Drivers and rarity of the strong 1940s westerly wind event in the Amundsen Sea, West Antarctica

Ice loss in the Amundsen Sea Embayment is occurring primarily via exposure to warm ocean water, which varies in response to local wind variability. There is evidence that glacier retreat in this region was initiated in the mid-20th century, however the perturbation that may have triggered retreat remains unknown, leaving the climatic mechanisms driving retreat highly uncertain. A leading hypothesis is that a large atmospheric circulation anomaly in the Amundsen Sea occurred in the 1940s, driving a strong oceanic ice-shelf melting perturbation. However, the characteristics and drivers of this 1940s event remain poorly constrained, and the expected occurrence of such events in a natural climate has not yet been evaluated. We investigate this hypothesis using paleoclimate reconstructions and climate model simulations. The reconstructions show that a large multi-year westerly wind anomaly occurred from ~1938-1942, likely as a combined response to the very large El Niño event from 1940-1942 and variability sourced from outside the tropical Pacific starting years earlier. In climate model simulations we find evidence that events of similar magnitude and duration are unusual but may have occurred tens to hundreds of times throughout the Holocene. Our results suggest that the strong westerly event in the 1940s is unlikely to be exceptional enough to initiate glacier retreat on its own; naturally driven climatic/oceanic trends preceding the event or perhaps anthropogenically driven trends following the event are needed to explain retreat. Our analyses provide novel constraints on the significance of the 1940s westerly event in the Amundsen Sea and highlight outstanding uncertainties in our understanding of the mechanisms driving glacier retreat.