The SAM and Southern Ocean SST: Consistency and discrepancy between models and observations across timescales

Despite global warming, the Southern Ocean (SO) has experienced multidecadal surface cooling over recent decades. Previous studies have suggested that the SO SST trends may have been caused by the strengthening of surface westerlies associated with the positive trend of Southern Annular Mode (SAM), in large part forced by stratospheric ozone depletion in the late 20th century. Here we revisit this suggestion and re-examine the relationships between the SAM, surface zonal winds and SO SSTs across timescales, using a linear regression method and a low-frequency component analysis (LFCA). Both approaches show that while summertime SAM positive anomalies (corresponding to surface wind strengthening) can lead to some SST cooling as previously reported, this seasonal-to-interannual SAM modulation makes little contribution to the observed long-term SO SST cooling trends. This is because the fast response of SST to SAM is too weak to persist over a year and the long-term SAM trends and SST trends do not align with each other. Additionally, we show that global climate models are able to capture the observed interannual SAM-SST relationship, and yet generally fail to simulate the recently observed SO SST cooling trends. Our results suggest that the forced SAM trend is unlikely the key cause of the observed multi-decadal SO SST trends, pointing to a limited role of the Antarctic ozone hole in driving the long-term SO SST change via its impact on the SAM.