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Abstract The Antarctic continental shelf (ACS) hosts processes that impact the climate system globally, which has motivated ongoing efforts to characterize its state, circulation, and variability. However, the nature and consequences of eddies over the ACS, and their contributions to the budgets of heat and freshwater, remain systematically understudied. This study uses hydrographic measurements collected from instrumented seals, supported by a high‐resolution model of the southern Weddell Sea, to characterize eddies and their role in vertical heat transport around the entire ACS. A key finding is that eddies are ubiquitous, and exhibit frequent (2%–10% of hydrographic casts) occurrences of bulk Richardson numbers, indicative of submesoscale variability. However, along‐track density power spectra exhibit wavenumber dependences of , consistent with quasigeostrophic turbulence. Approximately of the points in the surface mixed layer satisfy conditions favorable for symmetric instability, although its prevalence is likely higher than this due to the relatively coarse resolution of the seal tracks. Vertical heat transports, estimated from a regional model‐calibrated parameterization of submesoscale restratification, are largest in shelf regions hosting dense water, which have previously been identified as key sites of warm water intrusions onto the ACS. These regions also exhibit the largest seasonal cycles, with elevated winter eddy activity and heat fluxes accompanying the formation of high salinity shelf waters. These findings indicate that eddies may contribute substantially to ACS heat and tracer budgets, and motivate further study of their role in determining the pathways and fate of heat that intrudes onto the ACS. 

Southern Ocean air–sea fluxes are a critical component of the climate system but are historically undersampled due to the remoteness of the region. While much focus has been placed on interannual flux variability, it has become increasingly clear that high-frequency fluctuations, driven by processes like storms and (sub-)mesoscale eddies, play a nonnegligible role in longer-term changes. Therefore, collecting high-resolution in situ flux observations is crucial to better understand the dynamics operating at these scales, as well as their larger-scale impacts. Technological advancements, including the development of new uncrewed surface vehicles, provide the opportunity to increase sampling at small scales. However, determining where and when to deploy such vehicles is not trivial. This study, conceived by the Air–Sea Fluxes working group of the Southern Ocean Observing System, aims to characterize the statistics of high-frequency air–sea flux variability. Using statistical analyses of atmospheric reanalysis data, numerical model output, and mooring observations, we show that there are regional and seasonal variations in the magnitude and sign of storm- and eddy-driven air–sea flux anomalies, which can help guide the planning of field campaigns and deployment of uncrewed surface vehicles in the Southern Ocean. 

Abstract West Antarctic Ice Sheet mass loss is a major source of uncertainty in sea level projections. The primary driver of this melting is oceanic heat from Circumpolar Deep Water originating offshore in the Antarctic Circumpolar Current. Yet, in assessing melt variability, open ocean processes have received considerably less attention than those governing cross-shelf exchange. Here, we use Lagrangian particle release experiments in an ocean model to investigate the pathways by which Circumpolar Deep Water moves toward the continental shelf across the Pacific sector of the Southern Ocean. We show that Ross Gyre expansion, linked to wind and sea ice variability, increases poleward heat transport along the gyre’s eastern limb and the relative fraction of transport toward the Amundsen Sea. Ross Gyre variability, therefore, influences oceanic heat supply toward the West Antarctic continental slope. Understanding remote controls on basal melt is necessary to predict the ice sheet response to anthropogenic forcing. 

Abstract Despite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high‐latitude Southern Ocean. This is due in part to a historical paucity of observations in this remote, turbulent, and seasonally ice‐covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly climatology of the mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different zones of the Southern Ocean—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon‐rich waters at the base of the mixed layer supplies DIC which is, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern Antarctic Circumpolar Current (ACC), which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind‐driven transport of DIC from the SIZ. These results stem from the first observation‐based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region. 

Abstract The Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelf‐iron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixed‐layer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone. 

Abstract The spring bloom in the Southern Ocean is the rapid‐growth phase of the seasonal cycle in phytoplankton. Many previous studies have characterized the spring bloom using chlorophyll estimates from satellite ocean color observations. Assumptions regarding the chlorophyll‐to‐carbon ratio within phytoplankton and vertical structure of biogeochemical variables lead to uncertainty in satellite‐based estimates of phytoplankton carbon biomass. Here, we revisit the characterizations of the bloom using optical backscatter from biogeochemical floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling and Southern Ocean and Climate Field Studies with Innovative Tools projects. In particular, by providing a three‐dimensional view of the seasonal cycle, we are able to identify basin‐wide bloom characteristics corresponding to physical features; biomass is low in Ekman downwelling regions north of the Antarctic Circumpolar Current region and high within and south of the Antarctic Circumpolar Current.