OS-4b - Sea ice modeling and prediction: Sea ice process understanding Climate models struggle to capture observed Antarctic sea-ice extent trends, which has critical consequences for sea-ice projections. To tackle this, it is often suggested to develop models at high spatial resolution. In this study we focus on the September sea-ice extent in prototype simulations for the “High Resolution Model Intercomparison Project” (HighResMIP). Scenario simulations (RCP8.5) are performed with the AWI Climate Model (AWI-CM) at “high-resolution” in both ocean and atmosphere (HR) and “low-resolution” (LR) under the same protocol. Compared to LR, HR predicts a much weaker warming signal around Antarctica in austral winter for the end of this century (2070-2099) relative to 1976-2005. This may partly be explained by the different sea-ice mean states in LR and HR. However, we also observe increased extent variability and strongly reduced rates of sea-ice decrease. Specifically, HR shows polynya-like features in the Weddell Sea in the pre-satellite era, followed by a stable September sea-ice extent until ~2050. “Mixed-resolution” runs (HR ocean with LR atmosphere, and vice versa) attribute this difference in trends and variability to the HR ocean, independent of the atmospheric resolution. Considering the observed stable Antarctic September sea-ice extent and the recent return of the Weddell polynya, we will discuss possible common mechanisms and implications for other modelling centres participating in HighResMIP.