Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing 1-3 , where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch 4-6 . While brine rejection from sea ice is thought to contribute to the lower branch 7 , the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport 8,9 . Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework 10 to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with di erential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of ∼22 × 10 6 m 3 s −1 . These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.The Southern Ocean State Estimate (SOSE) is an ice/ocean data assimilation produced for the time period January 2005 through December 2010. (See Methods and Supplementary Information for SOSE details and validation.) The bulk freshwater fluxes at the ocean surface south of 50 • S, as estimated by SOSE, are summarized in Fig. 1a. When sea ice forms, nearly all of the salt remains behind in the underlying seawater (a process called brine rejection); when the ice melts, liquid freshwater is returned to the ocean. Direct open-ocean precipitation minus evaporation (0.28 fwSv) and glacial ice melt (0.05 fwSv) are both smaller than net sea-ice melt (0.50 fwSv) and brine rejection (0.36 fwSv). (Bulk freshwater volume fluxes are given in units of freshwater sverdrups 11 , 1 fwSv = 10 6 m 3 freshwater s −1 3.15 × 10 4 Gt freshwater per year.) Melt exceeds brine rejection because sea ice incorporates snowfall at a rate of 0.14 fwSv. Moreover, wind-driven sea-ice transport creates a freshwater conveyor belt from the Antarctic coast to the open ocean 12 , leading to sharp gradients in freshwater flux. The spatial structure of the sea-ice redistribution is assessed in Fig. 1b by comparing the annual mean freshwater flux leaving the atmosphere, land and glaciers (left panel) with that entering the ocean (right panel); the difference is due to sea-ice freshwater redistribution (middle panel, vectors show the ice thickness transport). From the atmosphere, widespread precipitation over the Southern Ocean leads to a broadly distributed downward freshwater flux with a characteristic magnitude of 0.5 m yr −1 , and glacial ice melt provides a stronger freshwater source near the Antarctic coas...

High latitudes present extreme conditions for the measurement and estimation of air-sea and ice fluxes, limiting understanding of related physical processes and feedbacks that are important elements of the Earth's climate. we focus on the exchange of energy, momentum, and material between the ocean and atmosphere and between atmosphere and sea ice (the basic concepts defining surface fluxes are outlined in "Primer: What is an air-sea flux?"). Surface fluxes at high latitudes

The Walin water mass framework quantifies the rate at which water is transformed from one temperature class to another by air-sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air-sea heat fluxes (formation). Walin's framework provides a precise integral statement at the expense of losing spatial information. In this study the integrand of Walin's expression to yield transformation and formation maps is plotted and used to study the role of air-sea heat fluxes in the cycle of formation-destruction of the 188 6 18C layer in the North Atlantic.Using remotely sensed sea surface temperatures and air-sea heat flux estimates based on both analyzed meteorological fields and ocean data-model syntheses for the 3-yr period from 2004 to 2006, the authors find that Eighteen Degree Water (EDW) is formed by air-sea heat fluxes in the western part of the subtropical gyre, just south of the Gulf Stream. The formation rate peaks in February when the EDW layer is thickened by convection owing to buoyancy loss. EDW is destroyed by air-sea heat fluxes from spring to summer over the entire subtropical gyre. In the annual mean there is net EDW formation in the west to the south of the Gulf Stream, and net destruction over the eastern part of the gyre. Results suggest that annual mean formation rates of EDW associated with air-sea fluxes are in the range from 3 to 5 Sv (Sv [ 10 6 m 3 s 21). Finally, error estimates are computed from sea surface temperature and heat flux data using an ensemble perturbation method. The transformation/formation patterns are found to be robust and errors mostly affect integral quantities.

International audienceSubantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air-sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air-sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air-sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air-sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air-sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air-sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss