Response of Southern Hemisphere Western Boundary Current Regions to Future Zonally Symmetric and Asymmetric Atmospheric Changes.

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. We find that the zonally asymmetric component of atmospheric forcing, characterized by a pattern that is consistent across CMIP6 models, can explain more than 30% (>2°C) of the sea surface temperature (SST) warming found in the Tasman Sea and southern Australia region and a sizable fraction of warming in the Agulhas Current region. These changes in SST in both the Indian and Pacific basins are found to be primarily driven by increases in the advection of warm tropical water to the mid‐latitudes due to changes in the large‐scale subtropical ocean gyres, which in turn can largely be explained by changes in the mid‐latitude surface wind stress patterns. Plain Language Summary: Strong ocean currents are found on the western side of the ocean basins, which flow from the tropics toward the poles in both hemispheres. These western boundary currents (WBCs) have shown strong changes in the last few decades, which have resulted in amplified ocean warming in the poleward extensions of these boundary currents; these changes are projected to amplify further in the future. In the Southern Hemisphere, recent changes in the WBCs are thought to have been primarily driven by changes in the surface westerly winds that encircle Antarctica. These westerly winds are generally considered to be changing uniformly in all three ocean basins; however, recent studies have shown that there are strong regional variations both historically and in future projections. Here, we find that regional asymmetries in wind projections can account for about 30% of the projected warming in the Tasman Sea and the southern Australia region and a sizable fraction of warming in the Agulhas Current region. This amplified warming in the Indian and the Pacific Ocean basins is primarily driven by changes in ocean circulation. Key Points: Strong warming in the subtropical western boundary current extension regions is projected in the futureZonally asymmetric atmospheric changes can explain >30% of future warming in the south Australia region, driven by ocean circulation changesUnderstanding historical and future ocean changes require the use of regionally varying and not simply zonally symmetric wind forcing [ABSTRACT FROM AUTHOR]