Asymmetric Arctic and Antarctic Warming and Its Intermodel Spread in CMIP6

Abstract

Abstract Under the background of global warming, the Arctic region has warmed faster than the Antarctic, which is referred to as asymmetric Arctic and Antarctic warming. The new generation of model simulations from the CMIP6 offers an opportunity to identify the major factors contributing to the asymmetric warming and its intermodel spread. In this study, the preindustrial and abrupt-4 × CO2 experiments from 18 CMIP6 models are examined to extract the asymmetric warming and its intermodel spread. A climate feedback-response analysis method is applied to reveal the contributions of external and internal feedback processes to the asymmetric warming and its intermodel spread, by decomposing total warming into the partial temperature changes caused by individual factors. It is found that a seasonal energy transfer mechanism (SETM) dominates in both polar warmings. The direct consequence of the sea ice declining in response to the anthropogenic forcing is an increase in the effective heat capacity of the ocean surface layer. Such an increase in the effective heat capacity temporally withholds most of the extra solar energy absorbed during summer and then releases it during winter, contributing to stronger warming in winter. However, the background oceanic circulation in the Southern Ocean, namely, the Antarctic Circumpolar Current, continually transports energy equatorward, resulting in a suppressed SETM and surface warming in the Antarctic. The key factor that accounts for intermodel spread in the asymmetric warming is the difference in their strengths of SETM. The poleward atmospheric transport and water vapor feedback also contribute to the intermodel spread. Significance Statement The asymmetric Arctic and Antarctic warming, as a response to the increase in CO2, can regulate global atmospheric and oceanic circulations via meridional temperature gradient. Previous studies have all ascribed the key role of the ocean in the asymmetric warming over the two poles with a lack of comprehensive understanding of the roles of other feedback processes. This study emphasizes that oceanic circulation is the root of the asymmetric warming via suppressing sea ice retreat and the associated SETM in the Antarctic, instead of increasing ocean heat uptake and equatorward transport only. For the intermodel spread in the asymmetric warming over two poles, the differences in the strength of all processes involving the SETM among models are the prominent issue.