Impacts of the Drake Passage on the Atmospheric Circulation and Their Seasonal Differences

Historical observations and model studies reveal a weaker and slower warming in the Antarctic than in the Arctic. Previous works have contributed the weaker Antarctic warming to the strong westerlies and Antarctic Circumpolar Circulation (ACC), as eastward current could reduce the heat transport from the tropic to the Antarctic through anomalous Ekman northward heat transport. To reveal the role of ACC in the climate system, we cut off the ACC by closing the Drake Passage (DP) in the fully coupled Community Earth System Model (CESM). While previous studies have been focused on the climatological oceanic changes with DP closure, this study investigates the impacts of the DP on the climatological atmospheric circulation and their seasonal differences.

Two experiments, DP opened experiment and DP closed experiment, are performed with the CESM. Relative to the DP opened case, a warmer Antarctic with less sea ice cover but a colder Arctic with more sea ice cover in the DP closed case, resulted from weaker ACC and Atlantic Meridional Overturning Circulation. Especially, evident seasonal differences appear in the atmospheric changes, with the largest surface air temperature (SAT) change in winter, while there are smaller seasonal differences in ocean circulation changes.

Compared to the DP opened case, the anomalous southward heat transport by a weakened ACC leads to an increase in ocean heat content in the Antarctic in the DP closed case, causing less sea ice in summer. Hence, more solar radiation is absorbed and stored in the ocean. As the atmosphere is a heat source in summer, the increase in summer SAT is limited in the Antarctic. The stored energy, together with that from anomalous Ekman southward transport, is released to the atmosphere in the following winter, causing an increase in upward surface heat flux. Consequently, the Antarctic sea ice concentration decreases and the SAT increases more significantly in winter than in other seasons, accompanied with enhanced surface pressure. The mechanism for the Arctic climate change is similar with the Antarctic warming, but changes in an opposite direction.

When DP is closed, the westerlies in the Southern Hemisphere weaken as the meridional temperature gradients decrease, especially in winter. Conversely, the Hadley circulation in the Southern Hemisphere weakens more significantly in summer, which may result from the greater reduction in meridional temperature gradients in the low latitudes which weakens the subtropical high. In the Northern Hemisphere, because of the increased meridional temperature gradients, the westerlies become stronger and move poleward. 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.