Southern Ocean summer warming is regulated by storm-driven mixing

The Southern Ocean absorbs most of the excess heat resulting from climate change. However, climate projections show a persistent warm summer bias in its sea surface temperatures, indicating a limited understanding of the air–sea heat exchange mechanisms governing this region. Here we examine the impact of storms on the interannual variability of Southern Ocean surface temperatures during summer using in situ observations from underwater and surface robotic vehicles, climate reanalyses and satellite data. We show that synoptic-scale storms regulate summer sea surface temperatures through alteration of the effective heat capacity of the mixed layer and the entrainment of colder water from below. Storms reduce the summer ocean heat gain by limiting solar radiation reaching the surface. This effect is partially offset by a reduction in heat loss due to turbulent air–sea exchange. We also find that interannual variations in sea surface temperature during summer in the Southern Ocean are driven by changes in storm-mean wind speeds, which are linked to the Southern Annular Mode. Our results demonstrate a causal link between storm forcing and sea surface temperature variability, which is critical for reducing warming biases in climate models and improving future climate projections.