Slowing down of the summer Southern Hemisphere Annular Mode trend against the background of ozone recovery

Abstract

Observations show significant trends in Southern Hemisphere extratropical climate in the late 20th century, including a strong positive trend in the austral summer Southern Hemisphere Annular Mode (SAM) accompanied by warming of the Antarctic Peninsula and melting sea ice in the Bellingshausen Sea. Statistical analysis and model simulations have shown that these trends were driven mainly by Antarctic stratospheric ozone depletion. Here, results show that the widely reported summer SAM trend has flattened since around the year 2001 against the background of the ozone recovery, supporting results from previous model simulations that predicted a slowing down of the well-documented positive summer SAM trend. Four SAM indices based on different definitions from different datasets show consistency in this slowdown. Furthermore, changes in surface air temperature (SAT) in the Antarctic and sea-ice concentration (SIC) around the Antarctic are detected. Different from the SAM, in which the signs of trends only slow down but do not reverse after the ozone recovery, the signs of trends in Antarctic SAT and SIC have reversed. The warming of the Antarctic Peninsula has turned into a cooling trend, and the melting of sea ice in the Bellingshausen Sea has turned into an increasing trend. Additional diagnostics studies with observational and model data could go a long way towards enhancing our understanding of changes in Southern Hemisphere surface climate against the background of ozone recovery.摘要20世纪末期, 南半球热带外地区经历了显著的气候变化, 包括夏季南半球环状模(SAM)的显著上升趋势, 伴随着南极半岛的增暖和别林斯高晋海的海冰融化. 这些趋势主要是由20世纪末期南极平流层臭氧消耗所驱动的. 本文发现, 自2001年左右以来, 在南极平流层臭氧恢复的背景下, 观测到的夏季SAM的上升趋势已经趋于平缓, 验证了前人利用数值模拟预测的夏季SAM上升趋势减缓现象. 与SAM在臭氧恢复后趋势只减缓但没有逆转不同, 南极地表气温和海冰的趋势发生了逆转. 南极半岛由变暖趋势转为降温趋势, 别林斯高晋海域的海冰由融化趋势转为增多趋势.