On the mechanisms of late 20th century sea-surface temperature trends over the Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) plays an important role in global climate. Examination of climate model simulations shows that they match the observed late 20th century sea-surface temperature (SST) trends averaged over the ACC region quite well, despite underestimating the observed surface-wind increases. Motivated by recent theories of ACC response to variable wind and radiative forcing, we used two idealized models to assess contributions of various dynamical processes to the SST evolution in the region. In particular, a high-resolution channel model of ACC responds to increasing winds by net surface ACC warming due to enhanced mesoscale turbulence and associated heat transports in the mixed layer. These fluxes, modeled via increased diapycnic surface diffusivity in a coarse-resolution hybrid coupled model, partially offset zonally non-uniform surface cooling dominated by Ekman-transport anomalies. We argue that the combination of these opposing effects must be accounted for when estimating climate sensitivity in the ACC region.