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

West Antarctic outlet glaciers are in a stage of rapid retreat, modulated by wind-driven exposure to warm Circumpolar Deep Water (CDW). Retreat likely began in the mid-20th century, and is often attributed to increased CDW volume near the ice shelves forced by strengthening westerly winds over the continental shelf break. This westerly wind trend is a feature of some historical climate simulations but is not supported by proxy 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, driving anomalous warming and freshening near the ice shelves, increasing ice-shelf melting. The increased meltwater leads to strengthening of the undercurrent that supplies CDW, further enhancing ice-shelf melting. Our results highlight the importance of local northerly winds and associated sea ice changes on ice-shelf melting in West Antarctica. Proxy reconstructions show a significant historical northerly wind trend in this region (an extension of Amundsen Sea Low deepening), providing the atmospheric forcing that can explain the initiation of West Antarctic glacier retreat during the mid-20th century.