Zonally Asymmetric Increase in Southern Ocean Heat Content

Abstract A zonally symmetric perspective has proven to be very useful in studying the key role of the Southern Ocean for global ocean heat uptake. Despite this, reconstructions of changes in Southern Ocean heat content ΔOHC over the past few decades have revealed substantial deviations from symmetry. Here, we investigate the zonal asymmetry of ΔOHC and the processes driving it for the period 1979–2019 using an eddy-permitting ocean model forced by atmospheric reanalysis (ERA5). We find that the model successfully reproduces the observed zonal asymmetry in ΔOHC and that it is primarily due to zonal asymmetries in changing surface fluxes. Zonal asymmetries in mean ocean circulation play an important secondary role. Factorial simulations are used to attribute the asymmetry in ΔOHC to the different surface flux components (wind stress, heat, and freshwater fluxes), revealing strong regional differences. North of the Antarctic Circumpolar Current (ACC), we find roughly equal contributions from changes in wind stress and surface heat flux. Within the ACC, all three forcings play an important role, while south of it only wind stress and freshwater flux changes are important. Our findings argue for full consideration of the three-dimensional circulation to understand heat uptake and storage in the Southern Ocean, especially when considering multidecadal time scales where natural variations are as important as the long-term warming trend.