Storms regulate Southern Ocean summer warming

Sea surface temperature (SST) in the Southern Ocean is the fingerprint of ocean heat uptake ^1,2 and critical for air-sea interactions. However, Southern Ocean SST is biased warm in climate models, reflecting our limited understanding of the mechanisms that set its magnitude and variability ^3,4 . An important factor driving SST variability is synoptic-scale weather systems, such as storms ^3,5 , yet their impacts are difficult to directly observe. Using in-situ observations from underwater and surface robotic vehicles in the subpolar Southern Ocean, we show evidence that storms regulate the summer evolution of SST through altering the mixed layer effective heat capacity and entraining colder water from below. Through these mechanisms, we determine that interannual variations in Southern Ocean SST reflect changes in storm intensity and prevalence, which, in turn, are driven by the Southern Annular Mode. Our results demonstrate a causal link between storm forcing and SST variability, which has implications for addressing warming biases in climate models.