Driven by the increase in greenhouse gas emissions, the global surface atmospheric temperature (SAT) has been rising at a rate of 0.1K per decade with faster warming in the polar regions. Particularly, owing to the Arctic sea ice melting and the difference in land-sea distribution between the North Hemisphere and the South Hemisphere, the warming of the Arctic is several times larger than that of the Antarctic as seen from observations and model simulations. Previous studies focused on the weaker warming in the Antarctic region have emphasized that the suppression of Antarctic warming is due to the oceanic northward energy transport by the Antarctic Circumpolar Circulation (ACC). However, few studies have attempted to reveal the root of asymmetric surface warming between the Arctic and the Antarctic.

  This study is aimed at revealing the source of bipolar asymmetric warming. We perturb the ocean circulations by filling the Drake Passage (DP) with land, which slows down the ACC and the Atlantic Meridional Overturning Circulation (AMOC). Two distinct basic climate states, the DP_open and DP_closed states, are established by the fully coupled Community Earth System Model (CESM). Further experiments driven by an abrupt CO₂ doubling show that compared to the DP_open case, the bipolar asymmetric warming is amplified in the case of DP_closed with significantly stronger Arctic warming.

  The amplified bipolar asymmetric warming (partially) originates in the distinct climatology when the DP is closed. Relative to the DP_open case, there is a climatological warmer Antarctic with less sea ice cover but a cold Arctic with more sea ice cover, resulted from weaker ACC and AMOC in the DP_closed case. The distinct background states regulate the amplified asymmetric warming in four ways. First, more sea ice melts in the Arctic than in the Antarctic, which contributes to stronger warming in the Arctic through the ice feedback. Secondly, when the DP is closed, the AMOC weakens less significantly, reducing the cooling in the warming hole in the North Atlantic. Thirdly, stronger increases in water vapor transports and cloud formation in the Arctic further contribute to the warming asymmetry. Lastly, the weakened northward oceanic energy transport due to ACC cutoff does not contribute to the warming asymmetry significantly.