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In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss-a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air-sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability. [ABSTRACT FROM AUTHOR]

Surface ocean temperature and velocity anomalies at meso‐ and sub‐meso‐scales induce wind stress anomalies. These wind‐front interactions, referred to as thermal (TFB) and current (CFB) feedbacks, respectively, have been studied in isolation at mesoscale, yet they have rarely been considered in tandem. Here, we assess the combined influence of TFB and CFB and their relative impact on surface wind stress derivatives. Analyses are based on output from two regions of the Southern Ocean in a coupled simulation with local ocean resolution of 2 km. Considering both TFB and CFB shows regimes of interference, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress curl anomalies approach 10−5 N m−3, ∼20 times stronger than at mesoscale. The synergy of both feedbacks improves the ability to reconstruct wind stress curl magnitude and structure from both surface vorticity and SST gradients by 12%–37% on average, compared with using either feedback alone. Plain Language Summary: Surface ocean temperature and velocity anomalies at 0.1–100 km scales imprint their signatures on the surface wind stress, which in turn supplies the ocean with momentum. This process is called wind‐front interaction and typically referred to as thermal and current feedbacks when generated by temperature gradients and velocity gradients, respectively. Previously, studies using satellite observations and regional numerical models have studied either feedback in isolation; consideration of both feedbacks in tandem remains immature. Here, we present an approach that assesses both feedbacks' combined and relative impact on the surface wind stress, using output from an air‐sea coupled simulation. This approach allows us to identify constructive and destructive patterns of how the two feedbacks interact, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress derivative anomalies are 20 times stronger than observed previously at larger scales. Considering both feedbacks, reconstructions of wind stress derivatives are viable and have 10%–40% less error on average compared with using either feedback by itself. Contributions from either feedback in modifying wind stress fields vary temporally and can be related to physical properties such as surface wind speed and air‐sea temperature difference across the studied area. Key Points: Surface temperature gradients and vorticity anomalies at scales down to O(10) km impact wind stress curl and divergence jointlyWind‐front interactions at sub‐mesoscales are at least an order of magnitude stronger than at mesoscalesReconstruction of the wind stress curl using both thermal and current feedback is 10%–40% more accurate than relying on a single feedback [ABSTRACT FROM AUTHOR]

The export of Antarctic Bottom Water (AABW) supplies the bottom cell of the global overturning circulation and plays a key role in regulating climate. This AABW outflow must cross, and is therefore mediated by, the Antarctic Circumpolar Current (ACC). Previous studies present widely varying conceptions of the role of the ACC in directing AABW across the Southern Ocean, suggesting either that AABW may be zonally recirculated by the ACC, or that AABW may flow northward within deep western boundary currents (DWBC) against bathymetry. In this study the authors investigate how the forcing and geometry of the ACC influences the transport and transformation of AABWusing a suite of process-oriented model simulations. The model exhibits a strong dependence on the elevation of bathymetry relative to AABWlayer thickness: higher meridional ridges suppress zonal AABWexchange, increase the strength of flow in the DWBC, and reduce the meridional variation in AABWdensity across the ACC. Furthermore, the transport and transformation vary with density within the AABW layer, with denser varieties of AABW being less efficiently transported between basins. These findings indicate that changes in the thickness of the AABW layer, for example, due to changes in Antarctic shelf processes, and tectonic changes in the sea floor shape may alter the pathways and transformation of AABW across the ACC. [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]

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]

Standing meanders are a key component of the Antarctic Circumpolar Current (ACC) circulation system, and numerical studies have shown that these features may locally enhance subduction, upwelling, as well as lateral and vertical tracer transport. Yet, observational data from these regions remain sparse. Here, we present results based on measurements made by a group of autonomous platforms sampling an ACC standing meander formed due to the interaction of the Polar Front with the Southwest Indian Ridge. Two Seagliders were deployed alongside a Biogeochemical‐Argo float that was advected through the standing meander. In the high eddy kinetic energy region of the standing meander, the glider observations reveal enhanced submesoscale frontal gradients as well as heightened tracer variability at depth, as compared to the more quiescent region further downstream. Vertical gradients in spice and apparent oxygen utilization are reduced in the standing meander despite similarities in the large‐scale vertical stratification, suggesting greater ventilation of the surface ocean. These observations are consistent with numerical studies that highlight standing meanders as hotspots for ventilation and subduction due to enhanced mesoscale stirring and submesoscale vertical velocities. Our results emphasize the need to account for spatial heterogeneity in processes influencing air‐sea exchange, carbon export, and biogeochemical cycling in the Southern Ocean. Plain Language Summary: The Southern Ocean plays a vital role in taking up carbon dioxide and heat from the Earth's atmosphere. Yet, historically, this region has suffered from a lack of direct observations, making it difficult to quantify rates of exchange. In regions where the strongest currents of the Southern Ocean interact with underwater topography turbulence and mixing may be locally enhanced. Data collected from two different types of robotic vehicles, including steerable ocean gliders, were used to study one of these mixing "hotspots." The instrumented platforms provided expanded data as compared to typical Southern Ocean observational techniques. The results show that these turbulent regions allow water properties, such as temperature and oxygen, to mix vigorously. In particular, at these hotspots the exchange of waters between the ocean surface and interior increases. It is important to document the magnitude and regional patterns of this exchange because only surface waters interact with the atmosphere to take up or release carbon and heat, while interior waters store these properties for centuries or longer. Numerical models of the ocean have indicated that these turbulent regions of the Southern Ocean are particularly influential, and we provide some of the first observational evidence confirming these hotspots' importance. Key Points: Observations with high spatial and temporal resolution were collected from two ocean gliders following a Biogeochemical‐Argo floatPhysical and biogeochemical tracer variance is enhanced within a standing meander of the Antarctic Circumpolar CurrentThe standing meander shows evidence of heightened exchange of water properties between the ocean surface boundary layer and the interior [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]

The last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed. [ABSTRACT FROM AUTHOR]

Abstract: This study combines sea surface height (SSH) estimates of the ice‐covered Southern Ocean with conventional open‐ocean SSH estimates from CryoSat‐2 to produce monthly composites of dynamic ocean topography (DOT) and sea level anomaly (SLA) on a 50 km grid spanning 2011–2016. This data set reveals the full Southern Ocean SSH seasonal cycle for the first time; there is an antiphase relationship between sea level on the Antarctic continental shelf and the deeper basins, with coastal SSH highest in autumn and lowest in spring. As a result of this pattern of seasonal SSH variability, the barotropic component of the Antarctic Slope Current (ASC) has speeds that are regionally up to twice as fast in the autumn. Month‐to‐month circulation variability of the Ross and Weddell Gyres is strongly influenced by the local wind field, and is correlated with the local wind curl (Ross: −0.58; Weddell: −0.67). SSH variability is linked to both the Southern Oscillation and the Southern Annular Mode, dominant modes of southern hemisphere climate variability. In particular, during the strong 2015–2016 El Niño, a sustained negative coastal SLA of up to −6 cm, implying a weakening of the ASC, was observed in the Pacific sector of the Southern Ocean. The ability to examine sea level variability in the seasonally ice‐covered regions of the Southern Ocean—climatically important regions with an acute sparsity of data—makes this new merged sea level record of particular interest to the Southern Ocean oceanography and glaciology communities. [ABSTRACT FROM AUTHOR]

Along-stream variations in the dynamics of the Antarctic Circumpolar Current (ACC) impact heat and tracer transport, regulate interbasin exchange, and influence closure of the overturning circulation. Topography is primarily responsible for generating deviations from zonal-mean properties, mainly through standing meanders associated with regions of high eddy kinetic energy. Here, an idealized channel model is used to explore the spatial distribution of energy exchange and its relationship to eddy geometry, as characterized by both eddy momentum and eddy buoyancy fluxes. Variations in energy exchange properties occur not only between standing meander and quasi-zonal jet regions, but throughout the meander itself. Both barotropic and baroclinic stability properties, as well as the magnitude of energy exchange terms, undergo abrupt changes along the path of the ACC. These transitions are captured by diagnosing eddy fluxes of energy and by adopting the eddy geometry framework. The latter, typically applied to barotropic stability properties, is applied here in the depth-along-stream plane to include information about both barotropic and baroclinic stability properties of the flow. These simulations reveal that eddy momentum fluxes, and thus barotropic instability, play a leading role in the energy budget within a standing meander. This result suggests that baroclinic instability alone cannot capture the dynamics of ACC standing meanders, a challenge for models where eddy fluxes are parameterized. [ABSTRACT FROM AUTHOR]

The ocean's overturning circulation is inherently three-dimensional, yet modern quantitative estimates of the overturning typically represent the subsurface circulation as a two-dimensional, two-cell streamfunction that varies with latitude and depth only. This approach suppresses information about zonal mass and tracer transport. In this article, the authors extend earlier, zonally averaged overturning theory to explore the dynamics of a 'figure-eight' circulation that cycles through multiple basins. A three-dimensional residual-mean model of the overturning circulation is derived and then simplified to a multibasin isopycnal box model to explore how stratification and diabatic water mass transformations in each basin depend on the basin widths and on deep and bottom-water formation in both hemispheres. The idealization to multiple, two-dimensional basins permits zonal mass transport along isopycnals in a Southern Ocean-like channel, while retaining the dynamical framework of residual-mean theory. The model qualitatively reproduces the deeper isopycnal surfaces in the Pacific Basin relative to the Atlantic. This supports a transfer of Antarctic Bottom Water from the Atlantic sector to the Pacific sector via the Southern Ocean, which subsequently upwells in the northern Pacific Basin. A solution for the full isopycnal structure in the Southern Ocean reproduces observed stratification differences between Atlantic and Pacific Basins and provides a scaling for the diffusive boundary layer in which the zonal mass transport occurs. These results are consistent with observational indications that North Atlantic Deep Water is preferentially transformed into Antarctic Bottom Water, which undermines the importance of an adiabatic, upper overturning cell in the modern ocean. [ABSTRACT FROM AUTHOR]

Insight into the global ocean energy cycle and its relationship to climate variability can be gained by examining the temporal variability of eddy-mean flow interactions. A time-dependent version of the Lorenz energy diagram is formulated and applied to energetic ocean regions from a global, eddying state estimate. The total energy in each snapshot is partitioned into three components: energy in the mean flow, energy in eddies, and energy temporal anomaly residual, whose time mean is zero. These three terms represent, respectively, correlations between mean quantities, correlations between eddy quantities, and eddy-mean correlations. Eddy-mean flow interactions involve energy exchange among these three components. The temporal coherence about energy exchange during eddy-mean flow interactions is assessed. In the Kuroshio and Gulf Stream Extension regions, a suppression relation is manifested by a reduction in the baroclinic energy pathway to the eddy kinetic energy (EKE) reservoir following a strengthening of the barotropic energy pathway to EKE; the baroclinic pathway strengthens when the barotropic pathway weakens. In the subtropical gyre and Southern Ocean, a delay in energy transfer between different reservoirs occurs during baroclinic instability. The delay mechanism is identified using a quasigeostrophic, two-layer model; part of the potential energy in large-scale eddies, gained from the mean flow, cascades to smaller scales through eddy stirring before converting to EKE. The delay time is related to this forward cascade and scales linearly with the eddy turnover time. The relation between temporal variations in wind power input and eddy-mean flow interactions is also assessed. [ABSTRACT FROM AUTHOR]

The complex export pathways that connect the surface waters of the Weddell Sea with the Antarctic Circumpolar Current influence water mass modification, nutrient fluxes, and ecosystem dynamics. To study this exchange, 40 surface drifters, equipped with temperature sensors, were released into the northwestern Weddell Sea's continental shelf and slope frontal system in late January 2012. Comparison of the drifter trajectories with a similar deployment in early February 2007 provides insight into the interannual variability of the surface circulation in this region. Observed differences in the 2007 and 2012 drifter trajectories are related to a variable surface circulation responding to changes in wind stress curl over the Weddell Gyre. Differences between northwestern Weddell Sea properties in 2007 and 2012 include 1) an enhanced cyclonic wind stress forcing over the Weddell Gyre in 2012; 2) an acceleration of the Antarctic Slope Current (ASC) and an offshore shift of the primary drifter export pathway in 2012; and 3) a strengthening of the Coastal Current (CC) over the continental shelf in 2007. The relationship between wind stress forcing and surface circulation is reproduced over a longer time period in virtual drifter deployments advected by a remotely sensed surface velocity product. The mean offshore position and speed of the drifter trajectories are correlated with the wind stress curl over the Weddell Gyre, although with different temporal lags. The drifter observations are consistent with recent modeling studies suggesting that Weddell Sea boundary current variability can significantly impact the rate and source of exported surface waters to the Scotia Sea, a process that determines regional chlorophyll distributions. [ABSTRACT FROM AUTHOR]

The insensitivity of the Antarctic Circumpolar Current (ACC)'s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind-driven Ekman overturning-a framework verified in zonally symmetric circumpolar flows. Substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the ACC. Analysis of an eddy-resolving ocean GCM shows that the ACC can be broken into broad regions of weak eddy activity, where surface winds steepen isopycnals, and a small number of standing meanders, across which the isopycnals relax. Meanders are coincident with sites of (i) strong eddy-induced modification of the mean flow and its vertical structure as measured by the divergence of the Eliassen-Palm flux and (ii) enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions. Within meanders, the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence, providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification. Temporal fluctuations in these diagnostics are correlated with variability in both the Eliassen-Palm flux and bottom speed, implying a link to dissipative processes at the ocean floor. At larger scales, bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity, which links to variations in meander structure. From these results, it is proposed that the 'flexing' of standing meanders provides an alternative mechanism for reducing the sensitivity of the ACC's baroclinicity to changes in forcing, separate from an ACC-wide change in transient eddy characteristics. [ABSTRACT FROM AUTHOR]

In the modern climate, the ocean below 2 km is mainly filled by waters sinking into the abyss around Antarctica and in the North Atlantic. Paleoproxies indicate that waters of North Atlantic origin were instead absent below 2 km at the Last Glacial Maximum, resulting in an expansion of the volume occupied by Antarctic origin waters. In this study we show that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around Antarctica. A simple theory further suggests that these deep waters only came to the surface under sea ice, which insulated them from atmospheric forcing, and were weakly mixed with overlying waters, thus being able to store carbon for long times. This unappreciated link between the expansion of sea ice and the appearance of a voluminous and insulated water mass may help quantify the ocean's role in regulating atmospheric carbon dioxide on glacial-interglacial timescales. Previous studies pointed to many independent changes in ocean physics to account for the observed swings in atmospheric carbon dioxide. Here it is shown that many of these changes are dynamically linked and therefore must co-occur. [ABSTRACT FROM AUTHOR]

The Southern Ocean's Antarctic Circumpolar Current (ACC) naturally lends itself to interpretations using a zonally averaged framework. Yet, navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models. In this study, both observational and numerical results indicate that zonal asymmetries, in the form of topography, impact global flow structure and transport properties. The conclusions are based on a suite of more than 1.5 million virtual drifter trajectories advected using a satellite altimetry-derived surface velocity field spanning 17 years. The focus is on sites of 'cross front' transport as defined by movement across selected sea surface height contours that correspond to jets along most of the ACC. Cross-front exchange is localized in the lee of bathymetric features with more than 75% of crossing events occurring in regions corresponding to only 20% of the ACC's zonal extent. These observations motivate a series of numerical experiments using a two-layer quasigeostrophic model with simple, zonally asymmetric topography, which often produces transitions in the front structure along the channel. Significantly, regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic. Jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets. Eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering. The combination of high eddy kinetic energy and recirculation features enhances particle exchange. These results stress the complications in developing consistent circumpolar definitions of the ACC fronts. [ABSTRACT FROM AUTHOR]