Linking open-ocean polynyas and deep convection in the Southern Ocean across CMIP6 models

Abstract. Open-ocean polynyas (OOPs) and deep convection in the Southern Ocean are critical features of the global climate system, however, their representation and mutual dependence in climate models remain poorly understood. This study investigates the occurrence and coupling of OOPs and deep convection across 49 CMIP6 models using long-term pre-industrial control simulations. Our results reveal that while most models simulate both phenomena, their spatial and temporal co-occurrence varies substantially. Although deep convection is typically associated with surface salinification and heat loss, it does not always result in detectable polynyas. We identify two distinct regimes of OOPs across the ensemble: "deep OOPs", which are directly coupled to deep ocean convection, and "shallow OOPs", which form independently of deep mixing, likely driven by surface forcing or sea-ice divergence. The representation of these regimes is strongly influenced by the choice of ocean model component. These findings highlight the importance of process-based diagnostics in evaluating Southern Ocean overturning and suggest that the connection between surface polynyas and deep water formation is more complex than traditionally assumed in climate models.