An Assessment of the Southern Ocean Mixed-Layer Heat Budget

The mixed-layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from June 1, 2002 to May 31, 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer (AMSR-E) sea surface temperature measurements are available. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion and air-sea flux are largely consistent with the evolution of the mixed-layer temperature. The mixed-layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air-sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributors to the surface heat budget error are thought to be the air-sea heat fluxes, as only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. Sensitivity tests suggest that a proper representation of the mixed-layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed-layer depth.