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 4-6 km-resolution coupled simulation. 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-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 one alone.

A document by Vincent Verjans. Click on the document to view its contents.

Melt rates of West Antarctic ice shelves in the Amundsen Sea track large decadal variations in the volume of warm water at their outlets. This variability is generally attributed to wind-driven variations in warm water transport towards ice shelves. Inspired by conceptual representations of the global overturning circulation, we introduce a simple model for the evolution of the thermocline, which caps the water warm layer at the ice-shelf front. This model demonstrates that interannual variations in coastal polynya buoyancy forcing can generate large decadal-scale thermocline depth variations, even when the supply of warm water from the shelf-break is fixed. The modeled variability involves transitions between bistable high and low melt regimes, enabled by feedbacks between basal melt rates and ice front stratification strength. Our simple model captures observed variations in near-coast thermocline depth and stratification strength, and poses an alternative mechanism for warm water volume changes to wind-driven theories.

State-of-the-art climate models simulate a large spread in the mean-state Atlantic meridional overturning circulation (AMOC), with strengths varying between 12 and 25 Sv. Here, we introduce a framework for understanding this spread by assessing the balance between the thermal-wind expression and surface water mass transformation in the North Atlantic. The intermodel spread in the mean-state AMOC strength is shown to be related to the overturning scale depth: climate models with a larger scale depth tend to also have a stronger AMOC. Intermodel variations in the overturning scale depth are also related to intermodel variations in North Atlantic surface buoyancy loss and stratification. We present a physically-motivated scaling relationship that links the scale-depth variations to buoyancy forcing and stratification in the North Atlantic, and thus connects North Atlantic surface processes to the interior ocean circulation. These results offer a framework for reducing mean-state AMOC biases in climate models.

The biological pump, which removes carbon from the surface ocean and regulates atmospheric carbon dioxide, comprises multiple processes that include but extend beyond gravitational settling of organic particles. Contributions to the biological pump that arise from the physical circulation are broadly referred to as physical particle injection pumps; a synthetic view of how these physical pumps interact with each other and other components of the biological pump does not yet exist. In this study, observations from a quasi-Lagrangian float and ocean glider, deployed in the Southern Ocean’s subantarctic zone for one month during the spring bloom, offer insight into daily-to-monthly fluctuations in the mixed layer pump and the eddy subduction pump. Estimated independently, each mechanism contributes intermittent export fluxes on the order of several hundreds milligrams of particulate organic carbon (POC) per day. The float and the glider produce similar estimates of the mixed layer pump, with sustained weekly periods of export fluxes with a magnitude of 400 mg-POC-m-2-day-1. Export fluxes from the eddy subduction pump, based on a mixed layer instability scaling, occasionally exceed 500 mg-POC-m-2day-1, with some periods having strong inferred vertical velocities and others having enhanced isopycnal slopes. Regimes occur when a summation of the two pump estimates may misrepresent the total physical carbon flux. Disentangling contributions from different physical pump mechanisms from sparse data will remain challenging. Insight into how mesoscale stirring and submesocale velocities set the vertical structure of POC concentrations is identified as a key target to reduce uncertainty in global carbon export fluxes.

Oceanic macroturbulence is efficient at stirring and transporting tracers. The dynamical properties of this stirring can be characterized by statistically quantifying tracer structures. Here, we characterize the macroscale (1-100 km) tracer structures observed by two Seagliders downstream of the Southwest Indian Ridge (SWIR) in the Antarctic Circumpolar Current (ACC). These are some of the first glider observations in an energetic standing meander of the ACC, regions associated with enhanced ventilation. The small-scale density variance in the mixed layer (ML) was relatively enhanced near the surface and base of the ML, while being muted in the middle, suggesting the formation mechanism to be associated to ML instabilities and eddies. In addition, ML density fronts were formed by comparable contributions from temperature and salinity gradients, suggesting the dominant role of stirring, over air-sea interactions, in their formation and sustainability. In the interior, along-isopycnal spectra and structure functions of spice indicated that there is relatively lower variance at smaller scales than would be expected based on non-local stirring, suggesting that flows smaller than the deformation radius play a role in the cascade of tracers to small scales. These interior spice anomalies spanned across isopycnals, and were found to be about 3-5 times flatter than the aspect ratio that would be expected for O(1) Burger number flows like interior QG dynamics, suggesting the ratio of vertical shear to horizontal strain is greater than $N/f$. This further supports that small-scale flows, with high-mode vertical structures, stir tracers and impact tracer distributions.

Mesoscale eddies are a dominant source of spatial variability in the surface ocean and play a major role in the biological marine carbon cycle. Satellite altimetry is often used to locate and track eddies, but this approach is rarely validated against in situ observations. Here we compare measurements of a small (under 25 km radius) mode water anticyclonic eddy over the Procupine Abyssal Plain using CTD and ADCP measurements from 3 ships, 2 gliders, 2 profiling floats, and one Lagrangian float with those derived from sea level anomaly. In situ estimates of the eddy center were estimated from maps of the thickness of its central isopycnal layer, from ADCP velocities at a reference layer, and from the trajectory of the Lagrangian float. These were compared to three methods using altimetric SLA: one based on maximizing geostrophic rotation, one based on a constant SLA contour, and one which maximizes geostrophic velocity speed along the eddy boundary. All algorithms were used to select CTD profiles that were within the eddy. The in-situ metrics agreed to 97\%. The altimetry metrics showed only a small loss of accuracy, giving $>90$\% agreement with the in situ results. This suggests that current satellite altimetry is adequate for understanding the spatial representation of even relatively small mesoscale eddies.

Similar to most West Antarctic ice shelves, those in the Bellingshausen Sea have rapidly thinned by hundreds of cubic kilometers over the last decades yet they remain under-studied compared to other regions. The increased melting rates in the West Antarctic Peninsula (WAP) have been linked to warm Circumpolar Deep Water (CDW) that is able to access the continental shelf due to the absence of the Antarctic Slope Current. The exact pathways of CDW flowing on to the shelf and of meltwater flowing away from the ice shelves are essential to understanding the dynamics in this region and how it will change in the future. Here, we propose that the Bellingshausen Sea plays an important role in connecting circulation between the Amundsen Sea and the WAP and may influence water properties that circulate under floating ice shelves throughout the West Antarctica. Using a combination of hydrographic and isotopic data from a recent cruise to the Bellingshausen Sea (December 2018 to January 2019), multiple methods are applied to identify circulation pathways and to quantify glacial meltwater fractions. The meltwater measurements show that the Belgica and Latady Troughs are important pathways for CDW to reach the ice shelves, though almost twice as much meltwater is transported off the shelf via the Belgica Trough. CDW enters the shelf at the deepest part of the Belgica Trough, moving towards the coast along the trough’s eastern side. The largest meltwater fractions are found along the western flank of the Belgica and Latady Troughs. The meltwater signature can be tracked to the western edge of the Bellingshausen Sea, where it is then entrained into a boundary current system that flows over the continental slope towards the Amundsen Sea.

In the sea ice-impacted Southern Ocean, the spring melt of sea ice modifies the upper ocean. These modified waters subduct and enter the global overturning circulation. Submesoscale processes act to modulate the stratification of the mixed layer and therefore mixed layer properties. Sparse observations mean that the role of submesoscales in exchange across the base of the mixed layer in this region is not well constrained. The goal of this study is to determine the interplay between sea ice melt, surface boundary layer forcing, and submesoscale flows in regulating the mixed layer structure in the Antarctic Marginal Ice Zone. High-resolution observations suggest that fine-scale lateral fronts, representative of submesoscale mixed layer eddies (MLEs), arise from mesoscale gradients produced by northwards advecting sea ice meltwater. The strong salinity-driven stratification at the base of the mixed layer confined the MLEs to the upper ocean, limiting submesoscale vertical fluxes across the mixed layer base. This strong stratification prevents the local subduction by submesoscale flow of these modified waters, suggesting that the subduction site that links to the global overturning circulation does not correspond with the location of sea ice melt. However, the presence of MLEs enhanced the magnitude of lateral gradients through stirring and increased the potential for Ekman-driven cross-frontal flow to modulate the stability of the mixed layer and mixed layer properties. The inclusion, particularly of submesoscale Ekman Buoyancy Flux parameterizations, in coupled-climate models, may improve the representation of mixed layer heat and freshwater transport in the ice-impacted Southern Ocean during summer.

In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss—a stark reversal of sea ice expansion and surface cooling trends that prevailed over preceding decades. The most dramatic changes occurred in the austral spring of 2016 when Antarctic sea-ice extent (SIE) reached a record minimum as sea surface temperatures (SST) climbed to a near-record high. In late 2019, another circumpolar surface warming event spanned the Southern Ocean, albeit with no appreciable decline in Antarctic SIE. 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. The latter effect amplified the surface warming effect of air-sea heat fluxes during months of peak solar insolation. More generally, summertime SST across the Southern Ocean is sensitive to the timing of the springtime shoaling of the mixed layer, which is controlled by the strength and temporal variance of the circumpolar westerlies. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annual Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. Thus, future Southern Ocean surface warming extremes will depend on the evolution of regional mixed layer depths and interannual SAM variability.