Search

Atlantic time‐mean heat transport is northward at all latitudes and exhibits strong multidecadal variability between about 30°N and 55°N. Atlantic heat transport variability influences many aspects of the climate system, including regional surface temperatures, subpolar heat content, Arctic sea‐ice concentration and tropical precipitation patterns. Atlantic heat transport and heat transport variability are commonly partitioned into two components: the heat transport by the Atlantic Meridional Overturning Circulation (AMOC) and the heat transport by the gyres. In this paper we compare four different methods for performing this partition, and we apply these methods to the Community Earth System Model Large Ensemble at 34°N, 26°N and 5°S. We discuss the strengths and weaknesses of each method. The four methods all give significantly different estimates for the proportion of the time‐mean heat transport performed by AMOC. One of these methods is a new physically‐motivated method based on the pathway of the northward‐flowing part of AMOC. This paper presents a preliminary version of our method that works only when the AMOC follows the western boundary of the basin. All the methods agree that at 26°N, 80%–100% of heat transport variability at 2–10 years timescales is performed by AMOC, but there is more disagreement between methods in attributing multidecadal variability, with some methods showing a compensation between the AMOC and gyre heat transport variability. Plain Language Summary: Scientists often want to quantify how much heat is transported by the Atlantic Meridional Overturning Circulation (sometimes called the "Conveyor Belt" circulation) and how much heat is transported by the ocean's gyres. This paper compares some different methods for estimating the heat transport by the overturning circulation and by the gyres, including a new method that has not been used before. There are significant differences in the results of each method, and here we discuss the strengths and weaknesses of each method. The heat transport at about 30°N has a lot of variability: all the methods agree that short‐timescale variability (2–10 years timescale) is due to the overturning circulation. All methods agree that more than half of the time‐mean heat transport across 30°N is performed by the overturning, but the exact proportion varies from 65% to 120% depending on the method we use. Key Points: We compare four diagnostics for Atlantic Meridional Overturning Circulation (AMOC) heat transport that partition the flow into mass‐conserving gyre and overturning componentsA new physically‐motivated method is compared with the standard method that relies on zonally averaging the temperature and velocity fieldsAll four methods yield different estimates of the fraction of time‐mean heat transport performed by AMOC [ABSTRACT FROM AUTHOR]

In the setting of the thin-shell approximation of the Euler equations in spherical coordinates for oceanic flows with variable density on the spinning Earth, we study a vorticity equation for a pseudo stream function ψ , whereby the assumption of incompressibility allows us to express the density as a function of ψ . Via an elliptic comparison argument, we show that, under certain assumptions, the (explicit) solution in the case of zero rate of rotation (i.e., on a fixed sphere) in a bounded region with smooth boundary contained either in the Northern or in the Southern Hemisphere is an approximation, in a suitable sense, of the corresponding solution of the equation with positive rate of rotation in the same region. This provides new insight into the dynamics of ocean gyres. [ABSTRACT FROM AUTHOR]

This article discusses the isolation and taxonomic identification of 46 eukaryotic phytoplankton strains from the tropical Pacific Ocean. The study aimed to address the lack of knowledge about the physiology and distribution patterns of open ocean eukaryotic phytoplankton. The isolates were grouped into five major photosynthetic groups and have been deposited into the NCMA culture collection for further study. The article highlights the importance of studying these phytoplankton in the context of expanding oligotrophic ocean gyres and global warming. The document provides a list of diatoms, including their identification codes, measurements, and taxonomic information. The diatoms listed include Thalassiosira oceanica, Thalassiosira diporocyclus, Thalassiosira, Cylindrotheca closterium, Fragilariopsis, Nitzschia traheaformis, Nitzschia, and Pseudo-nitzschia. The measurements provided include length and width, and the number of individuals measured is also given. The document is a valuable resource for researchers studying diatoms and their taxonomy. Another document provides a list of different types of microorganisms found in a specific location, including diatoms, stramenopiles, dinoflagellates, green algae, haptophytes, and coccolithophores. Each entry includes information such as the species name, sample location, and genetic code. This information can be useful for researchers studying [Extracted from the article]

The climatological mean barotropic vorticity budget is analyzed to investigate the relative importance of surface wind stress, topography, planetary vorticity advection, and nonlinear advection in dynamical balances in a global ocean simulation. In addition to a pronounced regional variability in vorticity balances, the relative magnitudes of vorticity budget terms strongly depend on the length‐scale of interest. To carry out a length‐scale dependent vorticity analysis in different ocean basins, vorticity budget terms are spatially coarse‐grained. At length‐scales greater than 1,000 km, the dynamics closely follow the Topographic‐Sverdrup balance in which bottom pressure torque, surface wind stress curl and planetary vorticity advection terms are in balance. In contrast, when including all length‐scales resolved by the model, bottom pressure torque and nonlinear advection terms dominate the vorticity budget (Topographic‐Nonlinear balance), which suggests a prominent role of oceanic eddies, which are of O(10–100) $\mathcal{O}(10\mbox{--}100)$ km in size, and the associated bottom pressure anomalies in local vorticity balances at length‐scales smaller than 1,000 km. Overall, there is a transition from the Topographic‐Nonlinear regime at scales smaller than 1,000 km to the Topographic‐Sverdrup regime at length‐scales greater than 1,000 km. These dynamical balances hold across all ocean basins; however, interpretations of the dominant vorticity balances depend on the level of spatial filtering or the effective model resolution. On the other hand, the contribution of bottom and lateral friction terms in the barotropic vorticity budget remains small and is significant only near sea‐land boundaries, where bottom stress and horizontal viscous friction generally peak. Plain Language Summary: Vorticity provides a measure of the local rotation and shear of fluid flow. The analysis of physical processes contributing to ocean vorticity has proven fundamental to our understanding of how those processes drive ocean flows, ranging from large‐scale ocean gyres to boundary currents such as the Gulf Stream, which is tens of km in width. Furthermore, a vorticity analysis can inform us about the relative importance of different physical processes in generating flow structures having different length scales. In the present work, we perform a length‐scale dependent vorticity budget analysis using a coarse‐graining method to remove signals finer than a fixed length scale. We coarse‐grain the climatological mean vorticity budget terms over a range of length scales, and then compare the relative magnitudes to identify the dominant vorticity balances as a function of length scale. We find that the spatial structure of the meridional transport is mainly controlled by atmospheric winds, variations in ocean depth and the momentum transport by ocean currents. However, the relative magnitudes of these factors change drastically at different length scales. We conclude that physical interpretations of the primary vorticity balances are fundamentally dependent on the chosen length scale of the analysis. Key Points: Relative magnitudes of barotropic vorticity budget terms display significant length‐scale dependenceBottom pressure torque and wind stress curl control the depth‐integrated meridional flow at length scales larger than 1,000 kmNonlinear advection and bottom pressure torque dominate the barotropic vorticity budget at smaller length scales [ABSTRACT FROM AUTHOR]

The mesoscale activity of western boundary currents can induce changes in the oligotrophic conditions of the oceanic subtropical gyres. Off SE Brazil, recurrent meanders in the Brazil Current can modulate the pelagic ecosystem through eddy‐induced upwelling, as well as drive coastal upwelling and cross‐shelf exchanges. Despite their relevance, direct measurements of such meanders are scarce. At the end of the Spring of 2019, we acquired measurements of both physical and biogeochemical properties along a transect through one of these features, with 5–10 km spacing between stations. Displacements of the isopycnals at the meander edges arose from submesoscale filaments along the meander's rim, associated with the signature of shelf water masses. Chlorophyll distributions along the transect indicated phytoplankton responses to the different dynamic regimes in the meander: eddy stirring in the surface mixed layers versus eddy pumping in the subsurface waters, and mesoscale dynamics in the meander center versus submesoscale dynamics in its periphery. Subsurface phytoplankton biomass was enhanced in the meander center, likely due to eddy pumping raising the nutricline into the euphotic zone. Eddy pumping also appears to have contributed to a shift in the phytoplankton community composition to larger organisms. Our results indicate that the physical dynamics create different habitats across the meander, leading to spatial patchiness in the chlorophyll distributions, with different types of organisms in distinct parts of the meander. Plain Language Summary: Like rivers, ocean currents can meander along their paths. Off southeastern Brazil, the Brazil Current flows southward and meanders as it flows past capes in the region. These meanders are hundreds of kilometers wide (mesoscale) and can generate smaller features that are tens of kilometers wide (submesoscale). These multiscale processes impact the planktonic ecosystem by inducing nutrient inputs to the nutrient‐poor open ocean and consequently enhancing primary productivity. However, direct measurements of these features are scarce. We had the opportunity to survey one of these meanders in 2019, collecting physical and biogeochemical data along a single line through the meander from southwest to northeast. We found small‐scale distortions that indicated the presence of submesoscale features swirling around the meander's rim. Different parts of the meander showed distinct phytoplankton responses, with more phytoplankton in the center, below the surface, likely growing on nutrients pumped upward by the meander. This pumping process might have been correlated with a phytoplankton community shift from small to larger organisms. Overall, our study shows that mesoscale and submesoscale motions create a variety of habitats, leading to distinct chlorophyll distribution patterns in different parts of the meander. Key Points: We present the first physical‐biological measurements of a Brazil Current cyclonic meander drawing high chlorophyll shelf waters offshoreEddy pumping raised nutricline to the euphotic zone driving subsurface phytoplankton enhancement at eddy center and invisible to satellitesDistinct hydrodynamic regimes modulated the phytoplankton response in the center, rim, surface, and subsurface of the eddy [ABSTRACT FROM AUTHOR]

The ability to infer ocean chlorophyll- a concentrations (Chl a) from spaceborne instruments is key to assessments of global ocean productivity and monitoring of water quality. Here, we present a novel parametric algorithm, OCG, trained on a set of global in situ high-performance liquid chromatography (HPLC) data that leverages Level-3 remote sensing reflectance (R rs ) products from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite. The OCG algorithm leverages more bands than existing algorithms and also provides pixel-wise uncertainty assessments that enable the calculation of the probability of exceeding specific Chl a thresholds. This feature has significant implications for water quality management, particularly in monitoring harmful algal blooms. The OCG surpasses existing algorithms in bias and accuracy without overfitting, especially in coastal areas, where it outperforms the current standard product (CI OC3) by 20 % in median symmetric accuracy. Moreover, the OCG reduces the signed symmetric percentage bias (SSPB) in coastal regions from 41 % (CI OC3) to below 5 %. Globally, the OCG algorithm yields lower Chl a in coastal regions, the Southern Ocean and the Mediterranean Sea, and higher values in the open ocean, particularly in ocean gyres and polar regions. For the Chesapeake Bay and the Baltic Sea, for example, daily OCG estimates for 2002 to 2021 are, on average, 2.9 g/L and 3.7 g/L lower than CI OC3 estimates, respectively. The presented approach also shows great potential for other existing and upcoming sensors, enabling widespread application in remote sensing. [ABSTRACT FROM AUTHOR]

From the Eocene (∼50 million years ago) to today, Southern Ocean circulation has evolved from the existence of two ocean gyres to the dominance of the Antarctic Circumpolar Current (ACC). It has generally been thought that the opening of Southern Ocean gateways in the late Eocene, in addition to the alignment of westerly winds with these gateways or the presence of the Antarctic ice sheet, was a sufficient requirement for the transition to an ACC of similar strength to its modern equivalent. Nevertheless, models representing these changes produce a much weaker ACC. Here we show, using an eddying ocean model, that the missing ingredient in the transition to a modern ACC is deep convection around the Antarctic continent. This deep convection is caused by cold temperatures and high salinities due to sea‐ice production around the Antarctic continent, leading to both the formation of Antarctic Bottom Water and a modern‐strength ACC. Plain Language Summary: The evolution of ocean circulation from the early Southern Ocean around 50 million years ago to today has seen much debate over the past decades. The main characteristic of the modern Southern Ocean is the prevalence of the Antarctic Circumpolar Current (ACC), the world's strongest current. In the past it has been thought that the deepening of ocean gateways, and changes in the strength and location of winds, led to an ACC of similar strength to its modern equivalent. Nevertheless, ocean models simulating these changes typically reproduce an ACC with less than a third of the modern ACC's strength. Here we show that the missing ingredient in the transition to a modern ACC is deep convection around the Antarctic continent. Deep convection is due to a combination of cooling and increase in salinity by sea ice formation which allows for water at the ocean surface to become denser than the water below, leading to the mixing of the water column to great depths. This deep convection allows for the ocean to be energized, leading to a modern‐strength Antarctic Circumpolar Current. A cool climate around the Antarctic continent is therefore crucial for the development of a modern‐strength Antarctic Circumpolar Current. Key Points: Deep convection is able to drive a strong Antarctic Circumpolar Current (ACC)Surface cooling and sea ice formation are crucial drivers of this deep convectionACC similar to a rim current around convective sites, energizing ocean circulation [ABSTRACT FROM AUTHOR]

Our planet is a self-sustaining ecosystem powered by light energy from the sun, but roughly closed to matter. Many ecosystems on Earth are also approximately closed to matter and recycle nutrients by self-organizing stable nutrient cycles, e.g., microbial mats, lakes, open ocean gyres. However, existing ecological models do not exhibit the self-organization and dynamical stability widely observed in such planetary-scale ecosystems. Here, we advance a conceptual model that explains the self-organization, stability, and emergent features of closed microbial ecosystems. Our model incorporates the bioenergetics of metabolism into an ecological framework. By studying this model, we uncover a crucial thermodynamic feedback loop that enables metabolically diverse communities to almost always stabilize nutrient cycles. Surprisingly, highly diverse communities self-organize to extract ≈10% of the maximum extractable energy, or ≈100 fold more than randomized communities. Further, with increasing diversity, distinct ecosystems show strongly correlated fluxes through nutrient cycles. However, as the driving force from light increases, the fluxes of nutrient cycles become more variable and species-dependent. Our results highlight that self-organization promotes the efficiency and stability of complex ecosystems at extracting energy from the environment, even in the absence of any centralized coordination. [ABSTRACT FROM AUTHOR]

We investigate the origin of the equatorial Pacific cold sea surface temperature (SST) bias and its link to wind biases, local and remote, in the Kiel Climate Model (KCM). The cold bias is common in climate models participating in phases 5 and 6 of the Coupled Model Intercomparison Project. In the coupled experiments with the KCM, the interannually varying NCEP/CFSR wind stress is prescribed over four spatial domains: globally, over the equatorial Pacific (EP), the northern Pacific (NP), and the southern Pacific (SP). The corresponding EP SST bias is reduced by 100%, 52%, 12%, and 23%, respectively. Thus, the EP SST bias is mainly attributed to the local wind bias, with small but not negligible contributions from the extratropical regions. Erroneous ocean circulation driven by overly strong winds causes the cold SST bias, while the surface heat flux counteracts it. Extratropical Pacific SST biases contribute to the EP cold bias via the oceanic subtropical gyres, which is further enhanced by dynamical coupling in the equatorial region. The origin of the wind biases is examined by forcing the atmospheric component of the KCM in a stand-alone mode with observed SSTs and simulated SSTs from the coupled experiments. Wind biases over the EP, NP, and SP regions originate in the atmosphere model. The cold EP SST bias substantially enhances the wind biases over all three regions, while the NP and SP SST biases support local amplification of the wind bias. This study suggests that improving surface wind stress, at and off the equator, is a key to improve mean-state equatorial Pacific SST in climate models. [ABSTRACT FROM AUTHOR]

In tropical and subtropical ocean gyres, Trichodesmium colonies account for up to half of the total N2 fixation in the ocean, making it one of the most ecologically significant N2-fixing cyanobacteria. The processes whereby Trichodesmium colonies and large-scale surface blooms form have not been investigated thoroughly. In particular, the effects of fluid motion have not been included in previous studies. As the first step toward understanding the mechanical processes associated with Trichodesmium colony synthesis, we propose a shear-related flow-based growth model to enlighten how fluid dynamics affect bacteria colony formation and growth. To investigate the possibility that early growth characteristics are strongly dependent on the shear rate, a two-way coupled fluid-colony interaction is developed using the lattice Boltzmann method for a porous colony. This model captures the exponential growth trend during the colony formation phase found in experiments. Our results show that the flow field significantly impacts both the colony growth rate and shape. In pure shear conditions, colonies grow in circular shapes, whereas in uniform flow, they take filament-like forms. Additionally, the Reynolds number (Re) plays a crucial role in shaping the colonies, especially in uniform flow conditions where its effect is more profound. [ABSTRACT FROM AUTHOR]

The Kuroshio Extension (KE) flows eastward at the northern boundary of the North Pacific subtropical gyre. By transporting large amounts of seawater with heat, the KE contributes significantly to the formation of sea surface temperature (SST) fields. Recently, poleward shifts of major ocean gyres in the world ocean, including the North Pacific subtropical gyre, have been highlighted based on basin-scale changes in SST and sea surface height (SSH) distributions. However, a detailed investigation of the long-term meridional KE movement has not been presented. Investigation of KE path changes helps provide insights into long-term changes in the physical fields in the western North Pacific. In this study, we identified the KE path from satellite-derived SSH and surface current velocity data using a front identification method and showed that the KE migrated northward by approximately 210 km during 1993–2021. We further explored the cause of the northward KE shift based on atmospheric reanalysis data and numerical experiments using a high-resolution ocean general circulation model. It was revealed that the northward KE shift is mostly caused by the trend of wind stress curl in the North Pacific during 1993–2021. [ABSTRACT FROM AUTHOR]

The North Atlantic gyre experiences both a significant temperature rise at high latitudes and a considerable weakening of the geostrophic component of the Gulf Stream, which is reflected by the 64-year fundamental gyral Rossby wave (GRW). This singular behavior compared to the South Atlantic and South Indian Ocean gyres highlights a feedback loop of Arctic ice sheet melting on mid-latitude Atlantic Ocean temperature. The warming of the northern oceanic gyre at high latitudes due to the retreat of Arctic ice sheet via the Labrador Current decreases the thermal gradient between the high and low latitudes of the north Atlantic gyre. This results in a weakening of the geostrophic forces at the basin scale and a reduction in the amplitude of the GRWs. Reducing the amplitude of the variation of the upward and downward movement of the pycnocline modifies air–sea interactions, weakening vertical mixing as well as the evaporation processes and the departure of latent heat when the pycnocline rises. The resulting thermal anomaly stretching along the Gulf Stream from where it leaves the American continent is partly transferred to the Arctic sea ice via the drift current and thermohaline circulation, which contributes to the retreat of the ice sheet, and the closing of the feedback loop. The 64-year-period GRW should disappear around 2050 if its damping continues linearly, favoring an increasingly rapid warming of the ocean at mid-latitudes. These interactions are less acute in the southern hemisphere due to the circumpolar current. [ABSTRACT FROM AUTHOR]

The Cretaceous 'greenhouse' period (~145 to ~66 million years ago, Ma) in Earth's history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO2 variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs. Simulations using a coupled atmosphere-ocean model show that paleogeography-driven reduction in the intensity of surface ocean circulation explains much of the increase in the mid-latitudinal sea surface temperature gradient during the Cretaceous. [ABSTRACT FROM AUTHOR]

This study investigated the coupling of the wind-driven ocean gyres with the atmospheric westerly jet using an idealised, eddy-resolving, coupled model. An empirical orthogonal function analysis of the low-pass filtered data showed that the ocean gyre variability is dominated by meridional shifts of the western boundary current extension (WBCE) and changes in the strength of the subtropical inertial recirculation zone. On the other hand, the atmospheric potential vorticity (PV) variability is dominated by the growth of standing Rossby wave patterns, while its pressure variability is dominated by a zonally-asymmetric meridional shift of the atmospheric jet. Damping sea surface temperature (SST) variability in the atmosphere was shown to weaken its PV variability and reduce the zonal asymmetry of the jet-shift mode. Singular value decompositions revealed a positive feedback between meridional shifts of the WBCE and the growth of standing Rossby wave disturbances in the atmospheric jet. The atmosphere's response is controlled by shifts in the meridional eddy heat flux over the SST front which triggers the growth of baroclinic instabilities. This instability growth eventually leads to a large-scale, barotropic pressure response over the eastern ocean basin, or an aforementioned meridional shift of the atmospheric jet. Reduction in the atmospheric resolution inhibits the ability of atmospheric eddies to resolve length scales associated with meridional shifts of the SST front and WBCE. The lack of resolution consequently weakens the influence of ocean gyre variability on the atmospheric jet and reduces the strength of the positive feedback. [ABSTRACT FROM AUTHOR]

This paper investigates the spatial inhomogeneity of the time-averaged, quasigeostrophic, double-gyre circulation response to fixed, realistic, large-scale modes of wind-stress forcing. While the companion paper of this study focused on understanding the anatomy of low-frequency, midlatitude climate variability in an idealised, eddy-resolving coupled model, this paper looked at understanding the nature of the wind-induced ocean gyre response using an ocean-only configuration of the same model. Our analysis revealed two, time-averaged responses to an east–west dipole, wind-stress curl anomaly in the ocean basin. Firstly, wind-stress anomalies in the western ocean basin led to changes in relative strength of the inertial recirculation zones and jet-axis tilt. This is consistent with an advection-dominated, nonlinear adjustment of the ocean gyres to anomalous forcing. Secondly, wind-stress curl anomalies in the eastern ocean basin was found to induce a largely independent response involving meridional shifts of the western boundary current extension (WBCE). The effects of time-averaged advection in this region are weak and the discovery of westward-propagating Rossby waves along the WBCE revealed the response is more akin to a baroclinic Rossby wave adjustment. [ABSTRACT FROM AUTHOR]

Plastic pollution has not raised much attention until the 2000s, despite being manufactured for about a century. It is now considered one of the most substantial environmental issues. Here we investigate the presence of plastic contamination in 34 stranded animals on the coast of French Guiana, South America. Here we present information highlighting the magnitude of plastic contamination in marine coastal and pelagic tropical marine vertebrates on the Equatorial Atlantic coast of South America. All four species studied here are protected and emblematic vertebrates of the region, with a fragile conservation status, including the olive ridley turtle Lepidochelys olivacea, the green turtle Chelonia mydas and the leatherback turtle Dermochelys coriacea, and a small cetacean, the Guiana dolphin Sotalia guianensis. Macroplastics (polypropylene, polyethylene, terephthalate, and polystyrene) were detected in four juvenile green turtles. Microplastics (polypropylene, polyethylene, terephthalate, polystyrene, nylon, acrylates, polycarbonates) were found in 13 individuals (two dolphins, six olive ridleys, four green turtles, and the leatherback turtle). The sampled species have different diets, distribution patterns, and ages, suggesting widespread plastic pollution. The study area is located far from the oceanic gyres. However, they are influenced by the North Brazilian Current, the Amazon River, and other rivers of the Guianas. [ABSTRACT FROM AUTHOR]

We investigated the connectivity properties of an idealized western boundary current system separating two ocean gyres, where the flow is characterized by a well-defined mean circulation as well as energetic fine-scale features (i.e., mesoscale and submesoscale currents). We used a time-evolving 3D flow field from a high-resolution (HR-3D) ocean model of this system. In order to evaluate the role of the fine scales in connectivity estimates, we computed Lagrangian trajectories in three different ways: using the HR-3D flow, using the same flow but filtered on a coarse-resolution grid (CR-3D), and using the surface layer flow only (HR-SL). We examined connectivity between the two gyres along the western boundary current and across it by using and comparing different metrics, such as minimum and averaged values of transit time between 16 key sites, arrival depths, and probability density functions of transit times. We find that when the fine-scale flow is resolved, the numerical particles connect pairs of sites faster (between 100 to 300 d) than when it is absent. This is particularly true for sites that are along and near the jets separating the two gyres. Moreover, the connectivity is facilitated when 3D instead of surface currents are resolved. Finally, our results emphasize that ocean connectivity is 3D and not 2D and that assessing connectivity properties using climatologies or low-resolution velocity fields yields strongly biased estimates. [ABSTRACT FROM AUTHOR]

Using a probabilistic neural network and Lagrangian observations from the Global Drifter Program, we model the single particle transition probability density function (pdf) of ocean surface drifters. The transition pdf is represented by a Gaussian mixture whose parameters (weights, means, and covariances) are continuous functions of latitude and longitude determined to maximize the likelihood of observed drifter trajectories. This provides a comprehensive description of drifter dynamics allowing for the simulation of drifter trajectories and the estimation of a wealth of dynamical statistics without the need to revisit the raw data. As examples, we compute global estimates of mean displacements over 4 days and lateral diffusivity. We use a probabilistic scoring rule to compare our model to commonly used transition matrix models. Our model outperforms others globally and in three specific regions. A drifter release experiment simulated using our model shows the emergence of concentrated clusters in the subtropical gyres, in agreement with previous studies on the formation of garbage patches. An advantage of the neural network model is that it provides a continuous‐in‐space representation and avoids the need to discretize space, overcoming the challenges of dealing with nonuniform data. Our approach, which embraces data‐driven probabilistic modeling, is applicable to many other problems in fluid dynamics and oceanography. Plain Language Summary: Data collected using satellite tracked drifting buoys (drifters) have been used to estimate statistics of ocean dynamics. The estimation procedures employed are typically developed on a case‐by‐case basis and provide a limited characterization of the dynamics. We show that, by modeling the full probability distribution from which these statistics derive, we can estimate efficiently a range of statistics simultaneously and provide a more comprehensive description. To do this we leverage probabilistic neural networks. Our model provides a convenient means of estimating statistics, and can be used to simulate drifter trajectories. Our simulations reproduce the clustering behavior in ocean gyres seen in previous studies on the formation of garbage patches. Key Points: We use probabilistic neural networks to learn the transition probability density of surface drifters from observationsWe show that modeling the full transition density is an efficient way to estimate many dynamical statistics simultaneouslyOur model can also be used to simulate transport at the ocean near‐surface and achieves higher skill scores than existing models [ABSTRACT FROM AUTHOR]

The Kuroshio south of Japan is known to vacillate between a straight and a large meander (LM) path. Since 1950, eight LM events have been observed with different durations. The most recent/on‐going LM started in August 2017 and has become the longest event in record. By analyzing eddy‐resolving sea surface height data and by adopting a wind‐forced linear vorticity model, we demonstrate that the on‐going LM is maintained by an exceptionally stable dynamic state of the Kuroshio Extension (KE) forced by wind stresses across the Pacific basin. The highly‐stable KE system not only minimizes westward eddy perturbations from disrupting the upstream Kuroshio path, its strengthened southern recirculation gyre further helps to anchor the Kuroshio across the Izu Ridge. By contrasting the on‐going event to the 2004–2005 event, we argue that the LM duration is more sensitive to the wind‐forced KE dynamic state than the eastward Kuroshio transport south of Japan. Plain Language Summary: Occurrence of large meanders (LMs) by the Kuroshio south of Japan is a unique characteristic among the 5 western boundary currents in the wind‐driven subtropical ocean gyres. The LM occurrence and durations have both been observed to be irregular and chaotic. While processes initiating a LM event have been widely examined, the factors controlling the LM duration remain less explored. Two LM events with drastically different durations took place after the global eddy‐resolving sea surface height data from satellite altimeters became available. In contrast to the 2004–2005 LM event with a duration of 14 months, the on‐going LM started in August 2017 and, by persisting for >67 months, has become the longest among the eight recorded events since 1950. This study investigated the processes responsible for the differing durations of these two LM events. It is found that the wind‐forced stability of the Kuroshio Extension system east of the Izu Ridge plays a critical role in setting the LM's durations. This investigation is important because the Kuroshio LM impacts not only on fisheries south of Japan, but also synoptic weathers, such as typhoon paths, and climate in coastal areas facing the North Pacific Ocean. Key Points: The Kuroshio south of Japan is known to vacillate between straight and large meander (LM) pathsStarted in August 2017, the on‐going LM event has become the longest among the eight LM events detected since 1950This longevity is argued to be caused by highly stable dynamic state of the Kuroshio Extension after 2018 forced by the Pacific basin winds [ABSTRACT FROM AUTHOR]

This paper describes the new Regional Arctic Ocean/sea ice Reanalysis (RARE) with a domain that spans a subpolar/polar cap poleward of 45°N. Sequential data assimilation constrains temperature and salinity using World Ocean Database profiles as well as in situ and satellite SST, and PIOMAS sea ice thickness estimates. The 41-yr (1980–2020) RARE1.15.2 reanalysis with resolution varying between 2 and 5 km horizontally and 1–10 m vertically in the upper 100 m is examined. To explore the impact of resolution RARE1.15.2 is compared to a coarser-resolution SODA3.15.2, which uses the same modeling and data assimilation system. Improving resolution in the reanalysis system improves agreement with observations. It produces stronger more compact currents, enhances eddy kinetic energy, and strengthens along-isopycnal heat and salt transports, but reduces vertical exchanges and thus strengthens upper ocean haline stratification. RARE1.15.2 and SODA3.15.2 are also compared to the Hadley Center EN4.2.2 statistical objective analysis. In regions of reasonable data coverage such as the Nordic seas the three products produce similar time-mean distributions of temperature and salinity. But in regions of poor coverage and in regions where the coverage changes in time EN4.2.2 suffers more from those inhomogeneities. Finally, the impact on the Arctic of interannual temperature fluctuations in the subpolar gyres on the Arctic Ocean is compared. The influence of the subpolar North Pacific is limited to a region surrounding Bering Strait. The influence of the subpolar North Atlantic, in contrast, spreads throughout the Nordic seas and Barents Sea in all three products within two years. Significance Statement: The Arctic Ocean/sea ice system plays crucial roles in climate variability and change by controlling the northern end of the oceanic overturning circulation, the equator to pole air pressure gradient, and Earth's energy balance. Yet the historical ocean observation set is sparse and inhomogeneous, while ocean dynamics has challengingly fine horizontal and vertical scales. This paper introduces a new Regional Arctic Ocean/sea ice Reanalysis (RARE) whose goal is to use the combined constraints of mesoscale ocean dynamics, historical observations, surface meteorology, and continental runoff in a data assimilation framework to reconstruct historical variability. RARE is used to produce a 41-yr ocean/sea ice reanalysis 1980–2020 whose results are described here. [ABSTRACT FROM AUTHOR]

A current along a sloping bottom gives rise to upwelling, or downwelling Ekman transport within the stratified bottom boundary layer (BBL), also known as the bottom Ekman layer. In 1D models of slope currents, geostrophic vertical shear resulting from horizontal buoyancy gradients within the BBL is predicted to eventually bring the bottom stress to zero, leading to a shutdown, or "arrest," of the BBL. Using 3D ROMS simulations, we explore how the dynamics of buoyancy adjustment in a current‐ridge encounter problem differs from 1D and 2D temporal spin up problems. We show that in a downwelling BBL, the destruction of the ageostrophic BBL shear, and hence the bottom stress, is accomplished primarily by nonlinear straining effects during the initial topographic encounter. As the current advects along the ridge slopes, the BBL deepens and evolves toward thermal wind balance. The onset of negative potential vorticitymodes of instability and their subsequent dissipation partially offsets the reduction of the BBL dissipation during the ridge‐current interaction. On the upwelling side, although the bottom stress weakens substantially during the encounter, the BBL experiences a horizontal inflectional point instability and separates from the slopes before sustained along‐slope stress reduction can occur. In all our solutions, both the upwelling and downwelling BBLs are in a partially arrested state when the current separates from the ridge slope, characterized by a reduced, but non‐zero bottom stress on the slopes. Plain Language Summary: Surface winds pump mechanical energy into the large‐scale circulation of the ocean at an average rate of between 0.8 and 1 TW. This wind‐input occurs at large, so‐called synoptic scales spanning thousands of kilometers. Absent dissipative pathways, this steady energy input would cause uncontrolled spinup of the ocean gyres. For decades it has been assumed that friction at the seabed has an important role in the eventual turbulent dissipation of the ocean kinetic energy. In the 1990s, theoretical models suggested that turbulence could be wholly suppressed on sloping bottom bathymetry due to the rearrangement of density surfaces within the bottom boundary layer—a mechanism called buoyancy adjustment. Here we revisit this problem using modern 3D simulations of currents encountering a ridge. We find that although the bottom stress can be markedly reduced on topographic slopes, the mechanism through which it occurs is quite different than that in simplified 1D and 2D models. Flow "deformation," or straining effects during the topographic encounter play a more important role in weakening the bottom stress than buoyancy adjustment. Furthermore, geometric effects like curvature, and flow instabilities can partially offset the reduction in dissipation caused by suppression of bottom boundary layer turbulence. Key Points: A complex interplay of buoyancy adjustment, instabilities, and curvature effects influences oceanic bottom boundary layers evolutionNonlinear strain effects contribute significantly in weakening the bottom stress during the initial current‐ridge encounterThe onset of negative potential vorticity, and barotropic instabilities downstream partially offsets the reduced boundary dissipation [ABSTRACT FROM AUTHOR]

Climate model hindcasts are analyzed to reveal the impacts of the North Atlantic Oscillation (NAO) on the North Atlantic subpolar ocean, which exhibits variability on seasonal to decadal timescales. The ocean response to a single winter NAO event is separated into fast and slow responses. The fast response persists over winter–spring seasons, during which wind stress and heat flux anomalies associated with the NAO rapidly modify ocean temperatures via changes in Ekman transport and ocean‐atmosphere heat exchanges. The slow response persists for 3–4 years, during which overturning and gyre circulations redistribute opposing‐signed surface temperature anomalies created by the NAO. This redistribution modifies east‐west temperature contrasts altering the meridional heat transport associated with gyres and changing the strength of the overturning circulation. Hence, the fast and slow responses lead to opposing‐signed subpolar temperature anomalies in time from the competing effects of local forcing and horizontal heat convergence. Plain Language Summary: The North Atlantic Oscillation (NAO) index is one of the most common metrics to characterize atmospheric regimes in the northern hemisphere, revealing how different patterns of atmospheric winds and air‐sea fluxes over Europe and North America affect the ocean state. In the present research, coupled climate model simulations are utilized to study the impacts of particular atmospheric events on the subpolar North Atlantic Ocean. Using NAO indices during the winter season, sets of multiple simulations having similar types of atmospheric events are created to carry out a composite analysis. The analyses reveal that the impact of atmospheric events on the oceans can last up to 4–5 years (or even longer in some cases). The fast response is controlled by the effect of the local winds and air‐sea heat fluxes, while the slow response is related to the role of the meridional overturning circulation and ocean gyres in transporting heat anomalies from the subtropics to the subpolar ocean. The combination of these fast and slow responses leads to the sign reversal of the subpolar surface temperature anomalies in time. Key Points: Subpolar ocean changes due to the North Atlantic Oscillation (NAO) are separated into competing fast, seasonal and slow, inter‐annual responsesFast response is related to the anomalous Ekman transport and air‐sea heat exchange associated with the NAOSlow response is due to the redistribution of subtropical temperature anomalies via overturning and gyre circulations [ABSTRACT FROM AUTHOR]

Western boundary currents (WBCs) form the narrow, fast-flowing poleward return flows of the great subtropical ocean gyres and are sources of rapidly varying mesoscale eddies. Accurate simulation of the vertical structure, separation latitude, and ocean heat content of WBCs is important for understanding the poleward transport of heat in the global ocean. However, state estimation and forecasting in WBC regions, such as the East Australian Current (EAC), the WBC of the South Pacific subtropical gyre, is challenging due to their dynamic nature and lack of observations at depth. Here we use observing system simulation experiments to show that subsurface temperature observations in a high eddy kinetic energy region yield large improvement in representation of key EAC circulation features, both downstream and ∼ 600 km upstream of the observing location. These subsurface temperature observations (in concert with sea surface temperature and height measurements) are also critical for correctly representing ocean heat content along the length of the EAC. Furthermore, we find that a more poleward separation latitude leads to an EAC and eddy field that is represented with far reduced error, compared to when the EAC separates closer to the Equator. Our results demonstrate the importance of subsurface observations for accurate state estimation of the EAC and ocean heat content that can lead to marine heatwaves. These results provide useful suggestions for observing system design under different oceanographic regimes, for example, adaptive sampling to target high energy states with more observations and low energy states with fewer observations. [ABSTRACT FROM AUTHOR]

We identify and explore the fundamental differences in the dynamics of mesoscale vortices in eastward background (EB) parts in mid-latitude ocean gyres and in westward background (WB) return flows. In contrast to eddy behavior in EB flow, a systematic meridional drift of eddies in WB flow results in poleward expulsion of cold-core cyclones and equatorward expulsion of warm-core anticyclones from the unstable zone with a negative potential vorticity gradient (PVG). Consequently, heat can be transferred further by upper ocean vortices intrinsically coupled with deep opposite sign partners. Such structures can drift through the stable zone with positive PVG in both layers. This mechanism of lateral transfer is not captured by local models of homogeneous turbulence. The crossflow drift is related to the coupling of the upper vortices with opposite sign deep eddies shifted eastward. The abyssal vortices can be viewed as lee Rossby waves induced by their upper-layer partners and described analytically in the vicinity of latitude of marginal stability. Here, we show how such self-amplifying hetons, emerging in homogeneous turbulence, saturate when they approach locally stable regions of inhomogeneous currents. The presented results indicate that subtropical regions with return WB flows in the upper layer favor long-distance heat transport by spatially coherent eddies in accordance with observations and motivate the development of non-local parameterizations of eddy fluxes. [ABSTRACT FROM AUTHOR]

We investigate the connectivity properties between different ocean stations in an idealized open ocean model of a western boundary current system separating two ocean gyres. We applied a Lagrangian framework to compute trajectories from various dynamical setups: a high-resolution (1/9°) 3D velocity field reproducing a large range of the ocean fine-scale (i.e. mesoscale plus part of the submesoscale) dynamics, or a filtered velocity field on a coarse-resolution (1°) grid, and one limited to the surface 2D velocities. As ocean connectivity has been assessed in the published literature using different definitions, in this work we compare different metrics such as the average values of transit time and arrival depth between specified sample stations as well as the probability density functions (PDFs) of transit times and betweenness for the different dynamical setups. Our results indicate that almost none of the PDFs show Gaussian behaviour. When the fine-scale dynamics are taken into account, the numerical particles move and connect pairs of stations faster (between 100 days to 300 days) than when it is absent. This is particularly true, along and near the jets separating the two gyres. Moreover, the connectivity is facilitated when 3D instead of 2D velocities are considered. Finally, our results suggest that western boundary currents are characterized by high betweenness centrality values, which confirms its key role in controlling the transfer of particles in the double-gyre configuration. [ABSTRACT FROM AUTHOR]

The North Atlantic Subtropical Gyre (NASTG) has experienced the fastest expansion of oligotrophic waters worldwide in response to ocean warming. We study the trophic regime changes in the NASTG by using 21 years (1998–2018) of satellite chlorophyll‐a (CHL) data, complemented with other variables such as Sea Surface Temperature, optical backscatter coefficients (bbp), Secchi disk (zSD), and Mixed Layer Depth. To this aim, we describe the spatial/temporal variability of waters with the lowest CHL concentrations (CHL ≤ 0.04 mg m−3) with an inter‐annual variability analysis of key environmental variables. Main results demonstrate that these ultra‐oligotrophic waters are spatially expanding and increasing in frequency, shifting the NASTG to a dominant quasi‐permanent ultra‐oligotrophic condition, thus confirming the ongoing ocean desertification. Plain Language Summary: Oceanic subtropical gyres have a fundamental role in the global carbon budget due to their immense ecosystem size even if they are characterized by oligotrophic waters. In the last decades, the North Atlantic Subtropical Gyre (NASTG) has experienced the fastest expansion of oligotrophic waters worldwide with an enlargement in response to the ongoing ocean warming. Here, we study the trophic regime changes in the NASTG by using 21 years (1998–2018) of satellite phytoplankton chlorophyll‐a (CHL) data, complemented with other environmental variables such as Sea Surface Temperature, suspended particle concentration, water transparency, and Mixed Layer Depth. To this main goal, we describe the spatial and temporal variability of waters characterized by CHL ≤ 0.04 mg m−3 together with an inter‐annual variability analysis of the key oceanic variables introduced. The main findings demonstrate that these ultra‐oligotrophic waters are spatially expanding and increasing in frequency, shifting the NASTG to a dominant quasi‐permanent ultra‐oligotrophic condition, confirming the ongoing impactful ocean desertification. Key Points: The lowest chlorophyll‐a waters in the North Atlantic Subtropical Gyre (NASTG) are spatially expanding and increasing in frequency, as seen by 21 years of satellite dataNASTG core shows a shift to a dominant quasi‐permanent ultra‐oligotrophic conditionOcean warming's role in sharpening oligotrophy is more complex than local stratification increase only [ABSTRACT FROM AUTHOR]

This study examines microplastic (MP, 1–5 mm) densities in convergence zones in a coastal sea, the Seto Inland Sea, comparing them to those of non-convergence zones and other areas. Notably, Seto convergence zones exhibit MP densities 40 to 300 times higher than non-convergence zones, with an extraordinary density of 3.7 ± 6.3 pieces m−3, similar to densities found in Tokyo Bay as known a MP hotspot. The predominant polymer found was expanded polystyrene, varying seasonally and peaking in summer. Juvenile fish associated with driftweed in these convergence zones face a risk of long-term MP exposure, potentially up to four months. This large number of MPs found in coastal convergence zones is similar to accumulation zones formed in the gyres of open oceans, with strong implications for detrimental effects on coastal marine life. However, these MPs are autochthonous, and may be manageable through local marine plastic waste management. [Display omitted] • MPs are compared in Seto Inland Sea convergence and non-convergence zones. • Seto convergence zone MP levels are 300 times higher than in non-convergence zones. • Massive MPs numbers suggest potential marine life harm. • Expanded polystyrene is a particular problem locally but policy focus is feasible. [ABSTRACT FROM AUTHOR]

Edited by James McWilliams, University of California, Los Angeles, CA; received April 19, 2022; accepted September 3, 2022 The expansive gyres of the subtropical ocean account for a significant fraction of global organic carbon export fromthe upper ocean. In the gyre interior, verticalmixing and the heaving of nutrient-rich waters into the euphotic layer sustain local productivity, in turn depleting the layers below. However, the nutrient pathways by which these subeuphotic layers are themselves replenished remain unclear. Using a global, eddy-permitting simulation of ocean physics and biogeochemistry, we quantify nutrient resupplymechanisms along and across density surfaces, including the contribution of eddy-scale motions that are challenging to observe. We find that mesoscale eddies (10 to 100 km) flux nutrients from the shallow flanks of the gyre into the recirculating interior, through time-varying motions along density surfaces. The subeuphotic layers are ultimately replenished in approximately equal contributions by this mesoscale eddy transport and the remineralization of sinking particles.Themesoscale eddy resupply ismost important in the lower thermocline for the whole subtropical region but is dominant at all depths within the gyre interior. Subtropical gyre productivity may therefore be sustained by a nutrient relay, where the lateral transport resupplies nutrients to the thermocline and allows vertical exchanges to maintain surface biological production and carbon export. [ABSTRACT FROM AUTHOR]

Noting that 43 days is the minimum drifting time of the kelp rafts, the simulations confirmed that possible regions of origin could have been northern Tasmania (i.e., which have the nearest known populations of giant kelp), Wilsons Promontory, or somewhere along the pathway of the simulated particles. Keywords: barnacle; dispersal; Lepas; Macrocystis; wrack EN barnacle dispersal Lepas Macrocystis wrack 1 6 6 10/06/22 20221001 NES 221001 On 9 August 2020, two local marine naturalists (authors W. Marshall-Grey and J. Rankin) on the south coast of New South Wales, Australia noticed a significant amount of a large unfamiliar kelp washed up on a local beach. Giant kelp rafts wash ashore 450 km from the nearest populations and against the dominant ocean current Based on a general trajectory from northern Tasmanian to southern New South Wales, the maximum putative distance covered by the kelp rafts was ~600 km, which over 43 days equates to a drift speed of ~0.16 m/s. This is similar to other drift speed estimates for floating kelp rafts (~0.20 m/s, Fraser et al., 2010). [Extracted from the article]

Marine particles of different nature are found throughout the global ocean. The term "marine particles" describes detritus aggregates and fecal pellets as well as bacterioplankton, phytoplankton, zooplankton and nekton. Here, we present a global particle size distribution dataset obtained with several Underwater Vision Profiler 5 (UVP5) camera systems. Overall, within the 64 µm to about 50 mm size range covered by the UVP5, detrital particles are the most abundant component of all marine particles; thus, measurements of the particle size distribution with the UVP5 can yield important information on detrital particle dynamics. During deployment, which is possible down to 6000 m depth, the UVP5 images a volume of about 1 L at a frequency of 6 to 20 Hz. Each image is segmented in real time, and size measurements of particles are automatically stored. All UVP5 units used to generate the dataset presented here were inter-calibrated using a UVP5 high-definition unit as reference. Our consistent particle size distribution dataset contains 8805 vertical profiles collected between 19 June 2008 and 23 November 2020. All major ocean basins, as well as the Mediterranean Sea and the Baltic Sea, were sampled. A total of 19 % of all profiles had a maximum sampling depth shallower than 200 dbar, 38 % sampled at least the upper 1000 dbar depth range and 11 % went down to at least 3000 dbar depth. First analysis of the particle size distribution dataset shows that particle abundance is found to be high at high latitudes and in coastal areas where surface productivity or continental inputs are elevated. The lowest values are found in the deep ocean and in the oceanic gyres. Our dataset should be valuable for more in-depth studies that focus on the analysis of regional, temporal and global patterns of particle size distribution and flux as well as for the development and adjustment of regional and global biogeochemical models. The marine particle size distribution dataset is available at 10.1594/PANGAEA.924375. [ABSTRACT FROM AUTHOR]

The global heterogeneity in export productivity after the Cretaceous‐Paleogene (K‐Pg) mass extinction is well documented, with some sites showing no change on geologic timescales, some demonstrating sustained decline, and a few showing a somewhat surprising increase. However, observational data come from sites so widespread that a key outstanding question is the geographic scale of changes in export productivity, and whether similar environments (e.g., open ocean gyres) responded similarly or whether heterogeneity is unrelated to environment. To address this, we developed three new Ba/Ti export productivity records from sites in the Gulf of Mexico and Caribbean which, combined with published data from a fourth site in the Chicxulub Crater itself, allow us to reconstruct regional changes in post K‐Pg export productivity for the first time. We find that, on a regional scale, export productivity change was homogenous, with all four sites showing a ∼300 Kyr period of elevated export production just after the boundary, followed by a longer period of decline. Interestingly, this interval of elevated export production appears to coincide with the post K‐Pg global micrite layer, which is thought to at least partially have been produced by blooms of carbonate‐producing cyanobacteria and other picophytoplankton. Global comparison of sites shows that elevated export productivity appears to have been most common in oligotrophic gyres, which suggests that changing plankton ecology evidenced by the micrite layer altered the biological pump, leading to a temporary increase in export production in these settings. Plain Language Summary: Primary producers are the base of the food chain; this group was severely damaged by the environmental effects associated with the Cretaceous‐Paleogene mass extinction. Determining how primary production recovered after this calamity is an important foundation for understanding how ecosystems recovered. Most previous work has focused on a process called export production, whereby organic carbon produced by phytoplankton is transferred to the ocean interior (some of which sinks to the seafloor and is buried). This work has shown that although most parts of the ocean recorded a decline in export production after the extinction event, some regions actually showed an increase. However, it was not clear on what geographic scale these differences occurred, or what caused them. We generated three new records of export production from a single region, the Gulf of Mexico/Caribbean Sea, and found a consistent increase in export production at each site for the same period of time after the extinction event. Comparison with other sites with increased export production shows that many are from open ocean gyres, suggesting that these regions were predisposed to increased export production in the earliest Paleocene because they were characterized by low productivity prior to the extinction. Key Points: Post K‐Pg export productivity was elevated across the Caribbean‐Gulf of Mexico region for ∼300 KyrAt sites with a clearly defined micrite layer, the end of micrite deposition coincides with the top of the highest productivity intervalElevated post K‐Pg export productivity appears to be a feature of oligotrophic low latitude open ocean sites [ABSTRACT FROM AUTHOR]

We investigate the impact of Arctic sea ice loss on the Atlantic meridional overturning circulation (AMOC) and North Atlantic climate in a coupled general circulation model (IPSL‐CM5A2) perturbation experiment, wherein Arctic sea ice is reduced until reaching an equilibrium of an ice‐free summer. After several decades we observe AMOC weakening caused by reduced dense water formation in the Iceland basin due to the warming of surface waters, and later compensated by intensification of dense water formation in the Western Subpolar North Atlantic. Consequently, AMOC slightly weakens in deep, dense waters but recovers through shallower, less dense waters overturning. In parallel, wind‐driven intensification and southeastward expansion of the subpolar gyre cause a depth‐extended cold anomaly ∼2°C around 50°N that resembles the North Atlantic "warming hole." We conclude that compensating dense water formations drive AMOC changes following sea ice retreat and that a warming hole can develop independently of the AMOC modulation. Plain Language Summary: To investigate the impact of Arctic sea ice loss due to anthropogenic warming on climate, we have designed experiments using an atmosphere‐ocean coupled general circulation model reducing the Arctic sea ice albedo in order to match future sea ice reduction. The induced changes in the summer Arctic sea ice reflect projections of an ice‐free summer Arctic. To examine the impact of a reduced Arctic sea ice state we investigate the Atlantic meridional overturning circulation (AMOC), due to the long timescales involved in ocean circulation. In response to Arctic sea ice decline, our results indicate that the decline in dense waters in the eastern subpolar region are partly compensated by an increase in the western subpolar North Atlantic, resulting in a modest decline and then recovery of AMOC. Our results further reveal that an anomalous warming hole feature forms as a result of the changes in atmospheric circulation and ocean gyres, independent of the strength of AMOC. Key Points: Atlantic meridional overturning circulation (AMOC) response involves compensation between changes in eastern (decline) and western (increase) subpolar North Atlantic deep convectionReduced sea ice results in southeastward expansion of the North Atlantic subpolar gyre and a negative North Atlantic Oscillation‐like patternA North Atlantic warming hole develops independently of AMOC modulation [ABSTRACT FROM AUTHOR]

Downstream of Drake Passage, the Antarctic Circumpolar Current (ACC) veers abruptly northward along the continental slope of South America. This spins down the ACC, akin to the western boundary currents of ocean gyres. During this northward excursion, the mean potential vorticity (PV) increases dramatically (decreases in magnitude) by up to a factor of 2 along mean geostrophic streamlines on middepth buoyancy surfaces. This increase is driven by drag near the continental slope, or by breaking eddies further offshore, and is balanced by a remarkably steady, eddy-driven decrease of mean PV along these northern circumpolar streamlines in the open ocean. We show how two related eddy processes that are fundamental to ACC dynamics—poleward buoyancy fluxes and downward fluxes of eastward momentum—are also concomitant with materially forcing PV to increase on the northern flank of a jet at middepth, and decrease on the southern flank. For eddies to drive the required mean PV decrease along northern streamlines, the ACC merges with the subtropical gyres to the north, so these streamlines inhabit the southern flanks of the combined ACC–gyre jets. We support these ideas by analyzing the time-mean PV and its budget along time-mean geostrophic streamlines in the Southern Ocean State Estimate. Our averaging formalism is Eulerian, to match the model's numerics. The thickness-weighted average is preferable, but its PV budget cannot be balanced using Eulerian 5-day averaged diagnostics, primarily because the z-level buoyancy and continuity equations' delicate balances are destroyed upon transformation into the buoyancy-coordinate thickness equation. Significance Statement: The Antarctic Circumpolar Current is the world's largest ocean current and a key controller of Earth's climate. As the westerly winds that drive this current shift poleward under global warming, it is vital to know whether the current will follow. To begin addressing this, we study the current's fundamental dynamics, and constraints, under present-day conditions. By analyzing angular momentum and stratification together, we show that the current is weakened near boundaries and strengthened by eddies elsewhere. The strengthening effects of eddies are isolated to the current by merging the current with oceanic gyres to the north. This gives a new perspective on why the current travels so far northward alongside South America, and may provide dynamical constraints on future changes. [ABSTRACT FROM AUTHOR]

Floating ice shelves buttress the Antarctic Ice Sheet, which is losing mass rapidly mainly due to ocean-driven melting and the associated disruption to glacial dynamics. The local ocean circulation near ice shelves is therefore important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf and resultant melting. However, the dynamics controlling the near-coastal circulation are not fully understood. A cyclonic (i.e. clockwise) gyre circulation (27 km radius) in front of the Pine Island Ice Shelf has previously been identified in both numerical models and velocity observations. Mooring data further revealed a potential reversal of this gyre during an abnormally cold period. Here we present ship-based observations from 2019 to the west of Thwaites Ice Shelf, revealing another gyre (13 km radius) for the first time in this habitually ice-covered region, rotating in the opposite (anticyclonic, anticlockwise) direction to the gyre near Pine Island Ice Shelf, despite similar wind forcing. We use an idealised configuration of MITgcm, with idealised forcing based on ERA5 climatological wind fields and a range of idealised sea ice conditions typical for the region, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf. The model driven solely by wind forcing in the presence of ice can reproduce the horizontal structure and direction of both gyres. We show that the modelled gyre direction depends upon the spatial difference in the ocean surface stress, which can be affected by the applied wind stress curl filed, the percentage of wind stress transferred through the ice, and the angle between the wind direction and the sea ice edge. The presence of ice, either it is fast ice/ice shelves blocking the effect of wind or mobile sea ice enhancing the effect of wind, has the potential to reverse the gyre direction relative to ice-free conditions. [ABSTRACT FROM AUTHOR]

Numerous assumptions and approximations are employed when translating satellite-derived radiance to surface remote sensing reflectance ($R_{\mathrm {RS}}$) for ocean color applications. Among these is the vicarious calibration coefficient ($g$) of the “long” near infrared band (NIR $_{\mathrm {L}}$) used for atmospheric correction. For this band, the prelaunch calibration has always been deemed sufficient [thus $g$ (NIR $_{\mathrm {L}}) =1.00$ ] as long as other bands are vicariously calibrated. Recent research, however, suggests that Moderate Resolution Imaging Spectroradiometer (MODIS)/Aqua $R_{\mathrm {RS}}$ time series is quite sensitive to $g$ (NIR $_{\mathrm {L}}$) (and associated vicarious gains in other bands). In this work, we assessed the sensitivity of Visible Infrared Imaging Radiometer Suite onboard the Suomi National Polar-orbiting Partnership satellite (VIIRS/SNPP) $R_{\mathrm {RS}}$ to NIRL calibration and compared our results to previous MODIS/Aqua and Sea-viewing Wide Field-of-View Sensor onboard OrbView2 (SeaWiFS)/OrbView2 analysis. In doing so, we note that $g$ (NIR $_{\mathrm {L}}$) sensitivities of mission-averaged $R_{\mathrm {RS}}$ time series are lower for VIIRS and SeaWiFS, relative to MODIS. At the scale of monthly climatologies (MCs), however, all sensors show prominent $g$ (NIR $_{\mathrm {L}}$) sensitivity with that of SeaWiFS being the most substantial. These findings informed simulation analyses, whereby we identified signal-to-noise ratio (SNR) and radiant path geometry, as well as their interaction, as having notable impacts on $g$ (NIR $_{\mathrm {L}}$) sensitivity. As such, $g$ (NIR $_{\mathrm {L}}$) sensitivity is a necessary consideration for reflectance uncertainty budgets, especially for sensors with higher NIR SNR or particular prevailing radiant path geometries. Given the geometry components embedded within $g$ (NIR $_{\mathrm {L}}$) sensitivity, such studies should be coupled with cross-sensor intercalibrations [e.g., using simultaneous same view (SSV) measurements] toward minimizing NIRL errors between satellite instruments, but such efforts will not completely remediate remaining cross-sensor biases in $R_{\mathrm {RS}}$. [ABSTRACT FROM AUTHOR]

In this survey article, we provide a mathematical description of oceanic and atmospheric flows, based on the incompressible Navier–Stokes equation (for the ocean), and the compressible version with an equation of state and the first law of thermodynamics for the atmosphere. We show that, in both cases, the only fundamental assumption that we need to make is that of a thin shell on a (nearly) spherical Earth, so that the main elements of spherical geometry are included, with all other attributes of the fluid motion retained at leading order. (The small geometrical correction that is needed to represent the Earth's geoid as an oblate spheroid is briefly described.) We argue that this is the only reliable theoretical approach to these types of fluid problem. A generic formulation is presented for the ocean, and for the steady and unsteady atmosphere, these latter two differing slightly in the details. Based on these governing equations, a number of examples are presented (in outline only), some of which provide new insights into familiar flows. The examples include the Ekman flow and large gyres in the ocean; and in the atmosphere: Ekman flow, geostrophic balance, Brunt–Väisälä frequency, Hadley–Ferrel–polar cells, harmonic waves, equatorially trapped waves. [ABSTRACT FROM AUTHOR]

We prove new existence criteria relevant for the non-linear elliptic PDE of the form Δ S 2 u = C − h e u Δ S 2 u = C − h e u , considered on a two dimensional sphere S 2 S 2 , in the parameter regime 2 ≤ C < 4 2 ≤ C < 4. We apply this result, as well as several previously known results valid when C < 2 C < 2 , to discuss existence of solutions of a particular PDE modelling ocean surface currents. [ABSTRACT FROM AUTHOR]

Isopycnal mixing of tracers is important for ocean dynamics and biogeochemistry. Previous studies have primarily focused on the horizontal structure of mixing, but what controls its vertical structure is still unclear. This study investigates the vertical structure of the isopycnal tracer diffusivity diagnosed by a multiple-tracer inversion method in an idealized basin circulation model. The first two eigenvalues of the symmetric part of the 3D diffusivity tensor are approximately tangent to isopycnal surfaces. The isopycnal mixing is anisotropic, with principal directions of the large and small diffusivities generally oriented along and across the mean flow direction. The cross-stream diffusivity can be reconstructed from the along-stream diffusivity after accounting for suppression of mixing by the mean flow. In the circumpolar channel and the upper ocean in the gyres, the vertical structure of the along-stream diffusivity follows that of the rms eddy velocity times a depth-independent local energy-containing scale estimated from the sea surface height. The diffusivity in the deep ocean in the gyres instead follows the profile of the eddy kinetic energy times a depthindependent mixing time scale. The transition between the two mixing regimes is attributed to the dominance of nonlinear interactions and linear waves in the upper and deep ocean, respectively, distinguished by a nonlinearity parameter. A formula is proposed that accounts for both regimes and captures the vertical variation of diffusivities better than extant theories. These results inform efforts to parameterize the vertical structure of isopycnal mixing in coarse-resolution ocean models. Plain Language Summary Ocean mesoscale eddies mix momentum, heat, carbon and other tracers, which impacts the ocean environment and Earth's climate. The mixing of tracers by mesoscale eddies is mostly measured on the surface, but the observation of the mixing in the ocean interior is rare. It is unclear how eddy mixing varies with depth. We estimated the mixing by mesoscale eddies in an idealized numerical model, which simulates ocean currents and eddies in a basin like the Atlantic Ocean. Mixing decreases with depth in the same rate as the characteristic swirling velocity of eddies in the upper ocean, while it decreases faster in the deep ocean. This is because as eddies become weak in the deep ocean they behave like waves rather than closed swirls. A method accounting for this vertical variation is proposed which recovers the vertical structure of eddy mixing over full depth, once the characteristic swirling velocity is available. [ABSTRACT FROM AUTHOR]

Gyres are prominent surface structures in the global ocean circulation that often interact with the sea floor in a complex manner. Diagnostic methods, such as the depth‐integrated vorticity budget, are needed to assess exactly how such model circulations interact with the bathymetry. Terms in the vorticity budget can be integrated over the area enclosed by streamlines to identify forces that spin gyres up and down. In this article we diagnose the depth‐integrated vorticity budgets of both idealized gyres and the Weddell Gyre in a realistic global model. It is shown that spurious forces play a significant role in the dynamics of all gyres presented and that they are a direct consequence of the Arakawa C‐grid discretization and the z‐coordinate representation of the sea floor. The spurious forces include a numerical beta effect and interactions with the sea floor which originate from the discrete Coriolis force when calculated with the following schemes: the energy conserving scheme; the enstrophy conserving scheme; and the energy and enstrophy conserving scheme. Previous studies have shown that bottom pressure torques provide the main interaction between the depth‐integrated flow and the sea floor. Bottom pressure torques are significant, but spurious interactions with bottom topography are similar in size. Possible methods for reducing the identified spurious topographic forces are discussed. Spurious topographic forces can be alleviated by using either a B‐grid in the horizontal plane or a terrain‐following vertical coordinate. Plain Language Summary: Gyres are large scale circulations in the world ocean that often interact with the sea floor. It is important to develop a method to assess how the representation of the sea floor in models affects gyre circulations. By calculating how model forces generate vorticity (the tendency to rotate) in the flow, we are able to determine the forces acting with and against the gyre circulation. We apply this method to results from a simplified double gyre model and the Weddell Gyre in a realistic global model. We show that spurious forces which emerge from the layout of the model grid play an important role in the presented gyre circulations. The spurious forces originate from the calculation of the Coriolis acceleration in the model. In previous studies, it has been argued that gyre circulations interact with the sea floor primarily by forming pressure gradients; here we show that contributions from pressure gradients are significant, but the spurious forces are similar in size and also emerge from interactions with the sea floor. We discuss possible approaches to reduce the identified spurious forces by considering alternative grid layouts. The spurious forces can be alleviated by using a B‐grid or a terrain‐following vertical coordinate. Key Points: The vorticity budget is used to identify forces spinning gyres up and down when integrated over the area enclosed by streamlinesSpurious topographic forces and a numerical beta effect emerge from the Coriolis acceleration when using a C‐grid with z‐coordinatesThe identified spurious forces are significant in both an idealized gyre configuration and the Weddell Gyre in a realistic global model [ABSTRACT FROM AUTHOR]

The dynamics involved in the variability of temperature, salinity, and density distributions in the upper layer of the Indo‐Pacific sector of the Southern Ocean were studied using Acoustic Doppler Current Profiler and Conductivity Temperature Depth data from a mini triangle trans‐ocean buoy network mooring deployed at 60°S and 140°E between January 15th and 19th December 2012, together with hydrographic casts, reanalysis outputs, and satellite imagery. Spectral analysis confirmed that near‐inertial internal waves are a major factor transporting energy downward to the lower boundary of the mixed layer (ML), with 12.5 J m−3 or 45% of the mean surface energy being able to reach deeper than the ML depth during one of the analyzed events. During a period of net heat loss from the ocean surface to the atmosphere, most of the energy was concentrated below the maximum shear layer, at around 80 m depth. Internal wave events were found to be linked to scalar diffusivity increases of up to two orders of magnitude. A box model clarified that surface cooling plays the most significant role in mixing surface waters during winter, but it is not enough to explain variability below 100 m depth, where horizontal advection and eddy enhancement are also important in modulating warm water flux. Our results elucidate how wind energy might enhance mixing, even in ice‐covered regions, while influencing depths greater than previously thought. Plain Language Summary: We compared temperature, salinity, and ocean‐current data recorded by a 2012 Antarctic expedition to satellite data and numerical models to determine the mechanisms behind the seasonal variation of these parameters in the Southern Ocean. Three main influencing factors were identified: Cooling of the ocean surface during winter, eddies, and near‐inertial internal waves. We found that internal waves are especially important for increasing mixing and their energy accumulates at around 80 m depth during winter. These results imply that wind might increase mixing, even in ice‐covered regions, and also reach deeper than previously thought. However, at layers deeper than 100 m from the ocean surface, the movement of underwater polar fronts, instigated by the strengthening of oceanic gyres, is essential for explaining variability, indicating a hidden underwater pathway guiding Antarctic waters during winter. Key Points: Net heat‐flux variability at the ocean surface is not sufficient to explain seasonal temperature variability below 100 mSeasonal variability of temperature and salinity below 150 m might be tied to strengthened warm‐core eddiesPassage of near‐inertial internal waves enhanced mixing across the lower boundary of the mixed layer at the mooring location [ABSTRACT FROM AUTHOR]

High concentrations of microplastic particles are reported across the Arctic Ocean–yet no meaningful point sources, suspension timelines, or accumulation areas have been identified. Here we use Lagrangian particle advection simulations to model the transport of buoyant microplastic from northern European rivers to the high Arctic, and compare model results to the flux of sampled synthetic particles across the main entrance to the Arctic Ocean. We report widespread dispersal along the Eurasian continental shelf, across the North Pole, and back into the Nordic Seas; with accumulation zones over the Nansen basin, the Laptev Sea, and the ocean gyres of the Nordic Seas. The equal distribution of sampled synthetic particles across water masses covering a wide time frame of anthropogenic influence suggests a system in full saturation rather than pronounced injection from European sources, through a complex circulation scheme connecting the entire Arctic Mediterranean. This circulation of microplastic through Arctic ecosystems may have large consequences to natural ecosystem health, highlighting an ever-increasing need for better waste management. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms that generate and maintain diversity in marine prokaryotic communities is one of the main challenges for contemporary marine microbiology. We here review how observational, experimental, and theoretical evidence converge on the conclusion that the marine pelagic community of heterotrophic prokaryotes consists of organisms with two main types of life strategies. We illustrate this dichotomy by SAR11 and Vibrio spp. as typical representatives of the two strategies. A theory for life strategy dichotomy exists in classical r/K-selection. We here discuss an additional dichotomy introduced by what we term S/L-selection (for Small and Large, respectively). While r/K-selection focuses on the role of environmental disturbances, steady-state models suggest that high abundance at species level should be closely related to a low trade-off between competition and defense. We summarize literature indicating that the high availability of organic C is an essential environmental factor favoring Vibrio spp. and suggest that the essence of the generalized L-strategy is to reduce the competition-predator defense trade-off by using non-limiting organic C to increase size. The "streamlining" theory that has been suggested for the S-strategist SAR11 proposes the opposite: that low trade-off is achieved by a reduction in size. We show how this apparent contradiction disappears when the basic assumptions of diffusion-limited uptake are considered. We propose a classification scheme that combines S/L and r/K-selection using the two dimensions of organic C availability and environmental disturbance. As organic C in terrestrial runoff and size of the oligotrophic oceanic gyres are both changing, habitat size for both S- and L-strategists are affected by global change. A theory capturing the main aspects of prokaryote life strategies is therefore crucial for predicting responses of the marine microbial food web to climate change and other anthropogenic influences. [ABSTRACT FROM AUTHOR]

The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre–ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing. Significance Statement: This study explores the impact of seafloor bathymetry on the dynamics of subpolar gyres in the Southern Ocean. The subpolar gyres are major circulation features that connect the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. This work provides deeper insight for how the submarine ridges that exist along the northern periphery of these gyres shape the vertical distribution of tracers and overturning circulation in these regions. These findings highlight an underappreciated yet fundamentally important topographical constraint on the three-dimensional cycling of heat and carbon in the Southern Ocean—processes that have far-reaching implications for the global climate. Future work should explore how the presence of these ridges affect the time-evolving response of the Southern Ocean to changes in surface conditions. [ABSTRACT FROM AUTHOR]

Pine Island Ice Shelf (PIIS) buttresses the Pine Island Glacier, the key contributor to sea-level rise. PIIS has thinned owing to ocean-driven melting, and its calving front has retreated, leading to buttressing loss. PIIS melting depends primarily on the thermocline variability in its front. Furthermore, local ocean circulation shifts adjust heat transport within Pine Island Bay (PIB), yet oceanic processes underlying the ice front retreat remain unclear. Here, we report a PIB double-gyre that moves with the PIIS calving front and hypothesise that it controls ocean heat input towards PIIS. Glacial melt generates cyclonic and anticyclonic gyres near and off PIIS, and meltwater outflows converge into the anticyclonic gyre with a deep-convex-downward thermocline. The double-gyre migrated eastward as the calving front retreated, placing the anticyclonic gyre over a shallow seafloor ridge, reducing the ocean heat input towards PIIS. Reconfigurations of meltwater-driven gyres associated with moving ice boundaries might be crucial in modulating ocean heat delivery to glacial ice. Glacial melt can modify heat transport, and therefore ocean processes, associated with ice front retreat, as revealed by direct observations from the Pine Island Bay region of Antarctica. [ABSTRACT FROM AUTHOR]

A series of numerical simulations with different forcing conditions are carried out, to investigate the roles played by buoyancy and wind forcing on the upper ocean gyres, and to contrast the laminar and eddying regimes. Model experiments show that the buoyancy-driven eastward geostrophic flow tends to suppress the formation of the wind-driven subpolar gyre, but the northward eddy heat transport can homogenize the subpolar water and reduce the meridional temperature gradient by about two-third, thus counteracting the buoyancy effect and saving the subpolar gyre. For the subtropical gyre, its transport is enhanced by eddy mixing, and the role of buoyancy forcing is very sensitive to the choice of diapycnal diffusivity. Our results suggest that eddy effects must be considered in the dynamics of the subpolar gyre, and vertical diffusivity should be selected carefully in simulating the basin-wide circulations. [ABSTRACT FROM AUTHOR]

The global distribution patterns of 14918 geo-referenced occurrences from 394 living brachiopod species were mapped in 5° grid cells, which enabled the visualization and delineation of distinct bioregions and biodiversity hotspots. Further investigation using cluster and network analyses allowed us to propose the first systematically and quantitatively recognized global bioregionalization framework for living brachiopods, consisting of five bioregions and thirteen bioprovinces. No single environmental or ecological variable is accountable for the newly proposed global bioregionalization patterns of living brachiopods. Instead, the combined effects of large-scale ocean gyres, climatic zonation as well as some geohistorical factors (e.g., formation of land bridges and geological recent closure of ancient seaways) are considered as the main drivers at the global scale. At the regional scale, however, the faunal composition, diversity and biogeographical differentiation appear to be mainly controlled by seawater temperature variation, regional ocean currents and coastal upwelling systems. [ABSTRACT FROM AUTHOR]

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2. The role of Southern Ocean gateways contributing to the Eocene-Oligocene climate transition is still debated. Here, the authors present high-resolution ocean simulations to show that gateways opening led to a reorganization of ocean circulation, heat transport and Antarctic surface water cooling. [ABSTRACT FROM AUTHOR]