Impact of Weakened Antarctic Circumpolar Current on the Northern Hemisphere Climate

Recent findings show a remarkable linkage between the Northern Hemisphere and Southern Hemisphere climates. Previous studies have focused on the impact of the climate change in the northern high-latitudes on that in the Southern Hemisphere, but few studies concerned the impact of Southern Ocean circulation on the Northern Hemisphere, especially the Arctic climate. In this study, we close the Drake Passage (DP) to slow down the Antarctic Circumpolar Circulation (ACC) in the fully coupled Community Earth System Model, to investigate the impact of weakened ACC on the Northern Hemisphere.

Two model experiments, DP opened and DP closed experiments, are performed. Relative to the DP opened case, a warmer Antarctic with less sea ice cover but a colder Arctic with more sea ice cover appear in the DP closed case resulting from weaker ACC and Atlantic Meridional Overturning Circulation (AMOC). Especially, the changes in surface air temperature in the two poles are largest in winter.

Compared to the DP opened case, the anomalous southward heat transport by weakened ACC is largest in winter, contributing to the winter amplification in the Antarctic. However, the seasonal difference in AMOC change is insignificant. To understand the winter amplification in the Arctic, we further analyze local surface heat flux changes in the Arctic. The anomalous downward longwave radiation and sensible and latent heat fluxes are stored in the ocean in summer and released to the atmosphere in the following winter. Although the ocean heat content warms the surface, the upward sensible and latent heat fluxes cool the surface more significantly in winter. This local atmosphere-ocean-ice interaction contributes to the winter amplification in the Arctic. 

When DP is closed, the westerlies become stronger and move poleward in the Northern Hemisphere because of the increased meridional temperature gradients, especially in winter. The change in surface temperature also contribute to the weakening of Aleutian Low in winter. The warming in the Antarctic and the cooling in the Arctic leads to the notable weakening of Hadley circulation in the Southern Hemisphere. Additionally, compared to the DP opened case, the Intertropical Convergence Zone shifts southward and the Walker circulation and trade winds over the Pacific strengthen. These results shed light on understanding the interhemispheric interaction and the pole-to-pole connection.