Drivers of model-observation discrepancies in Southern Ocean SSTs and Antarctic sea ice trends*

Observed Southern Ocean sea surface temperatures (SSTs) cooled and Antarctic sea ice expanded from 1979 through 2015. However, climate models have largely been unable to reproduce this behavior. Here we examine the contribution of observed wind variability and meltwater from the Antarctic ice sheet and ice shelves to SST and Antarctic sea ice trends in climate model experiments. The free-running, CMIP6-class GISS-E2.1-G climate model can simulate regional cooling and neutral sea ice trends due to internal variability, but they are unlikely. Constraining the model to observed winds and meltwater fluxes from 1990 through 2021 gives SST variability and trends consistent with observations. We find that meltwater contributes more to the SST trend than winds, and winds contribute more to the sea ice trend than meltwater. However, while the constrained model captures much of the observed sea ice variability, discrepancies in sea ice trends remain.