The Key Role of the Southern Annular Mode During the Seasonal Sea Ice Maximum in Recent Antarctic Sea Ice Loss

Southern Hemisphere sea ice area (SH SIA) exhibited weak increases from the early 1980s until 2015 when it abruptly dropped, setting record low values in 2017, 2022, and 2023. The reasons for the rapid declines in SH SIA remain open to debate, with potential explanations ranging from changes in tropical Pacific climate, warming of the high latitude subsurface ocean, and contemporaneous variations in the extratropical atmospheric circulation. Here we provide novel insights into the role of the extratropical atmospheric circulation in driving year-to-year and long-term changes in Antarctic sea ice, with a focus on the influence of the Southern annular mode (SAM) on recent trends in SH sea ice area. The influence of the SAM on SH SIA exhibits a more pronounced seasonal variation than that indicated in previous work: during the annual sea ice minimum, anomalous circumpolar westerlies associated with the positive polarity of the SAM lead to increases in SH SIA that persistent for several months. In contrast, during the annual sea ice maximum, anomalous circumpolar westerlies associated with the positive polarity of the SAM lead to pronounced decreases in Antarctic sea ice that persist for up to a year. In terms of annual-mean SH SIA, by far the largest impacts arise from variations in the atmospheric circulation during the sea ice maximum. As a result, changes in the SAM during the sea ice maximum have had a marked impact on long-term changes in SH SIA. These linkages are robust in both observationally constrained data products and modeled data, with additional results exploring how this relationship changes as the mean state of the climate changes under global warming.