Oceanic Response to Changes in the Latitude of the Southern Hemisphere Subpolar Westerly Winds

Abstract

The oceanic response to imposed changes in the latitude of the subpolar westerly winds (SWWs) over the Southern Ocean is assessed in a global ocean model. The latitude changes are achieved by applying a zonally uniform zonal wind stress anomaly that is quasi-sinusoidal in latitude, with a positive (negative) band to the south (north) of about 50°S. This form of anomaly is chosen because it projects onto the Antarctic Oscillation, also known as the Southern Hemisphere annular mode, that is known to have a long-term trend. The response to both long-term trend and quasi-decadal periodic changes is examined in the latitude of the SWWs. In the long-term trend case, a 5.4° poleward shift of the SWWs over a 100-yr simulation is found to cause the poleward heat transport to increase by an average of 25% between 50°S and the equator. This change is primarily due to greater northward Ekman transport of cold water and its associated cooling of Subantarctic Mode Water (SAMW) by up to 0.5°C in the central-south Pacific. The authors also find that the rate of formation of Antarctic Intermediate Water increases as the SWWs shift poleward, resulting in cooling and freshening at intermediate depths. In the periodic experiment, where the SWWs axis has a range of 5.4° latitude, the poleward heat transport, North Atlantic Deep Water outflow and the overturning of Antarctic Bottom Water are all modulated by 20%?30%. Significant cooling is found at intermediate and upper-level water depths in the trend experiment and temperature fluctuations with a range of up to 0.4°C in the periodic experiment. These changes are of the same magnitude and form as that recently observed at intermediate depths in the Southern Ocean. The authors conclude that latitudinal shifts of the SWWs may play a significant role in generating observed temperature fluctuations at intermediate water depths.