Enhanced West Antarctic ice loss triggered by polynya response to meridional winds

Abstract

West Antarctic outlet glaciers are in a stage of rapid ice loss, modulated by wind-driven exposure to warm ocean water. Rapid ice loss probably began in the mid-twentieth century and is often attributed to warmer ocean conditions near the ice shelves driven by strengthening westerly winds at the continental shelf break. This westerly wind trend is a feature of some historical climate simulations but is not supported by proxy (for example, ice-core, tree-ring) observations. Here we present an ensemble of regional ocean simulations and proxy-constrained climate reconstructions and show that shelf-break westerlies are a poor indicator of ocean conditions near the ice shelves. Instead, cumulative northerly wind anomalies close coastal polynyas (open-water regions), driving anomalous warming and freshening near the ice shelves, increasing ice-shelf melting. The increased meltwater leads to strengthening of the undercurrent that supplies warm water, further increasing melting. Our results highlight the importance of local northerly winds and associated sea ice changes for ice-shelf melting in West Antarctica. Proxy reconstructions show a significant historical northerly wind trend in this region, providing the atmospheric forcing that can explain the initiation of West Antarctic glacier retreat in the mid-twentieth century. Ocean simulations and proxy-constrained climate reconstructions suggest that the rapid retreat of West Antarctic outlet glaciers was initiated by local northerly wind trends over the twentieth century.