Zonal variations in the Southern Ocean heat budget

The three-dimensional structure of the upper ocean heat budget in the Antarctic Circumpolar Current (ACC) is investigated using the 1/6°, data-assimilating Southern Ocean State Estimate (SOSE) for 2005-2010. Air-sea heat flux from reanalyses and SOSE shows a largely unexamined zonally asymmetric pattern of ocean heat gain in the Indian and Atlantic sectors and ocean heat loss in the Pacific sector of the ACC. In the Atlantic and Indian sectors of the ACC, the surface ocean heat gain is primarily balanced by cooling by divergence of equatorward Ekman temperature transport driven by westerly winds. In the Pacific sector, surface ocean heat loss and cooling due to divergence of Ekman temperature transport are balanced by warming by divergence of geostrophic temperature transport, and a comparison shows this is similar to the dominant heat balance found in the subtropical Agulhas Return Current. Divergence of horizontal and vertical eddy advection of temperature are important for warming the upper ocean in localized regions downstream of major topographic features, while the mean divergence of vertical temperature advection is generally a weak cooling term. Our results show that topographic steering and zonal asymmetry in air-sea exchange lead to substantial zonal asymmetries in the upper ocean heat budget which are masked in the zonally-averaged picture. Determining how ocean dynamics affect spatial patterns of surface heat fluxes is a key step to understanding the upper cell of the overturning circulation and the response of the overturning to future changes in buoyancy forcing.