Reversing seasonality of Southern Hemisphere Annular Mode trend in the 21st century

Abstract

The Southern Hemisphere Annular Mode (SAM) is the dominant mode of atmospheric circulation in the Southern Hemisphere extratropical region. In the late 20th century, the SAM underwent a remarkable positive trend, with the strongest trend occurring in austral summer because of Antarctic ozone depletion. In this study, we quantitatively explore changes in the seasonality of the SAM trend under the background of ozone recovery. As expected, accompanied by ozone recovery since 2001, the summer SAM trend has decreased, thus reducing the seasonal difference in the SAM trend between summer and winter. However, what was not expected and found in this study is that the winter SAM has meanwhile shown an increasing positive trend in the early 21st century, which further reverses the seasonality of the SAM trend, leading to a stronger trend in winter rather than summer, which is different from the seasonality in the late 20th century. The reason underlying the winter SAM changes is explored from the perspective of natural variability. Winter SAM exhibits clear multidecadal variability, and the recent increasing trend is a part of multidecadal oscillation. The multidecadal variability of winter SAM may partly originate from the Atlantic Multidecadal Oscillation. The seasonality of linear trend in extratropical surface climate also exhibits reversing characteristics since 2001. In the early 21st century, the acceleration of westerlies and the strengthening of Amundsen Sea Low are stronger in winter, reversing the previous seasonality in the late 20th century, in which the acceleration of westerlies and the strengthening of Amundsen Sea Low is stronger in summer.

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