of previously published modeled values. The volume of winter Labrador Shelf water (LShW) entering the GSL is computed by transport integration and compares well with integrated volumes that meet LShW temperature‐salinity criteria during an annual March survey of the GSL. Integrating over the whole year showed that on average 649 ± 397 km3 enters the GSL after the March surveys are conducted, independently of the volume that has entered up to March; this addition represents on average about a third of the total volume. Annual volumes of LShW calculated from transport suggest that flow through the SBI accounts for 12%–18% of the GSL winter surface mixed layer. Cross‐strait current shear may affect transport integration values, but sea surface temperature data suggests this bias is limited to the summer. Corrections are empirically derived to account for cross‐strait shear in our transport calculation. The corrected time series suggests that the average transport may be flowing toward the Labrador Shelf in July (0.6 ± 1.0 CI [0.1 1.1] dSv). Plain Language Summary: The Strait of Belle Isle (SBI) is the waterway at the northeastern end of the Gulf of St. Lawrence (GSL) that connects it to the Labrador Shelf, and where water may move in both directions. Despite its importance, there are not many recent measurements of water movement across this strait. This is because the SBI is remote from most routine oceanographic operations, and strong currents, sea ice, and icebergs complicate mooring design and restrict study site selection. Our study presents 15 years of new measurements obtained using contemporary technologies allowing for a better assessment of the contribution of Labrador Shelf waters to the GSL. In the future, they may also contribute to improving ocean models of the Northwest Atlantic. Key Points: A 15‐year Strait of Belle Isle transport time series is calculated from mooring‐based acoustic Doppler current profiler dataTransport is used to estimate Labrador Shelf Water contribution to the Gulf of St. Lawrence winter cold layerCorrecting for cross‐strait shear suggests that dominant July transport is toward the Labrador Shelf [ABSTRACT FROM AUTHOR]

Warm, salty Circumpolar Deep Water (CDW) is recognized as the primary driver for Antarctic glacial melt, but the mechanism by which it reaches the continental shelves remains highly uncertain from an observational standpoint. With the scarcity of eddy flux estimation in the Antarctic margin, we quantify the isopycnal diffusivity of CDW using hydrographic variability and satellite altimetry under the mixing length framework. For comparison, the spiciness and thickness are used as isopycnal tracers, and the two tracers yield qualitatively similar estimates. Over the Antarctic Circumpolar Current (ACC), variation of mixing length is generally aligned with the jet‐induced mixing suppression, including in areas where topography blocks the jet's influence. In contrast, the mixing length does not depend on the mean flow in the subpolar zone, likely reflecting the relatively quiescent flow regime. The estimated isopycnal diffusivity ranges from 100 to 500 m2 s−1 south of the ACC. The eddy diffusivity tends to be enhanced where the gradient of isopycnal thickness becomes small and CDW intrudes onshore. The cross‐slope eddy CDW flux is estimated, and the associated onshore heat flux across is calculated as ∼3.6 TW in the eastern Indian sector. The eddy heat flux and coastal solar heating are generally balanced with cryospheric heat sinks including glacial melting and surface freezing, suggesting that the eddy advection is substantial for the onshore CDW flux. The thickness field is essential for determining mixing length and eddy fluxes in the subpolar zone, however this is not the case in the ACC domain. Plain Language Summary: The deep ocean is a significant source of heat and salt for the Antarctic coasts. Thus, its behavior is relevant for a wide range of climate sciences, such as Antarctic glacial melt, sea ice variability, and global ocean overturning. The warm deep water is presumably transported to the Antarctic coasts by ocean eddies, but observational evidence has not been provided to date. By synthesizing ocean measurements and satellite altimetry, we estimate the eddy‐driven onshore transport of the deep water over the Antarctic margin. It is shown that shoreward heat transport by eddies is generally balanced with heat gain by solar heating and heat loss expected from surface freezing and glacial melt. This result indicates that eddy transport plays a fundamental role in the coastal heat supply. The thickness of the deep water primarily controls the ability to mix along density surfaces that bridge the open ocean to the continental shelves; more homogeneous thickness allows for easier mixing. Our results facilitate the possibility of predicting the diffusion rate of eddies using the thickness of the deep water. This idea is helpful in simulating deep‐water transport in global climate models, where sub‐grid and unresolved effects of eddies need to be prescribed. Key Points: Eddy diffusivity in the subpolar Southern Ocean is estimated to be 100–500 m2 s−1 based on hydrographic variability and satellite altimetryEddy heat flux towards the Wilkes Land is ∼3.6 TW, nearly balancing with surface freezing, glacial melt, and solar heatingThe thickness gradient controls mixing length and eddy diffusivity in the subpolar zone [ABSTRACT FROM AUTHOR]

Extratropical cyclones (ECs) produce comparable wave heights and hazardous sea states to those under hurricanes. With strong wind speeds and considerably larger diameters, the impact of ECs on the global wave climate is substantial. Because ECs occur more frequently, move more quickly, and exist in a large variety of sizes and shapes, these storms are not as well documented. Here, we present findings from a global EC tracking algorithm aimed at creating an EC database of wave producing storms. We focus on the major ocean basins of extratropical cyclone activity: The North Atlantic, North Pacific, and Southern Oceans. Our tracking algorithm uses mean sea level pressure fields from the European Centre for Medium‐Range Weather Forecasts (ECMWF) high resolution, advanced data assimilating, ERA5 reanalysis over the years 1979–2020. The 42 year‐long data set of thousands of cyclone tracks allows seasonal patterns and longer‐term trends of EC activity to be analyzed in these ocean regions. In addition, we employ 36 years of satellite radar altimeter‐derived significant wave height (Hs) data from 14 inter‐calibrated satellite missions to study the wave climate induced by the identified ECs in each basin. The resulting wave climatology compares well with previous results in the mid to high latitudes. We then analyze average wave heights under different EC characteristics, including cyclone translational speed and maximum winds, which have strong effects on wave height variability. Finally, a brief analysis of EC and wave climate under positive and negative phases of the North Atlantic Oscillation is presented. Plain Language Summary: Large storms which occur outside of the tropics, known as extratropical cyclones (ECs), create very large waves that can surpass that of tropical cyclones. These extreme sea states are responsible for coastal hazards including erosion and surge induced flooding. While tropical cyclones, or hurricanes, are well documented by devoted agencies, ECs are more difficult to track given their high occurrence, fast movement, and inconsistency of size and shape. Here we present the results from a tracking algorithm, applied over a 42 year period, aimed at documenting the many ECs which occur over the global oceans with specific focus placed on wave development within these storms. We use satellite measurements of wave heights within the tracked ECs to investigate wave growth and overall wave climate within these storms. In addition to studying the seasonality of ECs and associated wave activity, we show the regional influence of longer time‐scale climate variability on the same. Key Points: A new extratropical cyclone tracker is applied to 42 years of global reanalysis to investigate climatology and seasonality of stormsOcean basin composite analyses of satellite‐derived significant wave height observations show wave dependence on cyclones characteristicsThe North Atlantic Oscillation climate modes show strong influence on cyclone activity and induced wave climate in the North Atlantic [ABSTRACT FROM AUTHOR]

Ocean eddies influence regional and global climate by mixing and transporting heat, carbon, nutrients, and other properties. One of the most recognizable and ubiquitous features of oceanic variability are mesoscale eddies (vortices and meanders/jets) with spatial scales of tens to hundreds of kilometers. Vortices resemble Gaussian‐like features in sea surface height which locally affect near‐surface wind, cloud properties, and rainfall patterns; we refer to these Gaussian‐like eddies as spatially coherent eddies. Although spatially coherent eddies are ubiquitous, their climatology, seasonality, and long‐term temporal evolution is yet to be explored. Here, we examine the kinetic energy (KE) contained in spatially coherent eddies and present their temporal variability using satellite observations between 1993 and 2020. Around half of the transient KE contained by eddies corresponds to spatially coherent eddies. A strong seasonal cycle is observed in the spatially coherent eddy field, and correlates with the wind forcing at time‐lags of 3–6 months. The seasonal correlation between wind forcing and spatially coherent eddies with diameters smaller than 120 km is faster (∼3 months), than that of coherent eddies with larger diameters (∼6 months). These lags between different spatially coherent eddy scales are consistent with the transfer of energy from small to large scales (inverse energy cascade). Our analysis highlights the relative importance of the spatially coherent eddy field in the KE budget, revealing a lagged response between the spatially coherent eddy and forcing at different time‐scales, and showcases the seasonality, and multidecadal trends of coherent eddy properties over the global ocean. Plain Language Summary: Spatially coherent vortices are a ubiquitous feature of ocean variability observable from satellites. These eddies are crucial in ocean dynamics as they interact with the atmosphere and some can transport properties over long distances. Our study investigates the seasonal, interannual, and long‐term changes in the number and intensity of spatially coherent vortices, by automatically identifying individual vortices over the available satellite altimeter record. The seasonal cycle suggests a transition from numerous, smaller, and weaker spatially coherent vortices, to fewer, larger, and stronger spatially coherent vortices over the season. In addition, a long‐term adjustment of the spatially coherent field is identified with possible links to long‐term changes in the climate system. Key Points: Coherent eddies contain around half of the total surface ocean eddy kinetic energyThe seasonal cycle of the abundance and amplitude of spatially coherent eddies reveals a 3–6 months lag to wind forcingThe seasonal lag between the number and the amplitude of spatially coherent eddies suggests a role for the inverse energy cascade [ABSTRACT FROM AUTHOR]

Shelfbreak exchange processes have been studied extensively in the Middle Atlantic Bight. An important process occurring during stratified conditions is the Salinity Maximum Intrusion. These features are commonly observed at the depth of the seasonal pycnocline, and less frequently at the surface and bottom. Data collected from NOAA's National Marine Fisheries Service Ecosystem Monitoring program as well as data collected from the fishing industry in Rhode Island show that the middepth intrusions are now occurring much more frequently than was reported in a previous climatology of the intrusions (Lentz, 2003, https://doi.org/10.1029/2003JC001859). The intrusions have a greater salinity difference from ambient water and penetrate large distances shoreward of the shelf break relative to the earlier climatology. The longer term data from the Ecosystem Monitoring program indicates that the increase in frequency occurred in 2000, and thus may be linked to a recent regime shift in the annual formation rate of Warm Core Rings by the Gulf Stream. Given the increased frequency of these salty intrusions, it will be necessary to properly resolve this process in numerical simulations in order to account for salt budgets for the continental shelf and slope. Plain Language Summary: An important exchange process between waters of the continental slope and continental shelf is the Salinity Maximum Intrusion. These have been studied before but there is little information on their along‐shelf scale, how they are generated, or their broad impact on shelf ecosystems. Observations from both a multi‐decadal data set from the National Marine Fisheries Service as well as community science data collected by the commercial fishing industry over 5 years show that the frequency of these intrusions has increased significantly since 1998. In addition, the intrusions appear to be saltier and penetrate further onshore than previous reports. Future work is necessary to determine how the increasing frequencies of this important process may be affecting continental shelf heat and salt balances as well as broader impacts to the continental shelf ecosystem. Key Points: Observations over several decades show substantial increase of frequency of middepth Salinity Maximum IntrusionsThere is significant inter‐annual and month‐to‐month variability in the frequency and occurrence of Salinity Maximum IntrusionsSalinity Maximum Intrusions now commonly extend up to 100 km shoreward of the shelfbreak and profiles may contain multiple intrusions [ABSTRACT FROM AUTHOR]

Previous studies conducted based on reanalysis data sets have suggested that high‐frequency (≤30‐day) wind variability has a significant impact on the climatology of surface latent heat flux (LHF). The present study quantifies the impact of high‐frequency winds using field measurements derived from the global tropical moored buoy array (GTMBA) and, in turn, evaluates the performance of existing reanalysis products on capturing the role of these high‐frequency winds. The buoy observations demonstrate that high‐frequency winds at the n‐day (n = 1, 2, ..., 30) timescale make a positive contribution to the annual climatology of LHF by nonlinearly enhancing the mean surface wind speed. Furthermore, this contribution increases as the timescale decreases, with maxima occurring at the 1‐day (subdaily) timescale. The subdaily winds contribute 4.6 W m−2, on average over all GTMBA sites. The largest contribution exceeds 10.0 W m−2 over the equatorial western Pacific Ocean. Hourly reanalysis products underestimate the contribution of the high‐frequency winds at timescales shorter than about a week, especially at the subdaily timescale, which is underestimated by approximately 50–60%. This finding suggests that an improvement in the performance of subdaily wind variability is needed to improve the fidelity of next‐generation reanalysis products. Plain Language Summary: The accurate estimation of heat exchanges between the ocean and the atmosphere is critical for the proper calculation of the global ocean heat budget. To date, due to uncertainties in the estimates of surface heat fluxes, most air‐sea flux products could not close the heat budget (they typically have excessive heat fluxes into the ocean). The latent heat flux (LHF) estimates introduce the largest uncertainty in the heat budget, and this uncertainty is partly due to poor performance of the high‐frequency variability of LHF‐related variables. The present study first examines the impact that high‐frequency (≤30‐day) winds have on the LHF climatology using high temporal resolution buoy measurements collected in tropical oceans. Then it moves to evaluate the performance of existing reanalysis products. The buoy observations demonstrate that the contribution of high‐frequency winds at the n‐day (n = 1, 2, ..., 30) timescale to the climatological annual mean LHF increases as the timescale decreases. Hourly reanalysis products were found to underestimate the contribution of high‐frequency winds at timescales shorter than about a week, especially at the 1‐day (subdaily) timescale. Improvement in the fidelity of subdaily vector winds in next‐generation reanalysis products will be helpful for reducing the excessive heat inputs into the ocean. Key Points: The contribution of n‐day (n = 1, 2, ..., 30) timescale winds to the latent heat flux climatology increases as the timescale decreasesExisting reanalysis products underestimate this contribution at timescales shorter than about 1 week, especially at subdaily timescaleImprovement in the fidelity of subdaily wind variability is needed for the development of next‐generation reanalysis products [ABSTRACT FROM AUTHOR]

Accelerated loss of the sea‐ice cover and increased human activities in the Arctic emphasize the need for skillful prediction of sea‐ice conditions at subseasonal to seasonal (S2S) timescales. To assess the quality of predictions, dynamical forecast systems can be benchmarked against reference forecasts based on present and past observations of the ice edge. However, the simplest types of reference forecasts—persistence of the present state and climatology—do not exploit the observations optimally and thus lead to an overestimation of forecast skill. For spatial objects such as the ice‐edge location, the development of damped‐persistence forecasts that combine persistence and climatology in a meaningful way poses a challenge. We have developed a probabilistic reference forecast method that combines the climatologically derived probability of ice presence with initial anomalies of the ice‐edge location, both derived from satellite sea‐ice concentration data. No other observations, such as sea‐surface temperature or sea‐ice thickness, are used. We have tested and optimized the method based on minimization of the Spatial Probability Score. The resulting Spatial Damped Anomaly Persistence forecasts clearly outperform both simple persistence and climatology at subseasonal timescales. The benchmark is about as skillful as the best‐performing dynamical forecast system in the S2S database. Despite using only sea‐ice concentration observations, the method provides a challenging benchmark to assess the added value of dynamical forecast systems. Plain Language Summary: The Arctic is becoming more ice free and seeing more human activities, which means it is important to have reliable forecasts of sea ice conditions weeks to months ahead. The accuracy of a forecast system is typically compared against reference forecasts based on present and past observations of the ice edge. However, most widely used references either simply maintain the current state or consider states at the same time of the year during previous years. Such simple benchmarks can lead to an overestimation of how "skillful" a forecast system is considered. For sea ice edge, creating a better reference forecast combining both historical and current observations can be challenging. We have addressed this challenge and developed a method where we find the historical probability of ice presence along the current ice edge and use this probability to predict ice presence at future dates. The new method clearly outperforms the simpler methods and remains slightly better than historical‐based forecasts even 2 months ahead. Despite using only observed sea ice concentration data (like the simpler benchmarks), the new benchmark is about as good as modern model‐based forecast systems. The method therefore provides a good reference to study how well the latest forecast systems are actually performing. Key Points: We have developed a new method that combines climatological sea‐ice probability and initial‐state anomaly to forecast sea‐ice presenceIce‐edge forecasts derived from this method can outperform climatological benchmarks at lead times of up to 2 monthsSpatial damped anomaly persistence forecasts have a higher predictive skill than most models from the subseasonal to seasonal database [ABSTRACT FROM AUTHOR]

Although submesoscale processes are revealed to have a strong seasonality in many regions of global ocean, their seasonal modulation in the northeastern South China Sea (NESCS) remains unexplored by observations. In this study, approaches to diagnose submesoscale kinetic energy (KE) and its conversion terms using data from a single mooring are proposed and validated. Based on these approaches, seasonal modulation of submesoscale KE in the NESCS is for the first time investigated using 8.7‐year‐long moored velocity and temperature data between August 2010 and April 2019. It is found that the monthly climatology of submesoscale KE shows an asymmetric annual cycle with a steep peak between December and February but a flat trough between April and October. The maximum and minimum of this annual cycle occur in January and May, respectively. Although the submesoscale KE rapidly decreases with depth, its seasonality is still evident at 300 m, which is far beneath the mixed layer. Statistical analysis suggests that the overall seasonality of submesoscale KE is shaped by mixed‐layer depth while its steep peak in winter is modulated by mesoscale strain rate. It therefore suggests that the combination of mixed‐layer instability and strain‐induced frontogenesis determines the detailed annual cycle of submesoscale KE. Energetics analysis demonstrates that submesoscales obtain their KE from the release of available potential energy but lose a small portion of its KE through inverse cascade. This submesoscale inverse KE cascade may play an important role in modulating the temporal variation of mesoscale KE in winter. Plain Language Summary: Although previous simulation‐based studies suggested that submesoscale processes have strong seasonal variations, their seasonal modulation in the northeastern South China Sea (NESCS) remains unexplored using long‐term observations. In this study, we first proposed approaches to diagnose submesoscale kinetic energy (KE) and its energy conversion terms using single‐mooring data. Then, seasonal modulation of submesoscale KE was investigated through applying these approaches to the 8.7‐year‐long moored velocity and temperature data in the NESCS. We found that climatologically, the submesoscale KE is strongest and weakest in January and May, respectively. Its annual cycle, however, is asymmetric with a steeper peak but a flat trough. The submesoscale KE rapidly decreases with depth but its seasonal variation can penetrate at least to 300 m. The observed annual cycle is likely determined by the combination of mixed‐layer baroclinic instability and strain‐induced frontogenesis, whose strength depends on the mixed‐layer depth and mesoscale strain rate, respectively. In the perspective of energetics, seasonality of submesoscale KE is primarily modulated by the release of available potential energy while the role of nonlinear KE transfer is minor. The nonlinear KE transfer from submesoscales to mesoscales, however, may play an important role in modulating the temporal variation of mesoscale KE in winter. Key Points: Approaches to diagnose submesoscale KE and its conversion terms using moored data were proposedClimatological annual cycle of the submesoscale KE in the NESCS was obtained based on 8.7‐year‐long moored dataSeasonal modulation mechanisms of the submesoscale KE in the NESCS were revealed [ABSTRACT FROM AUTHOR]

Sea ice plays a critical role in the Earth's climate system, including influencing ocean heat uptake, reflecting solar radiation, and contributing to dense water formation. Instrumental records of polar sea ice extent are only available since 1979, however. The short length of such records also limits our knowledge of polynya variability, which can reflect large‐scale atmospheric and climate changes. Ice core proxy records can extend these observations, but require further development and regional validation. We compare chloride and methanesulfonic acid concentrations from two new firn cores from the Canadian Arctic with satellite‐derived observations of regional sea‐ice concentration and polynya variability from 2002 to 2014. The sub‐annual resolution of these cores allows for detailed investigation of how regional sea‐ice concentration is recorded in the ice at Prince of Wales Icefield (POW), Ellesmere Island and Devon Ice Cap (DIC), Devon Island, Nunavut. Over the period 2002–2014 we find that the primary sources of marine aerosols to POW are polynyas within Arctic Canada and the Canada basin of the Arctic Ocean, whereas the primary sources of marine aerosols to DIC are a broader region of the Queen Elizabeth Islands, Baffin Bay, and the Arctic Ocean. Marine aerosol sources to the two core sites are distinct, reflecting different moisture source regions and, likely, differing transport pathways. Air mass back trajectory results support the satellite‐derived results. Glaciochemical records from this dynamic, warming region may provide a proxy for reconstructing North Water polynya and other regional polynya and shore‐lead variability prior to the satellite era. Plain Language Summary: Sea ice plays a critical role in the Earth's climate system, yet instrumental records of polar sea ice extent are only available since 1979. The chemistry of ice cores can extend these observations farther back in time, but need to be validated over the satellite era first. In this study, we compare marine aerosol records from two new cores from the Canadian high Arctic with satellite‐derived observations of regional sea‐ice concentration from 2002 to 2014. Ice cores from this vital region are sparse, and recent surface warming across the Queen Elizabeth Islands heightens the urgency to develop histories of regional sea‐ice and environmental variability. We find that ice core records from this dynamic and warming region may provide a proxy for reconstructing sea‐ice variability prior to the satellite era in certain key regions of the Canadian Arctic Archipelago. Key Points: Marine aerosols to Prince of Wales, 2002–2014, are derived from Canadian Arctic polynyas and the Canada basin of the Arctic OceanMarine aerosols to Devon Ice Cap are derived from broader regions of the Queen Elizabeth Islands, Baffin Bay, and the Arctic OceanMarine aerosol sources to the two core sites are largely distinct, reflecting different moisture source regions and transport pathways [ABSTRACT FROM AUTHOR]

Simulations of the Argentine Basin have large uncertainties associated with quantities such as air‐sea exchanges of heat and carbon in current generation climate models and ocean reanalysis products. This is due to the complex topography, profound undersampling until recent years, and strong currents and mixing of subpolar and subtropical water masses in the basin. Because mixing of water masses is important here, model resolution is hypothesized to play an important role in estimating ocean quantities and determining overall budgets. We construct three regional ocean models with biogeochemistry at 1/3°, 1/6°, and 1/12° resolutions for the year 2017 to investigate heat and carbon dynamics in the region and determine the effect of model resolution on these dynamics. Initial conditions and boundary forcing from BSOSE (the Biogeochemical Southern Ocean State Estimate (Verdy & Mazloff, 2017), https://doi.org/10.1002/2016JC012650) and atmospheric forcing from ERA5 are used. The models are evaluated for accuracy by comparing output to Argo and BGC‐Argo float profiles, BSOSE, and other reanalyses and mapped products. We then quantify the effect of resolution on model upper ocean heat and carbon transport and the associated air‐sea exchanges. We determine that increasing the resolution from 1/3° to 1/12° enhances the upward vertical transport and surface exchanges of heat but causes no significant effect on surface carbon fluxes despite enhancing downward transport of anomalous DIC. Plain Language Summary: The Argentine Basin is a part of the Atlantic sector of the Southern Ocean that is poorly observed and poorly represented in models. Current climate models have a wide range of estimates for the fluxes of heat and carbon between the atmosphere and ocean in this region. We construct three models of the Argentine Basin that differ only in resolution to see the effect this has on the air‐sea exchanges of these quantities. We compare our results with other models and observations and determine that increasing the resolution has an enhancing effect on the vertical movement and surface exchanges of heat but has no significant effects on carbon fluxes. Key Points: Three regional biogeochemical models of the Argentine Basin with increasing resolution are run to study the effect on fluxes and transportsUpward heat transport and surface heat flux are enhanced in the eddying modelSurface carbon fluxes do not differ by resolution but eddies transport anomalous carbon downwards [ABSTRACT FROM AUTHOR]

The existence of a deep countercurrent in the subtropical northwest Pacific, named Subtropical Deep Countercurrent (STDCC), has been revealed by moored current‐meter data obtained from a buoy system (∼21.6°N, ∼132.8°E) during 4 years (2010–2014). The climatological average of Argo‐based absolute geostrophic velocities and ocean reanalyzes have further confirmed the spatial picture of the eastward‐flowing STDCC. STDCC is identified as a time‐mean feature in the latitudinal range of 19°–25°N with a meridional scale of 200–300 km that persistently appears from 125° to 180°E and slightly shifts to the north as it goes east. Vertically, STDCC is mostly found below 1,000 m with its core located between the depths of 1,250 and 2,000 m, which corresponds to the undercurrent's meridional pattern with increasing depth northward and a meridional space of 2°–4°. Nevertheless, STDCC is distinctive where it lies along ∼27.6 − σθ, which is deeper than the North Equatorial Undercurrent's density layer (∼27.0 − σθ). The mean velocity of STDCC at 2,000 m observed by mooring measurements was approximately 0.2 ± 0.15 cm s−1 with a maximum speed of approximately 5.59 cm s−1. The ocean reanalyzes also support the existence of STDCC with mean velocity of approximately 0.2–0.6 cm s−1 west of 145°E and can be as large as 1.6 cm s−1 at the eastern region. The results show that STDCC is closely related to the westward‐propagating eddies. Our finding emphasizes that the convergence of the cyclonic and anticyclonic eddies possibly generates time‐mean zonally elongated eastward flows forming STDCC feature. Plain Language Summary: Information about the deep circulation in the subtropical northwest Pacific is very limited owing to a lack of observational evidence. By using direct current measurements for 4 years (2010–2014), this study provides a description of the first observation of the newly identified deep eastward flows, named Subtropical Deep Countercurrent (STDCC). Argo‐based absolute geostrophic currents and ocean reanalyzes have further confirmed the existence of this current, which found around 19°–25°N at 125°–180°E longitude and core's depths of between 1,250 and 2,000 m. The maximum mean velocity of STDCC at 2,000 m observed by mooring measurements was approximately 0.2 ± 0.15 cm s−1 with a maximum speed of approximately 5.59 cm s−1, which is relatively weaker than the tropical North Equatorial Undercurrent (NEUC) and the upper‐layer Subtropical Countercurrent. In comparison, the model results show that STDCC has typical velocity of approximately 0.2–0.6 cm s−1 west of 145°E and greater velocity at the eastern region reaches 1.6 cm s−1. The westward‐propagating eddies have been suggested as being the possible dynamics behind the STDCC's generation. Key Points: A newly identified deep countercurrent has been found in the subtropical northwest Pacific from observations and ocean reanalyzesSubtropical Deep Countercurrent (STDCC) is identified in the range of 19°N–25°N, and 125°E to 180°E with its core depths between 1,250 and 2,000 mThe generation of STDCC is strongly associated with the westward‐propagating eddies [ABSTRACT FROM AUTHOR]

This study examines the potential impact of subsurface potential temperature and current conditions on the sea surface temperature (SST) and rainfall over the tropical region using Climate Forecast System Version 2 (CFSv2) ensemble seasonal reforecasts for 1958–2014. The climatological difference of earlier period (1958–78; P58‐78) and later period (1994–2014; P94‐14) in January initialized reforecasts (JIR) depicts mild‐warm SST over the equatorial eastern Pacific in January, but its magnitude becomes larger over there from May to August. The difference between JIR P58‐78 and P94‐14 depicts warm potential temperature (PT) up to 3°C over the equatorial central and eastern Pacific at depth from 155 to 70 m in January therefore 20°C isotherm of JIR P58‐78 in January tends to be deeper in the equatorial eastern Pacific than JIR P94‐14. The magnitude of equatorial undercurrent (EUC) in JIR P58‐78 is larger in the eastern Pacific at depth from 125 to 35 m from January to February than JIR P94‐14. Therefore, water upwelled to the surface through EUC was usually warm in January of JIR P58‐78. As lead months increase, the center of warm PT gradually moves upward in the eastern Pacific than its location in January, resulting in development of warm SST in the eastern Pacific in March. The difference between April initialized reforecasts P58‐78 and P94‐14 depicts warm PT up to 1.5°C in the eastern Pacific at depth from 135 to 35 m in April but as lead months increase, magnitude of warm PT gradually decreases, resulting in negligible difference SST over the eastern Pacific. Key Points: The difference between 1958–1978 and 1994–2014 in January initialized reforecasts depicts warm SST in summer over equatorial eastern PacificThe 20°C isotherm for period 1958–1978 in January of reforecasts tends to be deeper in equatorial eastern Pacific than period 1994–2014Difference between 1958–1978 and 1994–2014 January initial condition depicts warm potential temperature from 155 to 70 m in equatorial Pacific [ABSTRACT FROM AUTHOR]

A regional data‐constrained coupled ocean‐sea ice general circulation model and its adjoint are used to investigate mechanisms controlling the volume transport variability through Bering Strait during 2002 to 2013. Comprehensive time‐resolved sensitivity maps of Bering Strait transport to atmospheric forcing can be accurately computed with the adjoint along the forward model trajectory to identify spatial and temporal scales most relevant to the strait's transport variability. The simulated Bering Strait transport anomaly is found to be controlled primarily by the wind stress on short time scales of order 1 month. Spatial decomposition indicates that on monthly time scales winds over the Bering and the combined Chukchi and East Siberian Seas are the most significant drivers. Continental shelf waves and coastally trapped waves are suggested as the dominant mechanisms for propagating information from the far field to the strait. In years with transport extrema, eastward wind stress anomalies in the Arctic sector are found to be the dominant control, with correlation coefficient of 0.94. This implies that atmospheric variability over the Arctic plays a substantial role in determining Bering Strait flow variability. The near‐linear response of the transport anomaly to wind stress allows for predictive skill at interannual time scales, thus potentially enabling skillful prediction of changes at this important Pacific‐Arctic gateway, provided that accurate measurements of surface winds in the Arctic can be obtained. The novelty of this work is the use of space and time‐resolved adjoint‐based sensitivity maps, which enable detailed dynamical, that is, causal attribution of the impacts of different forcings. Plain Language Summary: An ocean circulation model, that was adjusted to match observations, is used to investigate what are the important factors controlling the oceanic flow of water through the Bering Strait. Results show that the flow through the strait is related to surface atmospheric winds over the Bering Sea Shelf (south of the strait) and the near coastal regions of the Arctic Ocean (north of the strait). In the model, knowledge of these winds over the preceding 1 month allows us to reconstruct most of the changes in the flow through the strait. A somewhat surprising result is that winds in the Arctic have a greater influence on the amount of water flowing through the Bering Strait than winds over any region of the Pacific Ocean or the Bering Sea. The connection between the winds and the flow through the strait is strong enough that interannual changes in the winds may be used to predict interannual change in the flow. This predictive skill opens up the prospect for an improved understanding of the causes and mechanisms of flow changes at this important Pacific‐Arctic gateway, provided that accurate measurements of surface winds over the Arctic can be obtained. Key Points: An adjoint sensitivity analysis is performed to quantify the role of atmospheric forcing on the variability of Bering Strait throughflowPrimary driver of the variability is the wind stress over the Bering Sea and Arctic shelves, on time scales matching shelf wave propagationImpact of precipitation, although consistent with steric flow control, yields insignificant variability on monthly to interannual time scales [ABSTRACT FROM AUTHOR]

The transport of warm Atlantic Waters north of Svalbard is one of the major heat and salt sources to the Arctic Ocean. The circulation pathways and the associated heat transport influence the variability in the Arctic sea ice extent, the onset of freezing, and marine ecosystems. We present observations obtained from research cruises and an autonomous underwater glider mission in summer and fall 2018, to describe the hydrographic structure, volume transport, and circulation patterns of the warm Atlantic Water Boundary Current between 12°E and 24°E north of Svalbard. The Atlantic Water volume transport reaches a maximum of 3.0 ± 0.2 Sv in October, with an intraseasonal variability of 1 Sv (1 Sv = 106 m3 s−1). During summer and late fall, we observed an Atlantic Water recirculation flowing westward (0.1–0.2 Sv) in the outer part of the section away from the shelf break. This counter current appears to be a part of an anticyclonic circulation in the Sofia Deep. The strength of the Atlantic Water recirculation and the Atlantic Water boundary current is very sensitive to the wind stress curl: The boundary current volume transport doubled in less than a week, corresponding to a transition from strongly negative (−10−6 N m−3) to strongly positive (10−6 N m−3) wind stress curl over the Sofia Deep. A previously unknown, deep bottom‐intensified current is observed to flow parallel to the boundary current, between the 1,500 and 2,000 m isobaths. Historical data in the region support the presence of the bottom‐intensified current. Plain Language Summary: Atlantic Water enters the Arctic Ocean along two pathways, through the Barents Sea and through Fram Strait west of Svalbard. Being warmer and saltier than the Arctic Ocean, the Atlantic Water is the main heat and salt source for the Arctic Ocean. The circulation pathways and the heat transported by the Atlantic Water influence the variability of the Arctic sea ice and the marine ecosystems. We present observations from research cruises and an autonomous underwater glider mission in summer and fall 2018, to describe the Atlantic Water structure and circulation between 12°E and 24°E north of Svalbard (an extension of the Fram Strait inflow). The Atlantic Water volume transport reaches a maximum of 3.0 ± 0.2 Sv in October, with an intraseasonal variability of 1 Sv (1 Sv = 106 m3 s−1). During summer and late fall, we observed Atlantic Water flowing westward (a counter current), north of the common eastward flowing Atlantic Water current. Observations suggest that the counter current is part of a clockwise circulation in the Sofia Deep, north of Svalbard. Our observations also reveal a previously undescribed deep, cold current flowing eastward into the Arctic Ocean. Historical data in the region support the presence of this deep current. Key Points: The average Atlantic Water inflow in summer and fall 2018 was 2.6 Sv, reaching 3 Sv in OctoberA recirculation in the Sofia Deep transports (0.1–0.2 Sv) Atlantic Water westwardA deep, bottom‐intensified current on the lower continental slope carries cold waters into the Arctic [ABSTRACT FROM AUTHOR]

At the base of the marine food web, phytoplankton are an essential component of the Arctic Ocean ecosystem and carbon cycle. Especially after sea ice retreats and light becomes more available to the Arctic Ocean each summer, phytoplankton productivity is limited by nutrient availability, which can be replenished by vertical mixing of the water column. One potential mixing mechanism is gale‐force wind associated with summer storm activity. Past studies show that sustained high winds (>10 m s−1) impart sufficient stress on the ocean surface to induce vertical mixing, and it has been speculated that greater storm activity may increase net primary productivity (NPP) on a year‐to‐year timescale. We test this idea using a combination of satellite products and reanalysis data from 1998 to 2018. After controlling for the amount of open water, sea‐surface temperature, and wind direction, we find evidence that greater frequency of high‐wind events in summer is associated with greater seasonal NPP in the Barents, Laptev, East Siberian, and southern Chukchi Seas. This relationship is only robust for the Barents and southern Chukchi Seas, which are more strongly impacted by inflow of relatively nutrient‐rich water from the Atlantic and Pacific Oceans, respectively. In other words, stormier summers may have higher productivity in several regions of the Arctic Ocean, but especially the two inflow seas. Additionally, a recent rise in high‐wind frequency in the Barents Sea may have contributed to the simultaneous increase in NPP. Plain‐Language Summary: Microscopic phytoplankton are an essential component of the Arctic Ocean ecosystem because they are the base of the marine food web. Sea ice in the Arctic Ocean limits phytoplankton growth by reducing how much sunlight reaches the ocean surface, but sea ice is becoming less common in the Arctic Ocean over time. This increases the importance of another limit on phytoplankton: nutrients. Phytoplankton near the surface use up nutrients rapidly in spring, so summer growth is limited by the rate that nutrients are replenished. Past studies have shown that individual storms with strong winds can stir up enough nutrients from deeper waters to temporarily enhance phytoplankton activity. We find that this is not simply a localized or short‐term impact. Stormier summers tend to have higher productivity in several regions of the Arctic Ocean. We also find that the frequency of high‐wind events has increased in recent years, so although the decline in Arctic sea ice can explain much of the increase in phytoplankton growth over 1998–2018, an increase in storminess is also a likely part of the story. Key Points: Past studies suggest that wind‐induced mixing enhances Arctic Ocean phytoplankton productivity by increasing nutrient availabilityGreater frequency of summer high‐wind events is associated with enhanced NPP in the two inflow seas (Barents Sea and southern Chukchi Sea)Positive trend of NPP for the Barents Sea in recent years is partly explained by increased frequency of high‐wind events [ABSTRACT FROM AUTHOR]

This study investigates the impact of assimilation of Northwestern Tropical Pacific Ocean (NWTPO) moored currents observation system on low‐frequency (over a period of more than 90 days) current estimation. A low‐frequency scale‐selective ensemble optimal interpolation (EnOI) scheme is designed to assimilate the horizontally sparse moored currents into an eddy‐permitting ocean model. In this assimilation scheme, low‐frequency current signals are selected by removing intraseasonal signals in both observation and ensemble data, and a horizontal spatial filter is implemented to match the model forecast to the low‐frequency ensemble and observation. The assimilation of currents efficiently resolves simulated low‐frequency current displacements, especially for the Equatorial Undercurrent (EUC), the North Equatorial Countercurrent (NECC), and the North Equatorial Current (NEC). More realistic intensities of the NWTPO low‐frequency currents are also reproduced. With respect to the moored velocity profiles, the root‐mean‐square error (RMSE) of the simulated low‐frequency zonal velocity above a 500‐m depth is reduced from 10.9 to 5.8 cm/s using this low‐frequency EnOI scheme. The low‐frequency variation of NWTPO currents is reproduced by correcting the structures of different baroclinic modes. The increments of zonal velocities mainly correspond to the first baroclinic mode for the NECC and NEC, while for the EUC, they correspond to the leading three and higher‐order baroclinic modes. The improvements of the simulated stratification and pressure gradient remark the role of the assimilation of moored currents in adjusting available potential energy. Plain Language Summary: Ocean data assimilation is a mathematical tool that optimally combines ocean observation data with the numerical ocean model, which was widely applied for the ocean state estimation and forecasting. To better constrain the complex low‐frequency (over a period of more than 90 days) currents in the Northwestern Tropical Pacific Ocean (NWTPO), we set out to investigate the impact of assimilating NWTPO moored currents on the ocean state estimation. The assimilation of moored currents into an eddy‐permitting ocean model is implemented with a low‐frequency scale‐selective ensemble optimal interpolation (EnOI) scheme, where a horizontal spatial filter on the eddy‐permitting model forecast is designed to match the model forecast to the low‐frequency background error covariance and observation. The assimilation of moored currents shows the importance in correcting the simulated low‐frequency current displacements. By adjusting the structure of different baroclinic modes when assimilating the moored currents, the low‐frequency variation of the Equatorial Undercurrent (EUC), the North Equatorial Countercurrent (NECC), and the North Equatorial Current (NEC) can be efficiently reproduced. With respect to the observed vertical temperature profiles and satellite altimeter data, an improvement of simulated stratification and pressure gradient in the NWTPO is also presented. Key Points: The assimilation of currents efficiently resolves simulated low‐frequency current displacementsReproduced NWTPO low‐frequency currents are attributed to correction of the structure of different baroclinic modesA spatial filter is designed to weaken intraseasonal currents from the model forecast [ABSTRACT FROM AUTHOR]

A new method for generating synthetic salinity (SS) profiles in the Southwest Atlantic was developed and applied in data assimilation experiments. This method was based on the smallest integrated values of root mean square deviation (RMSD)—with respect to observations—to infer salinity through climatological data and by regressive methods on temperature (T) using a five‐order polynomial function (P5). In the 14 delimited subregions, the averaged RMSD of P5 was 45% smaller than interpolated climatological data. However, climatological salinity presented better results in the first top layers while P5 presented smaller errors in higher depths. Therefore, by joining the best that P5 and climatology may offer, a new hybrid approach was used to generate SS based on T from XBT profiles. The SS would allow more T profiles to be employed in the Oceanographic Modeling and Observation Network (REMO) data assimilation system, called RODAS, into the Hybrid Coordinate Ocean Model (HYCOM). The use of SS estimates has potential to improve model outputs, in which the presence of the T‐S pair is quite necessary. Three integrations were performed: one run without assimilation (FREE), one assimilating sea surface temperature, Argo profilers, and sea level anomaly (RODAS) and one similar to RODAS, but with added XBTs with SS (RODAS_XBT). The inclusion of XBT data in the HYCOM + RODAS system improved the position and magnitude of the Brazil Current (BC). It was shown that SS is feasible for producing ocean reanalysis and initial conditions for ocean forecast systems requiring very low computational cost. Key Points: A new hybrid approach for estimating salinity from temperature profiles in the Southwest Atlantic was developedThis method showed better responses than obtained only by regression with temperature data or by climatological dataData assimilation of XBTs with synthetic salinity into the HYCOM+RODAS was able to improve the system [ABSTRACT FROM AUTHOR]

We present a coupled three‐dimensional biological‐physical model for the Salish Sea and evaluate it by comparison to nitrate, silicate, and chlorophyll observations. It accurately reproduces nitrate concentrations with Willmott skill scores, root‐mean‐square error, and bias ranging from 0.84–0.95, 4.02–6.5 μM, and −2.33–1.84 μM, respectively, compared to three independent discrete sample data sets. A prominent feature of the model output is a tidal jet emanating from Discovery Passage producing a downstream plume of elevated surface nitrate. The signal is present from April to September, when surface nitrate is otherwise drawn down. It has a weak but statistically significant correlation to Discovery Passage tidal velocity (R=0.37, p<0.01). Within the turbulent jet and associated plume, the average rate of vertical nitrate supply due to mixing and advection across a depth of roughly 6 m is 0.46 μmol m−2 s−1 between 15 May and 20 August 2015, compared to 0.10 μmol m−2 s−1 for the northern Strait of Georgia as a whole. Close to Discovery Passage, where velocities and shear are strongest, the majority of the vertical nitrate flux is due to mixing. As velocities weaken downstream, vertical advection becomes more important relative to mixing, but vertical velocities also decrease. The tidal pulses out of Discovery Passage drive waves that contribute net upward nitrate flux as far south as Cape Lazo, 40 km away. The nitrate supply drives new production, consistent with existing observations. Similar dynamics have been described in many other tidally influenced coastal systems. Plain Language Summary: A three‐dimensional coupled physical‐biological model has been developed that simulates currents, nutrients, and phytoplankton in the Salish Sea. Comparisons with observations show that the model successfully represents key aspects of the nitrogen cycle in the Strait of Georgia, including the seasonal cycle and regional differences. The model is used to investigate mechanisms through which tidal currents lead to nutrient supply to the surface layer, where phytoplankton have enough light to grow, in the northern Strait of Georgia. Strong tidal flow through the narrow Discovery Passage leads to turbulence that mixes deep nutrients to the surface. Tidal currents then advect the surface nutrient plume further into the Strait of Georgia. Additional mixing and vertical motion also occur downstream, increasing the amount of nutrients supplied to the surface. Key Points: A physical‐biological model of the Salish Sea represents spring blooming and seasonal and spatial patterns in nutrients and chlorophyllA tidal jet drives mixing and transport, producing a surface nitrate plume, fueling significant summertime primary productionA wave associated with the jet propagates southward along the Vancouver Island coast and makes a smaller contribution to mixing and transport of nitrate [ABSTRACT FROM AUTHOR]

The climatological structure of the subpolar cyclonic circulation off East Antarctica is delineated with Argo float data from the past decade. Up to 40% of the float profiles in the seasonal ice zone have been without satellite positioning. We refined their position data as following the bathymetry to get appropriate positions in the continental margin. The error of the terrain‐following interpolation was estimated by using positioned data to be 23 ± 27 (78 ± 70) km for 90 (390) day period. Profiles with the under‐ice period shorter than 360 days are adopted. The float trajectories reveal the extent of the subpolar gyre adjoined to the westward Antarctic Slope Current to its south and the southernmost eastward jet of the Antarctic Circumpolar Current along 4,000 m isobath to its north. The subpolar circulation in the Australian‐Antarctic Basin comprises of a series of quasi‐barotropic subgyre circulations, which are bounded by bathymetric spurs in the continental slope. The temperature field reveals shoreward excursions of Circumpolar Deep Water associated with the subgyres, effectively supplying heat to the continental shelves. An along‐slope temperature variation up to 1°C in 27.7–27.8 kg/m3 σθ indicates an active cross‐slope exchange within the layer. Provided the velocity field and the water mass structure, the subsurface water mass exchange is likely accomplished by a combination of topographically controlled mean flow and the eddy transports. Our findings suggest that the bathymetry primarily determines the structure of the subpolar gyre. Plain Language Summary: Subpolar gyres in the Southern Ocean are clockwise circulations that separate cold shelf water from warm offshore water. These circulations are essential for the Antarctic climate since they control heat supply onto the continental shelves. In this study, the physical structure of the subpolar gyre off East Antarctica was investigated using autonomous profiling float data with a new methodology. We found that the subpolar gyre in the Australian‐Antarctic Basin is composed of a series of smaller circulations, which are called subgyres. Subgyres are associated with the bathymetry and likely regulate the water exchange across the continental slope. We refined the traditional picture of the basin‐scale subpolar gyre, and the circulation structure was described in the context of water transport from offshore to inshore. Key Points: The subpolar gyre in the Australian‐Antarctic Basin is comprised of a series of subgyre circulations regulated by the continental slopeCross‐slope water exchange likely occurs in the Circumpolar Deep Water density rangeUnder‐ice float data with a new methodology revealed a detailed ocean structure in the marginal ice zone [ABSTRACT FROM AUTHOR]

Using the Gauss‐Markov decomposition method, this study investigates the mean structure and seasonal variation of the tropical gyre in the Indian Ocean based on the observations of surface drifters. In the climatological mean, the clockwise tropical gyre consists of the equatorial Wyrtki Jets (WJs), the South Equatorial Current (SEC), and the eastern and western boundary currents. This gyre system redistributes the water mass over the entire tropical Indian Ocean basin. Its variations are associated with the monsoon transitions, featuring a typical clockwise pattern in the boreal spring and fall seasons. The relative importance of the geostrophic and Ekman components of the surface currents as well as the role of eddy activity were further examined. It was found that the geostrophic component dominates the overall features of the tropical gyre, including the SEC meandering, the broad eastern boundary current, and the axes of the WJs in boreal spring and fall, whereas the Ekman component strengthens the intensity of the WJs and SEC. Eddies are active over the southeastern tropical Indian Ocean and transport a warm and fresh water mass westward, with direct impact on the southern branch of the tropical gyre. In particular, the trajectories of drifters reveal that during strong Indian Ocean Dipole or El Niño‐Southern Oscillation events, long‐lived eddies were able to reach the southwestern Indian Ocean with a moving speed close to that of the first baroclinic Rossby waves. [ABSTRACT FROM AUTHOR]

This study investigates extreme phytoplankton blooms in the southern tropical Indian Ocean (TIO) in January–April 2011 using the extended ocean color products and in situ data. The amplitude of the blooms was approximately 4 times higher than the climatological value and 2 times higher than that in 1998–1999. The anomalous enhancement of surface chlorophyll‐a concentration (Chla) was associated with the strong upwelling Rossby waves that forced by the extraordinary strong wind stress curl in the southeastern TIO during 2010–2011 La Niña, which was much stronger than that of 1998–1999 La Niña. The Rossby waves uplifted the thermocline, which was shoaled by more than 70 m relative to the climatology. The results of the nonlinear 1.5‐layer reduced‐gravity model further suggest that the thermocline variations are mainly due to the wind stress curl over the interior TIO. In the vertical direction, the Argo data show a distinct upward and westward propagation of subsurface cooling, indicating that the upwelling of cold nutrient‐rich waters leads to the abnormally high Chla. The strong upwelling processes are also well captured by the Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction observations at 8°S, 80.5°E. Based on the climatological in situ nitrate data, the Rossby wave‐induced nitrate supply and potential new production at the mooring site are estimated. The potential f ratio ranges from 50% to 87% when the water brought to the surface is assumed to have originated from 60 to 200 m, thus indicating the dominant role of nutrient supply by the upwelling processes. Plain Language Summary: Taking advantage of extended ocean color products and observation from Argo and Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction, this study investigates extreme phytoplankton blooms in the southern TIO during the 2010–2011 La Niña event. The amplitude of the blooms was approximately 4 times higher than the climatological value and 2 times higher than that in another La Niña year 1998–1999. The anomalous enhancement of surface chlorophyll‐a concentration was associated with the strong upwelling Rossby waves that forced by the extraordinary strong wind stress curl in the southeastern TIO during 2010–2011 La Niña. The in situ data reveal the dominant role of Rossby wave‐induced upwelling in triggering the extreme blooms in the southern TIO. Key Points: The extreme high chlorophyll‐a in the southern TIO in 2011 is identified from two‐decade satellite observationThe in situ data reveal the dominant role of Rossby wave‐induced upwelling in triggering the bloomsLa Niña‐induced wind stress curl in the southeastern TIO is much stronger in 2010–2011 than that in 1998–1999, accounting for the Rossby waves [ABSTRACT FROM AUTHOR]

This study investigates the horizontal distribution of wave energy flux in the tropical Atlantic Ocean using shallow‐water model experiments for three gravest baroclinic modes forced by climatological winds. This is the first attempt to apply a unified diagnostic scheme to the analysis of energy fluxes associated with both Rossby waves (RWs) and Kelvin waves (KWs) in the tropical Atlantic Ocean. Those analyses were difficult in previous studies owing to the difference of equatorial and quasigeostrophic dynamics. The scheme yields the transfer route (i.e., trajectories by group velocity vector) of wave energy originating from both local wind forcing and boundary reflection. The meridional flux of wave energy crossing two zonal sections (at 3°N and at 2°S) is examined for understanding where the equatorial and off‐equatorial regions interconnect. Equatorward energy fluxes into the basin interior are found mainly on the zonal section in the Northern Hemisphere (at 3°N), indicating the radiation of RWs from the Guinea coast. At the equator, seasonal transition in May from eastward wind anomaly to westward anomaly yields a significant energy input at the central basin that radiates both RWs and KWs. The energy flux of these RWs arrives at the western boundary in September, followed by reflection of KWs to yield eastward transbasin energy fluxes in October‐December. The analysis also identifies an unexpected wind input in May at 4°S which induces both RWs and inertial gravity waves. Plain Language Summary: Propagation, diffraction, and reflection of large‐scale waves play an important role in tropical climate variations. Horizontal energy fluxes associated with Rossby and Kelvin wave interaction have been little investigated owing to the lack of an appropriate diagnostic scheme in previous studies. A recent study has developed a diagnostic scheme which can reveal the trajectory of the group velocity of both Rossby and Kelvin waves with satisfying coastal boundary conditions. This scheme has been used in the present study to investigate energetics in the output of climatological model experiments. At the equator, seasonal transition in May from eastward wind anomaly to westward anomaly yields a significant energy input at the central basin that radiates both Rossby and Kelvin waves. The energy flux of these Rossby waves arrives at the western boundary in September, followed by reflection of Kelvin waves to yield eastward transbasin energy fluxes in October‐December. The unified scheme may be used to understand the impact of tropical climate variations to off‐equatorial regions and vice versa in a future study. Key Points: The energy flux of large‐scale waves in the tropical Atlantic is determined using shallow‐water models with climatological wind forcingA unified scheme of energy flux for both Rossby and Kelvin waves is applied to identify their interactions via boundary reflectionThe scheme allows to quantify poleward energy flux along coastal boundaries and equatorward flux into the basin interior [ABSTRACT FROM AUTHOR]

Meltwater that pools on the surface of Arctic sea ice enhances solar absorption and accelerates further ice melt. The impact of melt ponds on energy absorption is controlled primarily through their influence on ice albedo, which is, in turn, governed in large part by the ponds' spatial coverage. This work seeks to observe the spatial coverage of melt ponds across the Arctic basin with sufficient accuracy to investigate pond‐albedo feedback and presents an improved technique to achieve this goal. We approach the problem by using the Open Source Sea Ice Processing algorithm to classify surface features in submeter resolution optical satellite imagery over select sites where such imagery is available. These data establish "true" estimates of pond coverage and the ponds' spectral reflectance. This information is then used to inform, improve, and test spectral unmixing and machine learning techniques that seek to determine melt pond coverage from more widely available, but lower resolution, optical satellite imagery (e.g., Moderate Resolution Imaging Spectroradiometer). The new machine learning approach improves accuracy from prior work and can contribute to improved efforts to validate melt pond models or understand trends in pond coverage. Nevertheless, we encounter and carefully document significant challenges to retrieving melt pond fractions from low‐resolution optical imagery. These limit accuracy to levels below that necessary for resolving climatologically important trends. We conclude that greatly expanding the collection of high‐resolution satellite imagery over sea ice is necessary to monitor melt pond coverage with the accuracy needed by the scientific community. Plain Language Summary: As temperatures in the Arctic rise during summer months, meltwater pools on the surface of sea ice. These ponds are darker than snow/ice and cause more sunlight to be absorbed. This creates a feedback cycle that accelerates ice melt. A primary factor in this cycle is the percentage of the ice surface that is covered by ponds. It is therefore important to observe the spatial coverage of ponds across the Arctic to better understand their impact sea ice loss. Due to the remoteness of the Arctic Ocean, it is necessary to utilize satellite imagery to investigate the properties of sea ice. In satellite imagery there is a trade‐off between resolution and spatial coverage: Images can be either "sharp," seeing many details of the surface but covering a small region, or "blurry" covering a large region but where each pixel contains many ponds. This work seeks to improve techniques (called spectral unmixing) for determining the fraction of each low‐resolution pixel that is a pond. We investigate the theoretical limits of spectral unmixing and conclude that it has several limitations that negatively impact its usefulness. Finally, we trial a new machine learning approach to the same problem that shows promising results. Key Points: Current methods of estimating pond coverage on sea ice from MODIS have high error and bias when compared to new high‐resolution data setsSpectral unmixing used to observe sea ice features fails to meet key assumptions that reduce its potential accuracyA new machine learning model that improves subpixel surface estimations from MODIS is presented [ABSTRACT FROM AUTHOR]

Ocean swells can be a significant coastal hazard, potentially causing coastal disasters with costly damage to infrastructure and tragic loss of lives. We have observed this in the coastal region of Toyama Bay in the Sea of Japan, which has been devastated by severe winter swell events (the so‐called YoriMawari‐nami; YM wave). An extreme swell event was recorded in February 2008 in which the wave height at a site in the bay reached 9+ m, which corresponds to 16 times the local climatological average. Substantial efforts by the Japanese coastal engineering community have been expended to reproduce YM wave events with phase‐averaged wave models but, to our knowledge, with no success. During an archetype YM event, we found that the Noto peninsula filtered the incoming wavefield such that in the bay incident waves were quasi‐monochromatic. The swell then refracted over a submarine canyon resulting in a bimodal directional distribution of wave energy. This initiated conditions favorable for coherent interference, violating the assumptions of the phase‐averaged approach. Coherent interference is the key mechanism for understanding the formation of the large amplitudes and for matching observed high space‐time variability. Our results demonstrate the significance of the coherent interference for wave statistics over coastal bathymetry; only by accounting for phase‐resolving phenomena were we able to adequately reproduce the observations. Plain Language Summary: Waves, which originate in strong storms in the north of the Sea of Japan, have recently devastated the coastal region of Toyama Bay. These type of events have been known for centuries, and the locals refer to them as YoriMawari‐nami. Engineers have struggled to understand the origin of YoriMawari‐nami events, though important for protecting our coasts, and they have been unable to reproduce them with typical models (phase‐averaged wave models). We used data to characterize these events, which are always long, old waves (swells) with unusually large wave heights compared to the past wave climate. We use wave models and ray‐tracing techniques to show how a swell arrives in the bay and bends over submarine canyons. The submarine canyon, like a prism separating light into different colors, separates out swells, and wave phases (the exact timing of an individual wave cycle) can no longer be ignored. In fact, the wave phases become crucial for understanding the enhancement of wave height. We were able to reproduce the observations with a phase‐resolving model, giving hope for predicting these events in the future. Key Points: The coasts of Toyama Bay in Japan have been devastated by severe swell events called YoriMawari‐namiWe characterize these events with data, ray tracing, and phase‐averaged and phase‐resolving modelsPhase‐coherent amplification emerged as the key mechanism for matching observations [ABSTRACT FROM AUTHOR]

Hydrographic data collected from research cruises, bottom‐anchored moorings, drifting Ice‐Tethered Profilers, and satellite altimetry in the Beaufort Gyre region of the Arctic Ocean document an increase of more than 6,400 km3 of liquid freshwater content from 2003 to 2018: a 40% growth relative to the climatology of the 1970s. This fresh water accumulation is shown to result from persistent anticyclonic atmospheric wind forcing (1997–2018) accompanied by sea ice melt, a wind‐forced redirection of Mackenzie River discharge from predominantly eastward to westward flow, and a contribution of low salinity waters of Pacific Ocean origin via Bering Strait. Despite significant uncertainties in the different observations, this study has demonstrated the synergistic value of having multiple diverse datasets to obtain a more comprehensive understanding of Beaufort Gyre freshwater content variability. For example, Beaufort Gyre Observational System (BGOS) surveys clearly show the interannual increase in freshwater content, but without satellite or Ice‐Tethered Profiler measurements, it is not possible to resolve the seasonal cycle of freshwater content, which in fact is larger than the year‐to‐year variability, or the more subtle interannual variations. The Beaufort Gyre centered in the Canada Basin of the Arctic Ocean is the major reservoir of fresh water in the Arctic. The primary focus of this study is on quantifying variability and trends in liquid (water) and solid (sea ice) freshwater content in this region. The Beaufort Gyre Exploration Program was initiated in 2003 to synthesize results of historical data analysis, design and conduct long‐term observations, and to provide information for numerical modeling under the umbrella of the FAMOS (Forum for Arctic Observing and Modeling Synthesis) project. The data collected from research cruises, moorings, Ice‐Tethered Profiler observations, and satellite altimetry document an increase of more than 6,400 km3 of liquid freshwater content from 2003 to 2018, a 40% growth relative to the climatology of the 1970s. This fresh water volume is comparable to the fresh water volume released to the sub‐arctic seas during the Great Salinity Anomaly episode of the 1970s. Thus, since the 2000s, the stage has been set for another possible release of fresh water to lower latitudes with accompanying climate impacts, including changes to sea ice conditions, ocean circulation, and ecosystems of the Sub‐Arctic similar to the influence of the Great Salinity Anomaly observed in the 1970s. Key Points: Beaufort Gyre freshwater content time series (2003–2018) from different data sets are updated, compared, and analyzedQualitative and quantitative estimates of factors and mechanisms driving freshwater content changes are providedIn 2003–2018, the major sources of accumulated fresh water were sea ice melt, Mackenzie River runoff, and Bering Strait transport [ABSTRACT FROM AUTHOR]

Impacts of salinity anomalies associated with the positive Indian Ocean Dipole (pIOD) are assessed through novel sensitivity experiments using a regional ocean model (Regional Ocean Model System) and detailed diagnostics of heat and momentum budget. During the pIOD, density stratification in the eastern equatorial Indian Ocean is enhanced due to anomalous surface freshening and subsurface saltening. This causes momentum inputs from the wind forcing to be more strongly trapped in the surface layer, and zonal and vertical current anomalies to be more confined to the upper layer. As a result, upward transports of cold water from below the thermocline to the surface layer are significantly suppressed, and the cooling in the eastern equatorial Indian Ocean is suppressed by as much as 1.0 °C. The above arguments are further corroborated by a set of sensitivity experiments using a linear continuously stratified ocean model, which can isolate the effect of stratification change caused by salinity anomalies associated with the pIOD in the Regional Ocean Model System simulation. Our results suggest that salinity does play an active role in the evolution of the pIOD, rather than being passively affected by large‐scale anomalous atmospheric and oceanic conditions. Plain Language Summary: Salinity modulates the upper ocean circulation and temperature by changing the density of seawater and dynamical/themodynamical processes that control them. Recent studies have demonstrated that the tropical Indian Ocean experiences significant salinity variation associated with the Indian Ocean Dipole (IOD), which is a dominant climate mode in the tropical Indian Ocean and exerts widespread impacts on the global climate. However, how these salinity anomalies affect the upper ocean properties and feedback onto the IOD are not quantitatively understood. Here, we investigate the roles played by salinity variation associated with the positive IOD (pIOD) by conducting a series of sensitivity experiments using a regional ocean model. The results from our experiments show that these pIOD‐related salinity anomalies act to shift the current anomalies upward and weaken the surface cooling by about 1.0 °C in the eastern equatorial Indian Ocean. This is because enhanced salinity stratification traps momentum imparted by wind to the upper layer and alters the structure of the equatorial circulation. The validity of this mechanism is supported by detailed heat and momentum budget analyses, as well as sensitivity experiments using a simplified ocean model. These results provide clear evidences that salinity plays an active role in the evolution of the pIOD. Key Points: Role of salinity in the evolution of the positive Indian Ocean Dipole is quantitatively assessed using a regional ocean modelSensitivity experiments reveal significant impacts of salinity anomalies on the upper ocean temperature and currents anomaliesEnhanced stratification and associated weakening of upwelling suppress the sea surface temperature cooling in the east by 1.0 °C [ABSTRACT FROM AUTHOR]

Many studies on future climate projection point out that with progressing of global warming, upper‐ocean density stratification will strengthen over this century, and consequently, global‐averaged ocean primary productivity will decrease. Observed long‐term changes in the stratification to date, however, still show large uncertainties of the change itself and its driver. Focusing on the vertical difference in the emergence of the global warming signals, we used only observational profiles to describe the spatiotemporal characteristic of long‐term trend and variability in the upper‐ocean stratification (defined as the density difference between the surface and 200‐m depth). Statistically significant strengthening of the stratification since the 1960s was detected in ~40% of the global ocean area. The global average increase in the stratification corresponds to 3.3–6.1% of the mean stratification. The strengthening trends considerably change depending on the regions and show dominant contribution from the tropical region. In addition to the well‐documented explanation of strengthening stratification, namely the surface intensification of global warming signal, we found that changes in subsurface temperature and salinity stratification associated with changes in atmospheric/ocean circulations significantly contribute to the long‐term change in the stratification and setting its regional difference. In midlatitude and high‐latitude ocean of the Northern Hemisphere, the long‐term trend exhibits noteworthy seasonality, which shows faster increase trend in the summer than in the winter. From the detrended time series, interannual variabilities correlated with a particular climate mode are detected in several ocean regions, suggesting that these variabilities are mainly driven by associated sea surface temperature variation. Plain Language Summary: In general, upper‐ocean stratification (i.e., vertical changes in sea water density) serves as a barrier between relatively warm and nutrient‐depleted surface water and relatively cool and nutrient‐rich subsurface water. As a consequence of global warming, globally averaged upper‐ocean thermal stratification is thought to be enhanced due to the surface intensification of the warming signal. Strengthened stratification will have a large impact on oceanic biological activity through changes in light availability and vertical nutrient supply to the upper‐ocean phytoplankton community. Because investigations of the change in stratification are limited in ocean regions and have employed relatively short‐term observational data, large uncertainties remain. In this study we investigated the changes in stratification with as great a spatial and temporal coverage as possible, using only historically collected profile data. We found strengthened stratification in the large part of the global ocean with large spatial heterogeneity. Changes in the vertical structure of salinity and subsurface temperature contribute to the changes in density stratification, in addition to the surface intensification of the global warming signal. Key Points: Statistically significant strengthening of the upper‐ocean stratification since the 1960s is detected in ~40% of global ocean areaThe global average increase in the stratification corresponds to 3.3–6.1% of the climatological meanIn addition to surface warming, changes in subsurface temperature and salinity structure significantly contribute to the long‐term change [ABSTRACT FROM AUTHOR]

Rising seas coupled with ever increasing coastal populations present the potential for significant social and economic loss in the 21st century. Relatively short records of the full multidimensional space contributing to total water level coastal flooding events (astronomic tides, sea level anomalies, storm surges, wave run‐up, etc.) result in historical observations of only a small fraction of the possible range of conditions that could produce severe flooding. The Time‐varying Emulator for Short‐ and Long‐Term analysis of coastal flood hazard potential is presented here as a methodology capable of producing new iterations of the sea‐state parameters associated with the present‐day Pacific Ocean climate to simulate many synthetic extreme compound events. The emulator utilizes weather typing of fundamental climate drivers (sea surface temperatures, sea level pressures, etc.) to reduce complexity and produces new daily synoptic weather chronologies with an auto‐regressive logistic model accounting for conditional dependencies on the El Niño Southern Oscillation, the Madden‐Julian Oscillation, seasonality, and the prior two days of weather progression. Joint probabilities of sea‐state parameters unique to simulated weather patterns are used to create new time series of the hypothetical components contributing to synthetic total water levels (swells from multiple directions coupled with water levels due to wind setup, temperature anomalies, and tides). The Time‐varying Emulator for Short‐ and Long‐Term analysis of coastal flood hazard potential reveals the importance of considering the multivariate nature of extreme coastal flooding, while progressing the ability to incorporate large‐scale climate variability into site specific studies assessing hazards within the context of predicted climate change in the 21st century. Plain Language Summary: Predicting extreme coastal flooding is a present‐day societal need and will only become more relevant as mean water levels increase due to sea level rise. However, the number of processes contributing to such events is too high for relatively short observational records to have measured all of the constructive combinations of waves, surge, wind, and sea level anomalies. We present a framework designed to create hypothetical combinations of relevant flood hazard potential processes by simulating the climate and weather patterns that drive coastal flooding. Including large‐scale oceanic and atmospheric patterns as the drivers of coastal hazards reveals the climate a coastal community is most vulnerable to, which will be increasingly more important to understand as the climate changes during the 21st century. Key Points: Stochastic climate emulator explicitly links climate patterns to local coastal flooding components (waves, surges, sea level anomalies, etc.)Thousands of years of synthetic total water levels are directly simulated via individual components producing long return period event magnitudesConservation of temporal structure and persistence of interannual, seasonal, and intra‐seasonal time scales [ABSTRACT FROM AUTHOR]

The Southern Ocean features regionally varying ventilation pathways that transport heat and carbon from the surface ocean to the interior thermocline on timescales of decades to centuries, but the factors that control the distribution of heat along these pathways are not well understood. In this study, we use a global ocean state estimate (ECCOv4) to (1) define the recently ventilated interior Pacific (RVP) using numerical passive tracer experiments over a 10‐year period and (2) use an adjoint approach to calculate the sensitivities of the RVP heat content (RVPh) to changes in net heat flux and wind stress. We find that RVPh is most sensitive to local heat flux and wind stress anomalies north of the sea surface height contours that delineate the Antarctic Circumpolar Current, with especially high sensitivities over the South Pacific Gyre. Surprisingly, RVPh is not especially sensitive to changes at higher latitudes. We perform a set of step response experiments over the South Pacific Gyre, the subduction region, and the high‐latitude Southern Ocean. In consistency with the adjoint sensitivity fields, RVPh is most sensitive to wind stress curl over the subtropical gyre, which alter isopycnal heave, and it is only weakly sensitive to changes at higher latitudes. Our results suggest that despite the localized nature of mode water subduction hot spots, changes in basin‐scale pressure gradients are an important controlling factor on RVPh. Because basin‐scale wind stress is expected to change in the coming decades to centuries, our results may have implications for climate, via the atmosphere/ocean partitioning of heat. Key Points: The heat content of the recently ventilated Pacific (RVPh) is strongly controlled by gyre circulationOn timescales shorter than 3–5 years, RVPh is most sensitive to midlatitude wind stress anomaliesOn timescales longer than 3–5 years, RVPh is most sensitive to midlatitude heat flux anomalies [ABSTRACT FROM AUTHOR]

The shallow Chukchi Sea is a gateway to the Arctic Ocean for Pacific‐origin waters. While a substantial portion of the Pacific‐origin waters flows through Barrow Canyon in the northeast corner of the Chukchi Sea, little is known on the hydrography of the surrounding regions in winter. We present profiles of wintertime hydrography on the Chukchi slope from an autonomous profiling instrument and mooring records in Barrow Canyon. The central and western sectors of Barrow Canyon in December 2016 to February 2017 were anomalously warm (∼0.5 °C warmer than the climatology) with the flow orienting toward the Arctic Ocean. Unlike the summertime warming near the surface, the warm outflow has a temperature maximum at 80 dbar, and this outflow modifies the water mass properties on the Chukchi slope 70 km north of the canyon. Based on our Barrow Canyon mooring records starting in 2002, this is the first time that such warming is recorded on the outflow in winter. We discuss that this is due to the combination of the Barrow Canyon outflow favorable wind pattern and warming in the southern Chukchi Sea (the Gulf of Alaska and the Bering Sea) before the winter. Key Points: The Barrow Canyon outflow was anomalously warm in the winter 2016–2017, and the warming reached below 100 dbar on the Chukchi slopeThe wind was the outflow favorable in the winter 2016–2017, and the south of the Chukchi was warm before the outflowThe sustained winds and the ocean south of the Chukchi Sea favor the warm Barrow Canyon outflow for the first time in our record [ABSTRACT FROM AUTHOR]

The Eastern Tropical North Atlantic Oxygen Minimum Zone (OMZ) is a biogeochemically important area. The low oxygen in this region is thought to be maintained by a balance between the slow mixing supply of O2 and its removal by respiration. We use data from 90 isopycnal RAFOS floats to characterize the mixing coefficients responsible for the supply of oxygen to the region. One group was ballasted to drift on the isopycnal where oxygen is at its minimum and the other about 300 m deeper. Using the record of the float positions at each 6‐hr interval, we calculate the relative dispersion of pairs of floats. The time derivative of this dispersion provides a diffusivity coefficient that captures the net effect of eddy‐driven mixing along each isopycnal. Float pairs deployed at shared locations but across the two target densities reveal that the influence of vertically sheared currents is to accelerate the dispersion by 10–15% relative to true isopycnal dispersion. Relative dispersion of the floats in the OMZ area obeyed the canonical four‐thirds power scaling, representative of two‐dimensional turbulence. At the length scale of the maximum energy‐containing eddy (approximately 100 km), the effective diffusivity is 1,400±500 m2/s in the zonal direction and 800±300 m2/s in the meridional. Within the uncertainty, the diffusivities on the two isopycnals are indistinguishable from one another. An idealized model suggests that meridional mixing across the large‐scale O2 gradient is the leading supply term of oxygen to the OMZ. Key Points: Ninety Lagrangian isopycnal floats tracked for 600 days in a region with very slow mean advection used to calculate isopycnal diffusivityIsopycnal diffusivities are nearly twice as large in the zonal than meridional directionMeridional diffusion acting on the large‐scale oxygen gradient is the primary source of oxygen balancing biological consumption [ABSTRACT FROM AUTHOR]

As decadal predictions become operational, the need to use, understand, and extract information from them becomes essential. A climate index is a simple diagnostic quantity that can be used to characterize integral aspects of a geophysical system such as circulation patterns, and thus can be used to evaluate decadal forecasts. One of the most studied and well documented regions of the World Ocean is the North Atlantic. The North Atlantic subpolar gyre is an important region for the modulation of European climate and where skillful predictions of up to a decade can be obtained. Ocean re‐analysis (ORA‐S4) data from 1959 to 2017 are used to introduce a new methodology to compute a climate index of the North Atlantic subpolar gyre that captures both the variability in its strength and shape, the latter was to our knowledge never investigated previously as part of the variability of the gyre on interannual to decadal time scales. The methodology reveals two states of the gyre (before and after 2000), the former is mainly driven by temperature and the latter by a combination of mechanisms that interact to sustain a relatively stable subpolar gyre in terms of strength. Plain Language Summary: Decadal predictions is a relatively new topic in climate science and is driven by the needs of industry and society to adapt to future climate states. To that end, elaborate earth system models provide forecasts that nevertheless need to be validated. Some regions of the world ocean are easier to predict such as the North Atlantic subpolar gyre. One way to validate the forecasts is through climate indeces. To that extent, we follow a new approach to define a climate index for the subpolar gyre that captures many features of the circulation that relate to the strength and the size of the gyre. Key Points: A climate index that is a measure of the changes in strength and in the shape of the North Atlantic subpolar gyre systemThe suggested methodology can be applied to various data sets and can be improved with data sets with higher spatial resolutionThe climate index distinguishes periods with different dynamical characteristics before and after 2000 [ABSTRACT FROM AUTHOR]

Normalized water‐leaving radiance spectra nLw(λ) at the near‐infrared (NIR) bands from six years of observations (2012–2017) with the Visible Infrared Imaging Radiometer Suite (VIIRS) on‐board the Suomi National Polar‐orbiting Partnership are used to derive both the power law slope η of the particle backscattering coefficient spectra bbp(λ) and the particle size power law slope ξ for turbid coastal and inland waters. Based on the fact that the absorption coefficient of sea water aw(λ) is generally much larger than those of the other constituents aiop(λ) at the NIR wavelengths in coastal and inland waters, a NIR‐based bbp(λ) algorithm for turbid waters has been used to derive the power law slope η. A three‐order polynomial function between η and ξ in Kostadinov et al. (2009, https://doi.org/10.1029/2009JC005303) is applied to calculate the particle size slope ξ. Seasonal and interannual variations of η and ξ in China's east coastal region, the Amazon River Estuary, the La Plata River Estuary, the Meghna River Estuary, and the Atchafalaya River Estuary are characterized and quantified. Except for the Amazon River Estuary, η and ξ have significant seasonal dependence in these highly turbid regions. Among the global highly turbid waters, the Meghna River Estuary shows the most significant seasonal variations in η and ξ with the largest particle size in the surface layer. The peaks of the climatology η and ξ are at ~ −1.2 and 3.2 in the Meghna River Estuary, respectively. Particle size parameters η and ξ are also shown to be related to the magnitude of nLw(λ) (or bbp(λ)) at the NIR bands with strong regional dependence. This study demonstrates that η and ξ increase from the inshore to offshore regions in these highly turbid waters, illustrating the more large particles in the inshore waters than the offshore ocean regions. In addition, wind and precipitation data in the Amazon River Estuary and Meghna River Estuary show that wind forcing is the major driver for the particle size distribution dynamics in these highly turbid water regions. Key Points: A NIR‐based PSD algorithm for turbid waters has been developed to characterize the particle size distribution in estuarine watersSeasonal and interannual variations of particle size distribution in the ocean's most turbid waters are characterized and quantifiedThe Meghna River Estuary shows the most pronounced seasonal variations of the PSD slope [ABSTRACT FROM AUTHOR]

Our understanding of the seasonal variability in the global wind wave field is revisited here using a novel analysis that resolves the directional wave spectra. Empirical orthogonal function analysis was applied to modeled wave spectral data from a WAVEWATCH III hindcast across a sparse global grid to identify the main patterns of the climatological wave spectral variability at each grid point. Prior methods have focused on the variability of two modes of the wave field—locally generated sea and the primary swell component. Our results also consider additional wave modes at each location, enabling us to track the passage of the less dominant swell modes. Consistent with existing climatological knowledge, the main modes of wave spectral variability at high latitudes are related to eastward propagating waves that disperse equatorward as swell following great circle paths. However, despite being the less energetic mode, the Northern Hemisphere generated swell is found to propagate into the Southern Hemisphere further than the more energetic Southern Hemisphere swell which propagates northward. In the equatorial zone, a complex multimodal wave climate is found, with the spectra variability modulated by remotely generated swell and higher‐frequency waves associated with the prevailing winds. The evolution of these patterns throughout the year is clearly depicted. Overall, our approach captures a more complete picture of the seasonal variability of the global wave field, by accounting for all the wave modes observed in the spectra at each location together with their temporal variability. Plain Language Summary: Waves generated by the wind blowing over the surface of the ocean are able to travel immense distances and reach distant coasts (e.g., it has been recognized since the 1950s that the coasts of California receive waves generated in the Southern Ocean). This means that the wave climate of a given region can be very complex, having waves generated by local winds but also from many remote places at once (multiple swell fields). In this study, we present a new method to examine seasonal variations in the global wave climate which accounts for the full directional wave spectra and includes wave systems with different frequencies and directions separately, as opposed to using integrated wave parameters, such as significant wave height or mean direction. Our results show how low‐frequency swell waves propagate across ocean basins from high to low latitudes of both hemispheres, higher‐frequency waves that are a product of the local winds, and the variation in the intensity of these signals throughout the year. We believe future wave studies can benefit from this approach, since a concise and yet more complete representation of the wave climate variability can be achieved. Key Points: Dominant modes of seasonal variability in global directional wind wave spectra were identifiedThe wave climate at middle to high latitudes is well described with one or two wave modes, but in the tropics three to four modes are neededThe rotation of the wave signals at high latitudes was captured, which could be beneficial for future studies [ABSTRACT FROM AUTHOR]

A global reference state of the subtropical mode water and permanent pycnocline properties for the 2000–2015 period is presented. The climatology is obtained from a pattern recognition algorithm applied to stratification profiles from the Argo global array. The stratification features are identified as permanent upper ocean pycnostad and pycnocline even when the seasonal pycnocline is developed. The climatology shows that both Northern Hemisphere subtropical gyres have a qualitatively very similar stratification structure. The permanent pycnoclines in the North Atlantic and North Pacific show two deep centers colocated with thick subtropical and subpolar mode waters. These centers coincide with modes in the density and stratification space. These deep pycnocline centers are separated by a region with a shallower and thinner permanent pycnocline that is located downstream of Western Boundary Current Extensions and upstream of Eastern Subtropical Fronts. This feature creates a remarkable double‐bowl pattern at the basin scale. In the subtropical gyres of the South Atlantic and South Pacific Oceans, the mode water and permanent pycnocline structures are characterized by two modes in the density and stratification space that, unlike in the Northern Hemisphere, do not necessarily correspond to deep and thick permanent pycnocline regions. In the subtropical gyre of the South Indian Ocean, a single mode is found to correspond to a single center in the western part of the gyre. This study also shows that away from these deep centers where the pycnocline depth almost follows isopycnals, the permanent pycnocline experiences significant thermohaline gradients that are not density compensated. Plain Language Summary: The vertical structure of the ocean is of great importance to understand the impact of climate changes on the global ocean because a larger density differences (i.e., increased stratification) between the upper and the deeper ocean can prevent anthropogenic excess of heat, and carbon, from reaching the abyssal ocean. Using data from autonomous Argo floats that sample the ocean properties (e.g., temperature and salinity) from the surface down to 2,000 m, we describe the vertical structure of the ocean's surface layers at midlatitudes, where heat is mostly stored in the upper 1,000 m. This new study shows for the first time (i) a strong dependence of the ocean properties of the surface layer with the ocean properties of the transition layer with the abyss and (ii) that this transition layer exhibits rapid changes in temperature and density when rising to the surface as well as in the center of the each oceans. These new results are a significant refinement to the classic depiction of the midlatitude ocean as a simple bowl of warm water separated from the abyss by a layer of constant density and thus provide an accurate benchmark for climate models to detect long‐term changes in the ocean vertical structure. Key Points: In each subtropical basin, the stratification pattern of a mode water pycnostad overlying a permanent pycnocline exists continuously throughout the gyreThe pycnocline has a double‐bowl pattern with two deep centers colocated with thick mode waters and shallower in the center of the gyreThe North Atlantic subtropical permanent pycnocline and mode waters are much deeper, thicker, and less stratified than those in other gyres [ABSTRACT FROM AUTHOR]

In 2017, Ice Atmosphere Ocean Observing System autonomous drifting platforms provided extensive physical and biogeochemical data in the upper 350 m of the western Eurasian Basin through their 8‐month drift across the Amundsen Basin, the Gakkel Ridge, the Nansen Basin and western Fram Strait. Comparison with WOA13 climatology indicates a fresher surface layer and shallower warm layer in 2017 than in 2005–2012. The Ice Atmosphere Ocean Observing System 2017 data feature two halocline eddies in the Amundsen Basin and two Atlantic Water (AW) mesoscale structures in the Nansen Basin. Analysis of the global (1/12)° Mercator Ocean operational system suggests that the halocline eddies resulted from instabilities in the frontal zone between fresher Makarov waters and saltier Eurasian waters. This frontal region appears to have shifted further southeast in 2017 (near 88°N, 10°E) compared to 2005–2012. The operational system depicts the large AW structure in the Nansen Basin (140 km crossed as far as 83.7°N, 34.5°E) as an AW meander from the Arctic Circumpolar Boundary Current, which turned into an anticyclonic eddy about a month after the platforms drifted away. The AW structure at 82.8°N, 3°W, northwest of the Yermak Plateau, corresponds to an AW recirculating branch detaching from the Yermak Plateau slope back toward Fram Strait. Plain Language Summary: In 2017, IAOOS (Ice Atmosphere Ocean Observing System) autonomous drifting platforms provided extensive physical and biogeochemical data in the upper 350 m of the Western Eurasian Basin through their 8‐month drift. The surface layer is fresher and the warm layer shallower in 2017 than in 2005‐2012. The IAOOS 2017 measurements document several medium scale ocean structures. Simulations from a high resolution ((1/12)°) model (Mercator Ocean operational model) provide insights on the nature and origin of these observed structures. Key Points: Autonomous profilers provide an extensive physical and biogeochemical characterization of the western Eurasian Basin upper 350 m in 2017The western Eurasian Basin exhibits a fresher surface layer and shallower warm Atlantic Water layer in 2017 than in 2005‐2012Mercator Ocean operational model outputs bring insights on the observed mesoscale structures in the halocline and warm layer [ABSTRACT FROM AUTHOR]

Interior, cross‐equatorial transports are unique phenomena of the Indian Ocean, fluxing heat and salt between hemispheres. We use real and simulated surface drifter trajectories to reveal three cross‐equatorial gyres in the northern Indian Ocean that are driven by the seasonally reversing monsoon winds. Simulated trajectories are computed by advecting hundreds of thousands of particles through a monthly drifter climatology. Gyres are depicted using probabilistic pathlines, defined as the most common trajectories that cross the equator within the Somali Current. Northward pathlines in boreal spring and summer split into two gyres: A 1‐year loop contained west of the Maldives, described as a wind‐driven overshoot of the Southern Hemisphere equatorial gyre, and a 2‐year figure‐of‐eight, encompassing the Great Whirl and reaching the eastern boundary via the equatorial Wytrki jet, to return westward within the North Monsoon Current. Both these gyres include cross‐equatorial flow, driven by southward Ekman drift during the southwest monsoon. Southward pathlines through the Somali Current in winter originate in the dissipating Great Whirl and feed eastward into the South Equatorial Counter Current. Eighty‐five percent of simulated trajectories participate in these gyres, with significant leakage into the Bay of Bengal (14%) and Mozambique Channel (8%). A striking result of our analysis is the lack of pathlines connecting the Somali Current and coastal upwelling regions with the Arabian Basin, in contrast to the continuous coastal current that seasonal streamlines depict. Geostrophic pathlines reveal slow connectivity between these regions below the surface. Consideration of upwelling and subduction is reserved for subsequent modeling studies. Plain Language Summary: The Somali Curent connects the Southern and Northern Hemispheres in the western Indian Ocean. Under the influence of the monsoon winds, the Somali Current reverses in direction and connects with different currents annually. Each basin of the Indian Ocean possesses different water mass characteristics, in temperature and salinity, for example. We use real and simulated drifters, devices flowing with the currents, to show the most important pathways followed by the water masses through the Somali Current. We find three gyres, or recirculation patterns, crossing the equator. Two of them are observed during the boreal spring and summer monsoon, when the Somali Current flows northward: a 1‐year loop remaining in the western part of the Indian Ocean and a 2‐year figure‐of‐eight extending from the coast of Somali to the eastern coast of India. The last gyre is observed in winter, when the Somali Current reverses to flow southward. Our approach uses a monthly resolution, thus focusing on quasi‐stationary features (e.g., the Great Whirl, a large anticyclone) and large‐scale mechanisms: geostrophy and wind‐induced motions, thus keeping aside smaller features such as mesoscale eddies. A striking result is the lack of connectivity between the Somali Basin and Arabian Basin. Key Points: Probabilistic pathlines are computed to trace the circulation of surface waters through the time‐evolving flow of the monsoonal Indian OceanThree cross‐equatorial gyres link the Somali Current with circulation around the equatorial gyre and Seychelles‐Chagos thermocline ridgeThe Somali Current is only weakly connected to the Arabian Basin via slow, geostrophic, eddying flows beneath the Ekman layer [ABSTRACT FROM AUTHOR]

Using the Estimating Circulation and Climate of the Ocean Phase II product, this study investigates the eddy characteristics and energetics in two dynamical regimes within the Kuroshio Extension (KE) region. Based on the empirical orthogonal function analysis, it is found that the decadal evolution of eddy kinetic energy in the KE region is characterized by a delayed negative correlation between the upstream and downstream. Besides the out‐of‐phase change in eddy activity levels, eddy characteristics and energetics in the upstream KE are also different from those in the downstream. In the upstream region, eddies are stretched in the zonal direction and the unstable state is characterized by strong eddy‐shedding processes. During these processes, cyclonic eddies are found to be formed in the first quasi‐stationary meander and finally pinched off from the KE jet. Energy budget illustrates that the eddy shedding processes are triggered by baroclinic instability, while barotropic instability becomes the dominated energy source after the meander is fully developed. Accompanied by the generation of cyclonic eddies, significant inverse energy cascade is detected. When the upstream KE is stable, the eddy activity is dominated by baroclinic instability and the inverse energy cascade occurs similarly. Distinct from the upstream, eddies in the downstream KE tend to be stretched in the meridional direction and the decadal variability of eddy kinetic energy in this region is mainly regulated by the upstream through lateral advection. Plain Language Summary: Based on classic physical oceanography theory, oceanic eddies are generated mainly through the vertical shear of background mean flow, i.e. baroclinic instability (BC). A stronger mean flow should favor BC and lead to more eddy generation. However, at decadal timescales, the correlation between the strength of the Kuroshio Extension (KE) jet and the regional eddy kinetic energy (EKE) level is negative, therefore the decadal variability of mesoscale eddy field in the KE system cannot be simply explained by the classic instability theory. This study explores in great depth the dynamical processes in the unstable state in the upstream KE and analyzes the evolution of eddy energetics during these processes. It is found that unstable events in the upstream KE are characterized by strong eddy‐shedding processes. During these events, cyclonic eddies are found to be formed in the first quasi‐stationary meander and finally pinched off from the KE jet. During these processes, both BC and barotropic energy transfer are important in the generation of eddies. This study will be a step toward the understanding of low‐frequency variability in strong oceanic current regions in the global ocean. Key Points: Energy‐weighted length scale of eddies in the KE region is about 250 km; in the upstream (downstream) region, eddies tend to be stretched in the zonal (meridional) directionIn the upstream, the unstable state is characterized by strong eddy‐shedding processes; strong cyclonic eddies are found to be pinched off from the first quasi‐stationary meander and are controlled by both baroclinic and barotropic instabilitiesIn the downstream, eddy activity is mainly regulated by the upstream through lateral advection [ABSTRACT FROM AUTHOR]

Abstract: Water properties and formation processes of Alaskan Coastal Winter Water (ACWW) over the eastern Chukchi shelf along the Alaska coast, the so‐called Barrow Canyon pathway, are examined using data from moorings, atmospheric reanalysis, satellite‐derived sea‐ice production (SIP), and a numerical tracer experiment. Along this pathway, Pacific Winter Water (PWW) can be modified to produce ACWW through SIP accompanied by production of cold, saline polynya water in the coastal polynyas, upwelling of warm Atlantic Water (AW), and mixing processes on the shelf. Three different types of ACWW are formed: (i) a mixture of AW and PWW, (ii) a mixture of AW and polynya water, and (iii) hypersaline polynya water. The northeasterly winds, correlated with the north‐south atmospheric pressure gradient between Beaufort High and Aleutian Low, are common triggers of polynya SIP episodes and AW upwelling in the Barrow Coastal Polynya (BCP). Due to the dual impact of northeasterly winds, PWW modification processes in the BCP are more complicated than what occurs elsewhere in the Chukchi Polynya. The impact of AW upwelling on the ACWW formation is most prominent in the BCP, usually centered along the coast. All types of ACWW are thought to be basically transported westward or northwestward with the Chukchi slope current and/or Beaufort Gyre and finally contribute to maintenance of the lower halocline layer especially over the Chukchi Borderland, Northwind Ridge, and southern Canada Basin. Even in the Pacific sector of the Arctic Ocean, ACWW properties are strongly influenced by both Atlantic‐origin and Pacific‐origin waters. [ABSTRACT FROM AUTHOR]

The El Niño and Southern Oscillation (ENSO) can induce large perturbations in freshwater flux (FWF, defined as precipitation minus evaporation) and ocean ecosystem in the tropical Pacific. However, how El Niño‐induced FWF can affect the tropical Pacific ecosystem (e.g., chlorophyll) is still unknown. Here, a series of ocean‐only experiments are performed using a coupled ocean‐physical biogeochemistry model forced by prescribed climatological wind stress. Interannual FWF anomalies observed during the 1997–1998 El Niño event are imposed onto the ocean model in the tropical Pacific, and the related positive FWF anomalies are specified to be varying in its intensity, and the corresponding sensitivity experiments are performed to examine the ocean ecosystem response. In general, chlorophyll increases with the intensities of the El Niño‐induced positive FWF anomalies at a certain strength but decreases when the positive FWF forcing is underrepresented (e.g., specified to be its half intensity). Additionally, chlorophyll in the eastern tropical Pacific keeps almost steady during the increasing stage of the El Niño‐induced interannual FWF forcing and then rapidly increases after the peak stage of the FWF forcing. The phytoplankton budget and diagnostic analyses are conducted to understand the behavior of chlorophyll in response to varying intensities of FWF forcing. Chlorophyll response to varying intensities of FWF forcing depends on the iron concentration in the mixed layer, suggesting that the ocean ecosystem response may be shifted from one regime to another when FWF forcing reaches a certain intensity. These results offer insights into biophysical interactions and chlorophyll‐induced feedback effects on ENSO. Plain Language Summary: Chlorophyll is used to represent changes in phytoplankton biomass, and its variability is significantly modulated by multiscale physical processes. In the tropical Pacific, ENSO can induce large perturbations in freshwater flux (FWF) and ocean ecosystem. Given that FWF forcing affects sea surface salinity and buoyancy flux, it is reasonable to ask the following question: Can El Niño‐induced FWF forcing affect the tropical Pacific ecosystem (e.g., phytoplankton and chlorophyll)? Here we examine the effect of FWF forcing on ocean biology using a coupled physics‐biogeochemistry model, in which FWF forcing is explicitly represented with varying intensity. Results suggest that chlorophyll concentration in the eastern equatorial Pacific tends to increase in response to El Niño‐accompanied positive FWF anomalies. However, a regime shift can occur when the FWF forcing intensity is underrepresented (e.g., specified to be its half intensity), with chlorophyll concentration tending to decrease instead. The behavior of chlorophyll in response to varying FWF forcing intensities is regulated by the iron concentration in the mixed layer, suggesting that the ocean ecosystem response may be shifted from one regime to another when FWF forcing reaches a certain intensity. These results offer insights into biophysical interactions in the tropical Pacific and chlorophyll‐induced feedback effects on ENSO. Key Points: Chlorophyll in the eastern equatorial Pacific tends to increase in response to El Niño‐accompanied positive freshwater flux (FWF) anomaliesA regime shift can occur when FWF forcing is weakly represented (e.g., its half intensity), with chlorophyll tending to decrease insteadThe behavior of chlorophyll response to different FWF forcing intensity is regulated by iron concentration in the mixed layer [ABSTRACT FROM AUTHOR]

We evaluate fields of Arctic Ocean precipitation from six atmospheric reanalyses: NASA MERRA, NASA MERRA2, NOAA CFSR/CFSv2, ECMWF ERA‐Interim, ECMWF ERA5, and JMAO JRA55. The study is motivated by recognition that precipitation fields from reanalyses can serve as the key input into snow on sea ice accumulation models, supporting retrievals of sea ice thickness from satellite altimeter systems. Time series of annual precipitation over the central Arctic Ocean correlate well between all reanalyses, and they all capture the basic spatial and seasonal patterns of Arctic precipitation. However, they differ strongly with respect to precipitation amount: CFSR and MERRA2 are wetter than the other reanalyses. All reanalyses depict that the majority of total annual precipitation over the central Arctic Ocean comes from small events, less than 1 mm/day. Validation is challenged by sparse observations, uncertain adjustments for gauge undercatch, and other issues. However, given that correlations with data from Russian North Pole drifting station records are all roughly equal and that they depict similar interannual variability at the regional scale, all of the reanalyses appear suitable for supporting retrievals of ice thickness (provided that appropriate bias corrections can be applied). However, only CFSR, MERRA2, ERA5, and JRA55 will continue to generate output beyond 2019. Plain Language Summary: While the thickness of the sea ice that floats atop the Arctic Ocean can be measured using satellite altimeters—instruments that measure the height that something projects above the surface—this assumes that one knows the mass of any snow cover that lies atop the ice. Estimating the overlying snow cover is a formidable task. One way forward is to use estimates of precipitation from a type of weather model known as an atmospheric reanalysis. We look at precipitation from six different reanalyses and find that all of them hold much potential for providing fields of precipitation. Key Points: Arctic Ocean precipitation from six global atmospheric reanalyses is compared against measured precipitationAll reanalyses capture known spatial and seasonal patterns of precipitation but differ in the amount of precipitationTime series of annual precipitation over the central Arctic Ocean from the six reanalyses are strongly correlated with each other [ABSTRACT FROM AUTHOR]

The generation and dissipation of North Pacific subsurface low potential vorticity water mass (SLPVW) affect various ocean‐atmosphere dynamic and thermodynamic processes on a wide range of time scales. Mesoscale oceanic eddies are believed to play an important role in the variation of SLPVW. National Centers for Environmental Prediction (NCEP)/NCEP‐Climate Forecast System Reanalysis coupled data set during 1979–2010 has been analyzed to investigate the different influences of mesoscale oceanic eddies on the SLPVW between winter and summer. It is found that the correlation between the sea surface height and SLPVW volume in winter is positive on the north of 30°N but negative on the south of 30°N, which is mainly caused by the eddy vorticity strength. The negative correlation value region has more strong anticyclones than the positive correlation value region, so the mixed layer depth in the negative (positive) correlation value region tends to be deeper (shallower) caused by stronger (weaker) anticyclones, which leads to the decreasing (increasing) of the low PV water mass under mixed layer depth (namely, SLPVW). Eddies also have obvious impacts on the subduction rate. The climatological eddy subduction rate can reach to 1.88 Sv in the NCEP‐Climate Forecast System Reanalysis, which accounts for 36.4% of the total subduction rate. In summer, the correlation between the sea surface height and SLPVW volume is remarkably negative. It is found that water diverges (converges) in the composite anticyclone (cyclone), which leads to the increasing (decreasing) of PV value in the composite anticyclone (cyclone), finally accelerates (hinders) the dissipation of SLPVW. Plain Language Summary: The study researched different influences of mesoscale oceanic eddies on the North Pacific subsurface low potential vorticity water mass (SLPVW) between winter and summer. The results revealed that eddies influence the formation of low potential vorticity water by changing turbulent heat flux in winter. Moreover, it is found that the relationship between oceanic eddies and SLPVW volume is influenced by the eddy vorticity strength. The mixed layer depth in the stronger (weaker) anticyclones tends to be deeper (shallower), which leads to the increasing (decreasing) of the low PV water mass but decreasing (increasing) of the low PV water mass under mixed layer depth (namely, SLPVW). The subduction rate varies greatly from year to year, suggesting the role of the interannual variation of eddies. In summer, the rotation of the eddies leads to the convergence and divergence, thus changing the stratification of the ocean and ultimately affecting the dissipation of SLPVW. The results might give the new insight to studies the role of mesoscale eddies on SLPVW, which have important implications for North Pacific climate scientists and predictions. Key Points: The relationship between oceanic eddies and SLPVW volume in winter is influenced by the eddy vorticity strengthThe subduction rate varies greatly from year to year, suggesting the role of the interannual variation of eddiesIn summer, water converges in the composite cyclone, which is beneficial to the retention of the SLPVW [ABSTRACT FROM AUTHOR]