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Dense overflows from marginal seas are critical pathways of oxygen supply to the Arabian Sea oxygen minimum zone (OMZ), yet these remain inadequately understood. Climate models struggle to accurately reproduce the observed extent and intensity of the Arabian Sea OMZ due to their limited ability to capture processes smaller than their grid scale, such as dense overflows. Multi‐month repeated sections by underwater gliders off the coast of Oman resolve the contribution of dense Persian Gulf Water (PGW) outflow to oxygen supply within the Arabian Sea OMZ. We characterize PGW properties, seasonality, transport and mixing mechanisms to explain local processes influencing water mass transformation and oxygen fluxes into the OMZ. Atmospheric forcing at the source region and eddy mesoscale activity in the Gulf of Oman control spatiotemporal variability of PGW as it flows along‐shelf off the northern Omani coast. Subseasonally, it is modulated by stirring and shear‐driven mixing driven by eddy‐topography interactions. The oxygen transport from PGW to the OMZ is estimated to be 1.3 Tmol yr−1 over the observational period, with dramatic inter‐ and intra‐annual variability (±1.6 Tmol yr−1). We show that this oxygen is supplied to the interior of the OMZ through the combined action of double‐diffusive and shear‐driven mixing. Intermittent shear‐driven mixing enhances double‐diffusive processes, with mechanical shear conditions (Ri < 0.25) prevailing 14% of the time at the oxycline. These findings enhance our understanding of fine‐scale processes influencing oxygen dynamics within the OMZ that can provide insights for improved modeling and prediction efforts. Plain Language Summary: The Arabian Sea hosts extremely low‐oxygenated waters at depth (oxygen minimum zone, OMZ). Understanding how the depth of this layer changes and what processes control its variability is vital to understanding how it will change under climate change and how it may affect the ecosystem. One of the ways the oxygen gets into this low‐oxygenated region is through the sinking of a water type called Persian Gulf Water (PGW). This water forms in the Persian Gulf and is hypersaline, warm, and well‐oxygenated. It leaves the Persian Gulf through the narrow Strait of Hormuz and flows around 200 m depth along the northern coast of Oman. High‐resolution ocean glider observations show the variability of its properties across different seasons and years. The amount of oxygen the PGW brings varies a lot over time, changing from year to year and between seasons. We estimate it to be 1.3 Tmol per year during the study period. We identify events, locations and mixing mechanisms that explain how the oxygen contained in PGW can mix with the surrounding low‐oxygen waters, hence increasing the OMZ's oxygen content. This knowledge can be useful for improving climate models and predicting how the OMZ might change in the future. Key Points: PGW is characterized by high spatiotemporal variability in its properties and transport that impacts the variability of the upper OMZOxygen contribution from PGW to the Arabian Sea OMZ is resolved for the first time as 1.3 Tmol yr−1Intermittent shear‐driven mixing at the PGW bottom boundary amplifies ventilation of the OMZ by salt fingering 14% of the time [ABSTRACT FROM AUTHOR]

At the southern boundary of the Antarctic Circumpolar Current (ACC), relatively warm ACC waters encounter the colder waters surrounding Antarctica. Strong density gradients across the southern boundary indicate the presence of a frontal jet and are thought to modulate the southward heat transport across the front. In this study, the southern boundary in the Weddell Sea sector at the prime meridian is surveyed for the first time in high resolution over 2 months during an austral summer with underwater gliders occupying a transect across the front on five occasions. The five transects show that the frontal structure (i.e. hydrography, velocities and lateral density gradients) varies temporally. The results demonstrate significant, transient (a few weeks) variability of the southern boundary and its frontal jet in location, strength and width. A mesoscale cold-core eddy is identified to disrupt the southern boundary's frontal structure and strengthen lateral density gradients across the front. The front's barrier properties are assessed using mixing length scales and potential vorticity to establish the cross-frontal exchange of properties between the ACC and the Weddell Gyre. The results show that stronger lateral density gradients caused by the mesoscale eddy strengthen the barrier-like properties of the front through reduced mixing length scales and pronounced gradients of potential vorticity. In contrast, the barrier-like properties of the southern boundary are reduced when no mesoscale eddy is influencing the density gradients across the front. Using satellite altimetry, we further demonstrate that the barrier properties over the past decade have strengthened as a result of increased meridional gradients of absolute dynamic topography and increased frontal jet speeds in comparison to previous decades. Our results emphasise that locally and rapidly changing barrier properties of the southern boundary are important to quantify the cross-frontal exchange, which is particularly relevant in regions where the southern boundary is located near the Antarctic shelf break (e.g. in the West Antarctic sector). [ABSTRACT FROM AUTHOR]

Quantifying the strength and efficiency of the Southern Ocean biological carbon pump (BCP) and its response to predicted changes in the Earth's climate is fundamental to our ability to predict long-term changes in the global carbon cycle and, by extension, the impact of continued anthropogenic perturbation of atmospheric CO2. There is little agreement, however, in climate model projections of the sensitivity of the Southern Ocean BCP to climate change, with a lack of consensus in even the direction of predicted change, highlighting a gap in our understanding of a major planetary carbon flux. In this review, we summarize relevant research that highlights the important role of fine-scale dynamics (both temporal and spatial) that link physical forcing mechanisms to biogeochemical responses that impact the characteristics of the seasonal cycle of phytoplankton and by extension the BCP. This approach highlights the potential for integrating autonomous and remote sensing observations of fine scale dynamics to derive regionally optimized biogeochemical parameterizations for Southern Ocean models. Ongoing development in both the observational and modelling fields will generate new insights into Southern Ocean ecosystem function for improved predictions of the sensitivity of the Southern Ocean BCP to climate change. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1–100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

At the Southern Boundary of the Antarctic Circumpolar Current (ACC), relatively warm ACC waters encounter the colder waters surrounding Antarctica. Strong density gradients across the Southern Boundary indicate the presence of a frontal jet and are thought to modulate the southward heat transport across the front. In this study, the Southern Boundary in the Weddell Sea sector at the Greenwich Meridian is surveyed for the first time in high resolution over an entire austral summer with underwater gliders transecting the front on 5 occasions. The 5 transects show that the frontal structure (i.e., hydrography, velocities and lateral density gradients) varies temporally. The results demonstrate significant, quite rapid (a few weeks) variability of the Southern Boundary and its frontal jet in location, strength and width. A mesoscale cold-core eddy is identified to disrupt the Southern Boundary's frontal structure and strengthen lateral density gradients across the front. The front's barrier properties are assessed using mixing length scales and potential vorticity to establish the cross-frontal exchange of properties between the Southern Boundary and the Weddell Gyre. The results show that stronger lateral density gradients caused by the mesoscale eddy strengthen the barrier-like properties of the front through reduced mixing length scales and pronounced gradients of potential vorticity. In contrast, the barrier-like properties of the Southern Boundary are reduced when no mesoscale eddy is influencing the density gradients across the front. Using altimetry, we further demonstrate that the barrier properties over the past decade have strengthened as a result of increased meridional gradients of absolute dynamic topography and increased frontal jet speeds in comparison to previous decades. Our results emphasize that locally and rapidly changing barrier properties of the Southern Boundary are important to quantify the cross-frontal exchange, which is particularly relevant in regions where the Southern Boundary is located near the Antarctic shelf break (e.g. in the West Antarctic Sector). [ABSTRACT FROM AUTHOR]

At the Southern Boundary of the Antarctic Circumpolar Current (ACC), relatively warm ACC waters encounter the colder waters surrounding Antarctica. Strong density gradients across the Southern Boundary indicate the presence of a frontal jet and are thought to modulate the southward heat transport across the front. In this study, the Southern Boundary in the Weddell Sea sector at the Greenwich Meridian is surveyed for the first time in high resolution over an entire austral summer with underwater gliders transecting the front on 5 occasions. The 5 transects show that the frontal structure (i.e., hydrography, velocities and lateral density gradients) varies temporally. The results demonstrate significant, quite rapid (a few weeks) variability of the Southern Boundary and its frontal jet in location, strength and width. A mesoscale cold-core eddy is identified to disrupt the Southern Boundary's frontal structure and strengthen lateral density gradients across the front. The front's barrier properties are assessed using mixing length scales and potential vorticity to establish the cross-frontal exchange of properties between the Southern Boundary and the Weddell Gyre. The results show that stronger lateral density gradients caused by the mesoscale eddy strengthen the barrier-like properties of the front through reduced mixing length scales and pronounced gradients of potential vorticity. In contrast, the barrier-like properties of the Southern Boundary are reduced when no mesoscale eddy is influencing the density gradients across the front. Using altimetry, we further demonstrate that the barrier properties over the past decade have strengthened as a result of increased meridional gradients of absolute dynamic topography and increased frontal jet speeds in comparison to previous decades. Our results emphasize that locally and rapidly changing barrier properties of the Southern Boundary are important to quantify the cross-frontal exchange, which is particularly relevant in regions where the Southern Boundary is located near the Antarctic shelf break (e.g. in the West Antarctic Sector). [ABSTRACT FROM AUTHOR]

Antarctic sea ice is prevalently seen as a major player in the climate system, but it is also an important factor in polar maritime safety. Remote sensing and forecasting of Southern Ocean sea ice at time scales suitable for navigation and research planning remain challenging. In this study, numerical sea ice drift forecasts are assessed from the perspective of informing shipping operations. A series of tests is performed to ascertain whether an operational global ocean and sea ice model and a simple free-drift model can provide accurate drift estimates over short lead times. Both approaches are evaluated against ice drift measurements from buoys deployed during two research cruises in the Southern Ocean marginal ice zone during winter and spring. The numerical forecast model was able to forecast sea ice trajectories over 24 h with an average position error of 16.6 km during winter and 9.2 km during spring. The simpler free-drift model, using empirically optimised wind scaling, returned an average position error of 15.9 and 9.3 km during winter and spring respectively. Model skill for both the dynamical and free-drift models is lower in winter than in spring. Free-drift model skill appears linked with sea ice consolidation, which may assist in determining when and where this approach is fit for purpose. Lingering uncertainties regarding the rheological representation of sea ice in the dynamical model and the quality of the wind and ocean forcing remain, potentially affecting model skill over tactical navigation time frames. [ABSTRACT FROM AUTHOR]

The subpolar Southern Ocean is a critical region where CO2 outgassing influences the global mean air-sea CO2 flux (FCO2). However, the processes controlling the outgassing remain elusive. We show, using a multi-glider dataset combining FCO2 and ocean turbulence, that the air-sea gradient of CO2 (∆pCO2) is modulated by synoptic storm-driven ocean variability (20 µatm, 1–10 days) through two processes. Ekman transport explains 60% of the variability, and entrainment drives strong episodic CO2 outgassing events of 2–4 mol m−2 yr−1. Extrapolation across the subpolar Southern Ocean using a process model shows how ocean fronts spatially modulate synoptic variability in ∆pCO2 (6 µatm2 average) and how spatial variations in stratification influence synoptic entrainment of deeper carbon into the mixed layer (3.5 mol m−2 yr−1 average). These results not only constrain aliased-driven uncertainties in FCO2 but also the effects of synoptic variability on slower seasonal or longer ocean physics-carbon dynamics. Storms dominate the subpolar Southern Ocean, where upwelling CO2 drives outgassing that impacts global CO2 budget, yet how storms modify this outgassing is unknown. Here, the authors present coupled atmosphere-ocean observations to show how storm-driven ocean mixing and circulation cause substantial CO2 variability and outgassing. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) dominate moisture transport globally; however, it is unknown what impact ARs have on surface ocean buoyancy. This study explores the surface buoyancy gained by ARs using high‐resolution surface observations from a Wave Glider deployed in the subpolar Southern Ocean (54°S, 0°E) between 19 December 2018 and 12 February 2019 (55 days). When ARs combine with storms, the associated precipitation is significantly enhanced (189%). In addition, the daily accumulation of AR‐induced precipitation provides a buoyancy gain to the surface ocean equivalent to warming by surface heat fluxes. Over the 55 days, ARs accounted for 47% of the total precipitation equating to 10% of the summer surface ocean buoyancy gain. This study indicates that ARs play an important role in the summer precipitation over the subpolar Southern Ocean and that they can alter the upper‐ocean buoyancy budget from synoptic to seasonal timescales. Plain Language Summary: Atmospheric rivers (ARs) are river‐like features in the sky that carry enormous amounts of water vapor from the tropics toward Antarctica and across the Southern Ocean. However, it is not well documented how this freshwater carried by ARs in the atmosphere impacts rainfall over the ocean. This study uses an uncrewed surface vehicle, called a Wave Glider, to measure the meteorology and surface ocean temperature and salinity. We show that ARs almost double rainfall, which directly impact the surface ocean buoyancy (i.e., rainfall adds freshwater and lightens the surface layer waters). These results provide evidence and highlight the importance of ARs in the Atlantic Southern Ocean when it comes to lightening the surface waters. Key Points: Atmospheric rivers (ARs) significantly increase precipitation (189%) in midlatitude cyclones over the Atlantic Southern OceanDuring intense AR‐driven rainfall events, precipitation contributes to daily buoyancy forcing equivalent to the surface heat fluxPrecipitation associated with ARs contributes cumulatively to 10% of the surface ocean buoyancy gain in summer [ABSTRACT FROM AUTHOR]

High‐resolution underwater glider data collected in the Gulf of Oman (2015–2016), combined with reanalysis data sets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique data set to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone. Plain Language Summary: Atmospheric forcings, such as wind and solar heating, and small‐scale ocean processes (1–10 km; e.g., eddies, fronts, filaments) modify the properties and the structure of the water column near the surface. These processes regulate the surface layer, creating a well‐mixed surface layer. The variation in these processes determine how the depth of this surface mixed layer changes through both time and space. This study investigates the variability of this layer during winter and spring in the Gulf of Oman using in situ observations and atmospheric data derived from models and observations. Episodic strong and dry winds from the northwest (Shamals) increase mixing and cause shorter‐term variability of the surface mixed layer. Concurrently, we find that the small‐scale ocean processes mainly shoal the mixed layer depth during winter. These processes are also important in determining the timing of the change from the deeper winter mixed layer to the shallower spring mixed layer, as we find a three‐week difference between the two observed years. The observations illustrate previously unquantified processes in the region that can impact coupling between the atmosphere, surface ocean, and deep ocean, with consequences for regional marine ecosystems. Key Points: Ocean glider observations reveal mixed layer variability that cannot be explained by seasonal warming aloneShamal winds dominate intraseasonal variability of the mixed layer in the Gulf of OmanSubmesoscale mixed layer eddies are responsible for 68% of the restratifying buoyancy flux in winter [ABSTRACT FROM AUTHOR]

The subpolar Southern Ocean is a critical region where CO2 outgassing influences the global mean air-sea CO2 flux (FCO2). However, the processes controlling the outgassing remain elusive. We show, using a multi-glider dataset combining FCO2 and ocean turbulence, that the air-sea gradient of CO2 (∆pCO2) is modulated by synoptic storm-driven ocean variability (20 µatm, 1–10 days) through two processes. Ekman transport explains 60% of the variability, and entrainment drives strong episodic CO2 outgassing events of 2–4 mol m−2 yr−1. Extrapolation across the subpolar Southern Ocean using a process model shows how ocean fronts spatially modulate synoptic variability in ∆pCO2 (6 µatm2 average) and how spatial variations in stratification influence synoptic entrainment of deeper carbon into the mixed layer (3.5 mol m−2 yr−1 average). These results not only constrain aliased-driven uncertainties in FCO2 but also the effects of synoptic variability on slower seasonal or longer ocean physics-carbon dynamics. Storms dominate the subpolar Southern Ocean, where upwelling CO2 drives outgassing that impacts global CO2 budget, yet how storms modify this outgassing is unknown. Here, the authors present coupled atmosphere-ocean observations to show how storm-driven ocean mixing and circulation cause substantial CO2 variability and outgassing. [ABSTRACT FROM AUTHOR]

Polynyas facilitate air–sea fluxes, impacting climate-relevant properties such as sea ice formation and deep water production. Despite their importance, polynyas have been poorly represented in past generations of climate models. Here we present a method to track the presence, frequency and spatial distribution of polynyas in the Southern Ocean in 27 models participating in the Climate Model Intercomparison Project Phase 6 (CMIP6) and two satellite-based sea ice products. Only half of the 27 models form open-water polynyas (OWPs), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWP, while other models unrealistically create OWPs nearly every year. In contrast, the coastal polynya area is overestimated in most models, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs is linked to changes in the regional hydrography, specifically the linkages between polynya activity with deep water convection and/or the shoaling of the upper water column thermocline. Models with an accurate Antarctic Circumpolar Current transport and wind stress curl have too frequent OWPs. Biases in polynya representation continue to exist in climate models, which has an impact on the regional ocean circulation and ventilation that should be addressed. However, emerging iceberg discharge schemes, more adequate vertical grid type or overflow parameterisation are anticipated to improve polynya representations and associated climate prediction in the future. [ABSTRACT FROM AUTHOR]

Proposals frommultiple nations to deploy air-sea flux moorings in the Southern Ocean have raised the question of how to optimize the placement of these moorings in order to maximize their utility, both as contributors to the network of observations assimilated in numerical weather prediction and also as a means to study a broad range of processes driving air-sea fluxes. This study, developed as a contribution to the SouthernOcean Observing System(SOOS), proposes criteria that can be used to determine mooring siting to obtain best estimates of net air-sea heat flux (Qnet). Flux moorings are envisioned as one component of a multiplatform observing system, providing valuable in situ point time series measurements to be used alongside satellite data and observations from autonomous platforms and ships. Assimilating models (e.g., numerical weather prediction and reanalysis products) then offer the ability to synthesize the observing system and map properties between observations. This paper develops a framework for designing mooring array configurations to maximize the independence and utility of observations. As a test case, within the meridional band from 358 to 658S we select eight mooring sites optimized to explain the largest fraction of the total variance (and thus to ensure the least variance of residual components) in the area south of 208S. Results yield different optimal mooring sites for lowfrequency interannual heat fluxes compared with higher-frequency subseasonal fluxes. With eight moorings, we could explain a maximum of 24.6% of high-frequency Qnet variability or 44.7% of low-frequency Qnet variability. [ABSTRACT FROM AUTHOR]

Submesoscale flows in the ocean are energetic motions, O(1–10 km), that influence stratification and the distributions of properties, such as heat and carbon. They are believed to play an important role in sea‐ice‐impacted oceans by modulating air‐sea‐ice fluxes and sea‐ice extent. The intensity of these flows and their response to wind forcing are unobserved in the sea‐ice regions of the Southern Ocean. We present the first submesoscale‐resolving observations in the Antarctic marginal ice zone (MIZ) collected by surface and underwater autonomous vehicles, for >3 months in austral summer. We observe salinity‐dominated lateral density fronts occurring at sub‐kilometer scales. Surface winds are shown to modify the magnitude of the mixed‐layer density fronts, revealing strongly coupled atmosphere‐ocean processes. We posture that these wind‐front interactions occur as a continuous interplay between front slumping and vertical mixing, which leads to the dispersion of submesoscale fronts. Such processes are expected to be ubiquitous in the Southern Ocean MIZ. Plain Language Summary: Satellite radar imagery shows evidence of 1–10 km eddies and jets in the ocean adjacent to the sea‐ice edge around Antarctica. We use field observations of temperature, salinity, and wind speed from autonomous robotic platforms deployed in the sea‐ice zone for >3 months. These measurements provide estimates of the surface ocean density fronts which are controlled primarily by lateral variations in salinity. We show that, during high wind speeds, these surface fronts temporarily dissipate, indicating an atmosphere‐ocean coupling occurring at the submesoscale. The fronts strengthen again during low wind speed, which is thought to be because the stirring of the fresher surface layer by mesoscale eddies leads to the generation of the submesoscale fronts. Providing in situ observations of such features improves our understanding of the small‐scale ocean and climate processes at play, such as how heat and carbon may exchange between the atmosphere and the ocean. Key Points: Using combined surface and underwater robotic observations, we observe haline‐dominated submesoscale fronts in the Antarctic MIZEnhanced wind speeds reduce the magnitude of lateral submesoscale fronts within the surface mixed layerSubmesoscale wind‐front interactions cause a continuous interplay between front slumping and vertical mixing, arresting lateral shear [ABSTRACT FROM AUTHOR]

Ocean stratification and the vertical extent of the mixed layer influence the rate at which the ocean and atmosphere exchange properties. This process has direct impacts for anthropogenic heat and carbon uptake in the Southern Ocean. Submesoscale instabilities that evolve over space (1–10 km) and time (from hours to days) scales directly influence mixed layer variability and are ubiquitous in the Southern Ocean. Mixed layer eddies contribute to mixed layer restratification, while down-front winds, enhanced by strong synoptic storms, can erode stratification by a cross-frontal Ekman buoyancy flux. This study investigates the role of these submesoscale processes on the subseasonal and interannual variability of the mixed layer stratification using four years of high-resolution glider data in the Southern Ocean. An increase of stratification from winter to summer occurs due to a seasonal warming of the mixed layer. However, we observe transient decreases in stratification lasting from days to weeks, which can arrest the seasonal restratification by up to two months after surface heat flux becomes positive. This leads to interannual differences in the timing of seasonal restratification by up to 36 days. Parameterizing the Ekman buoyancy flux in a one-dimensional mixed layer model reduces the magnitude of stratification compared to when the model is run using heat and freshwater fluxes alone. Importantly, the reduced stratification occurs during the spring restratification period, thereby holding important implications for mixed layer dynamics in climate models as well as physical–biological coupling in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Enhanced vertical velocities associated with submesoscale motions may rapidly modify mixed layer depths and increase exchange between the mixed layer and the ocean interior. These dynamics are of particular importance in the Southern Ocean, where the ventilation of many density classes occurs. Here we present results from an observational field program in southern Drake Passage, a region preconditioned for submesoscale instability owing to its strong mesoscale eddy field, persistent fronts, strong down-front winds, and weak vertical stratification. Two gliders sampled from December 2014 through March 2015 upstream and downstream of the Shackleton Fracture Zone (SFZ). The acquired time series of mixed layer depths and buoyancy gradients enabled calculations of potential vorticity and classifications of submesoscale instabilities. The regions flanking the SFZ displayed remarkably different characteristics despite similar surface forcing. Mixed layer depths were nearly twice as deep, and horizontal buoyancy gradients were larger downstream of the SFZ. Upstream of the SFZ, submesoscale variability was confined to the edges of topographically steered fronts, whereas downstream these motions were more broadly distributed. Comparisons to a one-dimensional (1D) mixing model demonstrate the role of submesoscale instabilities in generating mixed layer variance. Numerical output from a submesoscale-resolving simulation indicates that submesoscale instabilities are crucial for correctly reproducing upper-ocean stratification. These results show that bathymetry can play a key role in generating dynamically distinct submesoscale characteristics over short spatial scales and that submesoscale motions can be locally active during summer months. [ABSTRACT FROM AUTHOR]

Surface ocean wind datasets are required to be of high spatial and temporal resolution and high precision to accurately force or be assimilated into coupled atmosphere-ocean numerical models and to understand ocean-atmospheric processes. In situ observed sea surface winds from the Southern Ocean are scarce and, consequently, the validity of simulation models is often questionable. Multiple wind data products were compared to the first known high-resolution in situ measurements of wind speed from Wave Glider (WG) deployments in the Southern Ocean with the intent to determine which blended satellite or reanalysis product best represents the magnitude and variability of the observed wind field. Results show that the ECMWF reanalysis product is the most accurate in representing the temporal variability of winds, exhibiting consistently higher correlation coefficients with in situ data across all wind speed categories. However, the NCEP-DOE AMIP-II Reanalysis product matches in situ trends of deviation from the mean and performs best in depicting the mean wind state, especially during high wind states. The ECMWF product also leads to smaller differences in wind speeds from the in situ data, while CFSv2 showed slightly higher biases and a greater RMSE. The SeaWinds (SW) product consistently performed poorly at representing the mean or wind stress variability compared to those observed by the WG. Overall, the study shows autonomous surface vehicles provide valuable observations by which to validate, understand, and potentially assist in correcting satellite/reanalysis products, particularly in remote regions, where few in situ estimates exist. [ABSTRACT FROM AUTHOR]

The Global Ocean Observing System (GOOS) is deploying a holistic system to monitor the world's ocean; however, a major challenge is many regions are chronically under-sampled. One such region is the Southern Ocean, which is remote and a harsh region to sample. The importance of improving holistic sampling in this region is clear, given its disproportionate significance to Earth and the fact that the area is exhibiting rapid change. As the Southern Ocean is beyond the capability of any single nation, the international Southern Ocean research community recognized a need for improved international coordination, strategic planning, and eventual implementation of a Southern Ocean Observing System (SOOS). The focus of SOOS is on (1) designing a sustainable system that provides data for determining the status and change of the Southern Ocean, (2) standardizing measurements across national efforts, (3) providing a forum for opportunities to guide future investments, (4) developing a portal for open transparent access to data, and (5) supporting grassroots discussion to identify/design expeditions and technology development. This manuscript highlights current SOOS strategies to meet those needs. Critical lessons emphasize the need for providing value to users who are contributing content/strategy as volunteers and sustain a dedicated office to coordinate those efforts while providing documented value to those contributing time and expertise. [ABSTRACT FROM AUTHOR]

In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change. [ABSTRACT FROM AUTHOR]

Low levels of iron limit primary productivity across much of the Southern Ocean. At the basin scale, most dissolved iron is supplied to surface waters from subsurface reservoirs, because land inputs are spatially limited. Deep mixing in winter together with year-round diffusion across density surfaces, known as diapycnal diffusion, are the main physical processes that carry iron-laden subsurface waters to the surface. Here, we analyse data on dissolved iron concentrations in the top 1,000 m of the Southern Ocean, taken from all known and available cruises to date, together with hydrographic data to determine the relative importance of deep winter mixing and diapycnal diffusion to dissolved iron fluxes at the basin scale. Using information on the vertical distribution of iron we show that deep winter mixing supplies ten times more iron to the surface ocean each year, on average, than diapycnal diffusion. Biological observations from the sub-Antarctic sector suggest that following the depletion of this wintertime iron pulse, intense iron recycling sustains productivity over the subsequent spring and summer. We conclude that winter mixing and surface-water iron recycling are important drivers of temporal variations in Southern Ocean primary production. [ABSTRACT FROM AUTHOR]

The Brazil–Malvinas Confluence (BMC) is one of the most dynamical regions of the global ocean. Its variability is dominated by the mesoscale, mainly expressed by the presence of meanders and eddies, which are understood to be local regulators of air-sea interaction processes. The objective of this work is to study the local modulation of air-sea interaction variables by the presence of either a warm (ED1) and a cold core (ED2) eddy, present in the BMC, during September to November 2013. The translation and lifespans of both eddies were determined using satellite-derived sea level anomaly (SLA) data. Time series of satellite-derived surface wind data, as well as these and other meteorological variables, retrieved from ERA5 reanalysis at the eddies' successive positions in time, allowed us to investigate the temporal modulation of the lower atmosphere by the eddies' presence along their translation and lifespan. The reanalysis data indicate a mean increase of 78% in sensible and 55% in latent heat fluxes along the warm eddy trajectory in comparison to the surrounding ocean of the study region. Over the cold core eddy, on the other hand, we noticed a mean reduction of 49% and 25% in sensible and latent heat fluxes, respectively, compared to the adjacent ocean. Additionally, a field campaign observed both eddies and the lower atmosphere from ship-borne observations before, during and after crossing both eddies in the study region during October 2013. The presence of the eddies was imprinted on several surface meteorological variables depending on the sea surface temperature (SST) in the eddy cores. In situ oceanographic and meteorological data, together with high frequency micrometeorological data, were also used here to demonstrate that the local, rather than the large scale forcing of the eddies on the atmosphere above, is, as expected, the principal driver of air-sea interaction when transient atmospheric systems are stable (not actively varying) in the study region. We also make use of the in situ data to show the differences (biases) between bulk heat flux estimates (used on atmospheric reanalysis products) and eddy covariance measurements (taken as "sea truth") of both sensible and latent heat fluxes. The findings demonstrate the importance of short-term changes (minutes to hours) in both the atmosphere and the ocean in contributing to these biases. We conclude by emphasizing the importance of the mesoscale oceanographic structures in the BMC on impacting local air-sea heat fluxes and the marine atmospheric boundary layer stability, especially under large scale, high-pressure atmospheric conditions. [ABSTRACT FROM AUTHOR]