Localized Acceleration of the Antarctic Circumpolar Current

The influence of wind forcing on the strength of the Antarctic Circumpolar Current (ACC) is explored using realistic eddying simulations. The ACC transport is found to be set by both the zonal wind stress, at latitudes of Drake Passage, and by the curl of the wind stress, localized in the western South Atlantic. As such, for certain wind stress patterns, the ACC transport is dramatically increased, without any increase in the mean zonal wind stress. The mechanism by which local wind curl variations effect the strength of the ACC is as follows: In the western South Atlantic the ACC meanders over the shallowest topo- graphic features of its circumpolar path. In this region a positive wind curl induces vortex compression. This compression helps the ACC to move equator-ward and over the shallow topography. This interaction reduces the momentum transfer to the bottom for a given ACC transport. The total ACC transport then increases so that the momentum imparted at the sea surface is transferred to the sea floor. Local wind curl variations in the western South Atlantic are found to be far more effective at driving year to year fluctuations in ACC transport than the zonal wind stress. In addition, changes both in the pattern and in the magnitude of the Southern Hemisphere westerlies, concurrent with observed climate change, are likely to have both had an accelerating influence on the ACC of equivalent magnitude