Exploring the dynamics of Southern Hemisphere westerlies using abrupt warming and cooling experiments

The variations of Southern Hemisphere westerlies (SHW) influence ocean circulation and the global carbon cycle, and thus, play a crucial role in global climate. Earlier studies showed the SHW undergoes poleward-intensifying under anthropogenic climate change. However, it is controversial whether such a trend is driven by purely atmospheric processes or a coupled ocean-atmosphere response. Here, we investigate the dynamical mechanisms of SHW changes by providing three different scenarios under abrupt rising, long-term stabilizing, and abrupt decreasing CO2 forcing based on three numerical experiments. The results indicate that under abrupt rising CO2 forcing, the SHW strengthens and shifts poleward. As CO2 stabilizes, the SHW's intensity weakens but remains stronger than the cold climate. Its position remains at a higher latitude. Entering a cooling climate, forced by abruptly decreasing CO2, the intensity of SHW rapidly reduces, displaying a tendency for equatorward reversal. By comparing the behavior of SHW under the same atmospheric CO2 forcing, the contribution of ocean dynamics to SHW can be identified. Our results reveal that the location of SHW is aligned with the position of the oceanic meridional temperature gradient (MTG), while the intensity of SHW is coupled to the ocean-perturbed meridional heat imbalance. Both depend on the fast and slow response times of the subtropical and subpolar oceans to CO2 forcing, modulated by the downwelling and upwelling nature of these ocean regions. Our results reveal that the response of SHW to CO2 forcing depends on the background state of the Southern Ocean. This provides insights into the evolution of SHW.