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Melt rates of West Antarctic ice shelves in the Amundsen Sea track large decadal variations in the volume of warm water at their outlets. This variability is generally attributed to wind‐driven variations in warm water transport toward ice shelves. Inspired by conceptual representations of the global overturning circulation, we introduce a simple model for the evolution of the thermocline, which caps the warm water layer at the ice‐shelf front. This model demonstrates that interannual variations in coastal polynya buoyancy forcing can generate large decadal‐scale thermocline depth variations, even when the supply of warm water from the shelf‐break is fixed. The modeled variability involves transitions between bistable high and low melt regimes, enabled by feedbacks between basal melt rates and ice front stratification strength. Our simple model captures observed variations in near‐coast thermocline depth and stratification strength, and poses an alternative mechanism for warm water volume changes to wind‐driven theories. Plain Language Summary: Ice loss from the West Antarctic Ice Sheet contributes significantly to current and projected rates of global sea‐level rise. The ice sheet is primarily losing mass via glaciers that flow from the Antarctic continent into the Amundsen Sea, where floating ice shelves are exposed to much warmer ocean waters than elsewhere around Antarctica. In this work we present a simplified mathematical model for the volume of warm water at Amundsen Sea ice shelf fronts that reproduces observed patterns of warm water variability. The modeled variability relies on interactions between ice shelf melt and coastal polynyas, regions where enhanced wintertime sea‐ice production can trigger mixing that diverts heat carried by warm waters away from the ice shelf and into the atmosphere. Higher melt rates inhibit polynya convection, allowing more warm water into the ice shelf cavity and reinforcing a high melt state, whilst lower melt rates facilitate polynya convection, diverting heat away from the ice shelf and reinforcing a low melt state. Interannual variations in polynya sea‐ice production trigger shifts between these reinforcing states. Our results promote the importance of coastal processes in explaining observed variations in Amundsen Sea ice shelf melt, which have previously been attributed to remote wind patterns. Key Points: Rates of ocean‐driven Amundsen Sea ice shelf melt respond to variations in warm water transport to the coast and modification at the coastA simple Amundsen Sea continental shelf overturning model, based on water mass transformation, reveals bistable high and low melt regimesFeedbacks between glacial melt and polynya convection are central to the bistability and produce variability consistent with observations [ABSTRACT FROM AUTHOR]

In cold polar waters, temperatures sometimes drop below the freezing point, a process referred to as supercooling. However, observational challenges in polar regions limit our understanding of the spatial and temporal extent of this phenomenon. We here provide observational evidence that supercooled waters are much more widespread in the seasonally ice‐covered Southern Ocean than previously reported. In 5.8% of all analyzed hydrographic profiles south of 55°S, we find temperatures below the surface freezing point ("potential" supercooling), and half of these have temperatures below the local freezing point ("in situ" supercooling). Their occurrence doubles when neglecting measurement uncertainties. We attribute deep coastal‐ocean supercooling to melting of Antarctic ice shelves and surface‐induced supercooling in the seasonal sea‐ice region to wintertime sea‐ice formation. The latter supercooling type can extend down to the permanent pycnocline due to convective sinking plumes—an important mechanism for vertical tracer transport and water‐mass structure in the polar ocean. Plain Language Summary: Ocean water, which contains about 34 grams of salt per kilogram of seawater, generally freezes around −1.85°C. However, seawater can be cooled to even lower temperatures without turning into ice. This phenomenon is called supercooling. Supercooled water is found in the polar oceans, typically in regions where the ocean is in contact with ice, as is the case for the enormous seasonal sea‐ice region around Antarctica. But collecting measurements in this region under the thick ice cover during the dark and cold Antarctic winter is challenging. Here, we supplement rather sparse traditional ship‐based observations with data collected by autonomous floats and instrumented marine mammals to detect and analyze where, when, and how supercooled seawater forms in the Southern Ocean. We find widespread supercooling related to melting floating glaciers (ice shelves) along the Antarctic coast and sea‐ice formation. Our analysis enables us to detect sinking supercooled plumes from sea‐ice formation, which may be important for cooling the deep ocean and transporting constituents such as carbon, nutrients, or oxygen from the ocean's surface to deeper layers. Key Points: Potential and in situ supercooling occurs in large parts of the Southern Ocean seasonal sea‐ice zoneDeep coastal supercooling from below 100 m to the ocean bottom is associated with melting ice shelves and dense shelf water formationShallow supercooling is associated with sea‐ice formation and can penetrate as deep as the permanent pycnocline [ABSTRACT FROM AUTHOR]