Response of Southern Hemisphere western boundary current regions to
future zonally symmetric and asymmetric atmospheric changes
Abstract
Subtropical Western Boundary Currents (WBCs) are often associated with
hotspots of global warming, with certain WBC extension regions warming
3-4 times faster than the global mean. In the Southern Hemisphere strong
warming over the WBC extensions has been observed over the last few
decades, with enhanced warming projected into the future. This amplified
warming has primarily been linked to poleward intensification of the
mid-latitude westerly winds in the Southern Hemisphere. Changes in these
winds are often thought of as being zonally symmetric, however, recent
studies show that they contain strong zonal asymmetries in certain ocean
basins. The importance of these zonal asymmetries for the Southern Ocean
has not yet been investigated. In this study, we use an ocean-sea-ice
model forced by prescribed atmospheric fields to quantify the
contribution of projected zonally asymmetric atmospheric changes in
generating future ocean warming and circulation changes in the
subtropical WBC regions of the Southern Hemisphere. We find that the
projected zonally asymmetric component of atmospheric change can explain
more than 30% (>2°C) of the SST warming found in the
Tasman Sea and southern Australia region and a sizeable fraction of
warming in the Agulhas Current region. These changes in SST in both the
Indian and Pacific Ocean basins are found to be primarily driven by
changes in the large-scale subtropical ocean gyres, which in turn can
largely be explained by changes in the surface wind stress patterns.