Southern Ocean polynyas and dense water formation in a high-resolution, coupled Earth system model

Antarctic coastal polynyas produce dense shelf water, a primary source of Antarctic Bottom Water that contributes to the global overturningcirculation. This paper investigates Antarctic dense water formation in the high-resolution version of the Energy Exascale Earth System Model(E3SM-HR). The model is able to reproduce the main Antarctic coastal polynyas, although the polynyas are smaller in area compared toobservations. E3SM-HR also simulates several occurrences of open-ocean polynyas (OOPs) in the Weddell Sea at a higher rate than what the last50 years of the satellite sea ice observational record suggests, but similarly to other high-resolution Earth system model simulations. Furthermore,the densest water masses in the model are formed within the OOPs rather than on the continental shelf as is typically observed. Biases related tothe lack of dense water formation on the continental shelf are associated with overly strong atmospheric polar easterlies, which lead to a strongAntarctic Slope Front and too little exchange between on- and off-continental shelf water masses. Strong polar easterlies also produce excessivesouthward Ekman transport, causing a build-up of sea ice over the continental shelf and enhanced ice melting in the summer season. This, in turn,produces water masses on the continental shelf that are overly fresh and less dense relative to observations. Our results indicate that highresolution alone is insufficient for models to properly reproduce Antarctic dense water; the large-scale polar atmospheric circulation aroundAntarctica must also be accurately simulated.