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Recent Multi‐Decadal Southern Ocean Surface Cooling Unlikely Caused by Southern Annular Mode Trends
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Recent Multi‐Decadal Southern Ocean Surface Cooling Unlikely Caused by Southern Annular Mode Trends
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Journal Article
Recent Multi‐Decadal Southern Ocean Surface Cooling Unlikely Caused by Southern Annular Mode Trends
2023
Overview
Over recent decades, the Southern Ocean (SO) has experienced multi‐decadal surface cooling despite global warming. Earlier studies have proposed that recent SO cooling has been caused by the strengthening of surface westerlies associated with a positive trend of the Southern Annular Mode (SAM) forced by ozone depletion. Here we revisit this hypothesis by examining the relationships between the SAM, zonal winds and SO sea‐surface temperature (SST). Applying a low‐frequency component analysis to observations, we show that while positive SAM anomalies can induce SST cooling as previously found, this seasonal‐to‐interannual modulation makes only a small contribution to the observed long‐term SO cooling. Global climate models well capture the observed interannual SAM‐SST relationship, and yet generally fail to simulate the observed multi‐decadal SO cooling. The forced SAM trend in recent decades is thus unlikely the main cause of the observed SO cooling, pointing to a limited role of the Antarctic ozone hole. Plain Language Summary Despite increasing greenhouse gases, the Southern Ocean sea‐surface temperatures have cooled over the recent several decades. The cause of Southern Ocean cooling remains a puzzling feature of recent climate change. Earlier studies have proposed that this multi‐decadal cooling in the Southern Ocean has arisen in part from the strengthening of surface winds associated with a positive trend in a mode of climate variability known as the Southern Annular Mode (SAM). Here we employ a new statistical method to examine this proposed relationship in both observations and climate models. We found that SAM variability only changes Southern Ocean surface temperature on short‐term timescales and makes little contribution to observed long‐term trends. Our results thus suggest the SAM trend, via the strengthening of circumpolar westerlies, is unlikely to be the main cause of the observed long‐term Southern Ocean cooling. Key Points Austral summer Southern Annular Mode (SAM) anomalies affect Southern Ocean (SO) sea‐surface temperature (SST) only on seasonal to interannual timescales Multi‐decadal observed SAM trends make little contribution to observed Southern Ocean SST trends Global climate models (GCMs) capture the observed seasonal SAM‐SST relationship and yet fail to simulate the observed long‐term SO cooling
