The advent of under-ice profiling float and biologging techniques has enabled year-round observation of the Southern Ocean and its Antarctic margin. These under-ice data are often overlooked in widely used oceanographic datasets, despite their importance in understanding the seasonality and its role in sea ice changes, bottom water formation, and glacial melt. We develop a four-dimensional climatology of the Southern Ocean (south of 40°S and above 2,000 m) using Data Interpolating Variational Analysis, which excels in multi-dimensional interpolation and consistent handling of topography and advection. The climatology captures thermohaline variability under sea ice, previously hard to obtain, and outperforms other products in data fidelity with smaller root-mean-square errors and biases. Our dataset will be instrumental for investigating seasonality and for improving ocean models. This work further highlights the quantitative significance of under-ice data in reproducing ocean conditions, advocating for their increased use to achieve a better Southern Ocean observing system.

The advent of under-ice profiling float and biologging techniques has enabled year-round observation of the Southern Ocean and its Antarctic margin. These under-ice data are often overlooked in widely used oceanographic datasets, despite their importance in understanding seasonality and its role in sea ice changes, water mass formation, and glacial melt. We develop a monthly climatology of the Southern Ocean (south of 40°S and above 2,000 m) using Data Interpolating Variational Analysis, which excels in multi-dimensional interpolation and consistent handling of topography and horizontal advection. The climatology successfully captures thermohaline variability under sea ice, previously hard to obtain, and outperforms other observation-based products and state estimate simulations in data fidelity, with smaller root-mean-square errors and biases. To demonstrate its multi-purpose capability, we present a qualitative description of the seasonal variation, including 1) the surface mixed layer, 2) the water mass volume census, 3) the Antarctic Slope Front, and 4) shelf bottom waters. Particularly, the circumpolar variation in the extent of dense shelf water and the annual volume overturning rate of water masses are revealed for the first time. The present work offers a new monthly climatology of the Southern Ocean and the Antarctic margin, which will be instrumental in investigating the seasonality and improving ocean models, thereby making valuable winter observations more accessible. We further highlight the quantitative significance of under-ice data in reproducing ocean conditions, advocating for their increased use to achieve a better Southern Ocean observing system.

Warm, salty Circumpolar Deep Water (CDW) is recognized as the primary driver for Antarctic glacial melt, but the mechanism by which it reaches the continental shelves remains highly uncertain from an observational standpoint. With the scarcity of eddy flux estimation in the Antarctic margin, we quantify the isopycnal diffusivity of CDW using hydrographic variability and satellite altimetry under the mixing length framework. For comparison, the spiciness and thickness are used as isopycnal tracers, and the two tracers yield qualitatively similar estimates. Over the Antarctic Circumpolar Current (ACC), spatial variation of mixing length is generally aligned with the jet-induced mixing suppression theory, including its exception in the lee of the topography. In contrast, the mixing length does not depend on the mean flow in the subpolar zone, likely reflecting the relatively quiescent flow regime. The estimated isopycnal diffusivity ranges from 100 to 500 m2 s-1 south of the ACC. The eddy diffusivity tends to be enhanced where the gradient of isopycnal thickness becomes small and CDW intrudes onshore. The cross-slope eddy CDW flux is estimated, and the associated onshore heat flux across is calculated as ~3.6 TW in the eastern Indian sector. The eddy heat flux and coastal solar heating are generally balanced with cryospheric heat sinks including glacial melting and surface freezing, suggesting that the eddy advection is substantial for the onshore CDW flux. The thickness field is essential for determining mixing length and eddy fluxes in the subpolar zone, whereas the situation does not hold for the ACC domain.

Warm, salty Circumpolar Deep Water (CDW) has long been regarded as the climatological driver for Antarctica, but the mechanism of how it can reach the continental shelf remains unsettled. Motivated by the absence of observational eddy flux estimation in the Antarctic margin, we quantify isopycnal diffusivity of CDW by hydrographic records and satellite altimetry under the mixing length framework. For comparison, spiciness and thickness are used as the isopycnal tracer. Over the extent of the Antarctic Circumpolar Current (ACC), we find a general agreement with the mixing suppression theory and its exception in the lee of the topography as previously reported. In contrast, mixing length does not depend on mean flow to the pole, reflecting a stagnant flow regime in the Antarctic margin. Estimated isopycnal diffusivity ranges 100-500 m2 s-1 to the south of the ACC. Eddy diffusion is likely enhanced where the CDW intrusion is localized by the recirculating gyres, primarily attributable to the small gradient of isopycnal thickness. Volume transport is then estimated by the layer thickness gradient. Associated onshore heat flux across the continental slope by CDW is calculated as ~3.6 TW and ~1.2 TW in the eastern and western Indian sectors, respectively. The estimates are quantitatively consistent with cryospheric heat sinks by sea ice formation and ice shelf basal melt, suggesting that the isopycnal eddy diffusion is the leading cause of the onshore CDW intrusion. We emphasize that the thickness field is essential for determining the eddy fluxes in the Antarctic margin.

To clarify the impact of basal melting of the Antarctic ice sheet and biological productivity on biogeochemical processes in Antarctic coastal waters, concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA), inorganic nutrients, chlorophyll a, and stable oxygen isotopic ratios (δ18O) were measured from the offshore slope to the ice front of the Totten Ice Shelf (TIS) during the spring/summer of 2018, 2019, and 2020. Off the TIS, modified Circumpolar Deep Water (mCDW) intruded onto the continental shelf and flowed along bathymetric troughs into the TIS cavity, where it met the ice shelf base and formed a buoyant mixture with glacial meltwater. Physical oceanographic processes mostly determined the distributions of DIC, TA, and nutrient concentrations. However, DIC, TA, and nutrient concentrations on the surface of the ice front were decreased by photosynthesis and the dilution effect of meltwater from sea ice and the base of the ice shelf. The partial pressure of CO2 (pCO2) in surface water was reduced by photosynthesis and dilution, and the surface water became a strong CO2 sink for the atmosphere. The DIC and TA (normalized to salinity of 34.3 to correct for dilution effects) changed in a molar ratio of 106:16 because of phytoplankton photosynthesis. The decrease of pCO2 by more than 100 μatm with respect to mCDW was thus the result of photosynthesis. The nutrient consumption ratio suggested that enough iron was present in the water column to supply the surface layer via buoyancy-driven upwelling and basal melting of the TIS.