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Abstract

Mass loss from the West Antarctic Ice Sheet is currently dominated by discharge through ice shelves exposed to warm modified Circumpolar Deep Waters (mCDW). Basal melt rates in these regions track decadal scale oscillations in the depth of the thermocline separating mCDW from cooler near-surface waters. Prior work has attributed the observed thermocline depth variability to wind-driven variations in shoreward mCDW transport. Here, we show that transformation of mCDW in coastal polynyas also impacts mCDW thickness variability, and we compare the importance of this mechanism to shelf break wind forcing.We introduce a simple overturning circulation model for the continental shelf that distinguishes between thermocline depth variability associated with mCDW supply and mCDW transformation. This model indicates that surface buoyancy fluxes within coastal polynyas can generate large decadal variations in thermocline depth in the absence of variable mCDW inflow, posing an alternative mechanism for observed mCDW thickness variability. The modeled variability takes the form of transitions between bistable high and low melt regimes, made possible by feedbacks between basal ice melt and stratification at the ice front. Our simple model can explain both the magnitude of observed thermocline depth variations and their lagged correlation with stratification strength, which are not fully accounted for in work focussed on wind forcing. The work suggests a previously underestimated role for coastal polynya dynamics in setting the melt rates of West Antarctic ice shelves and demonstrates the utility of framing the transport of ocean heat towards ice shelves in terms of an overturning circulation.