Investigating the drivers of glacier retreat in West Antarctica using proxy-data assimilation and numerical modeling

Thesis (Ph.D.)--University of Washington, 2023Outlet glaciers in West Antarctica are rapidly retreating and contributing to sea level rise. Ice loss is primarily occurring via wind-driven incursions of warm circumpolar deep water melting the ice shelves that buttress the glaciers. The leading hypothesis is that the current stage of retreat was triggered by a wind-driven change in ocean conditions that occurred in the mid-20th century. However, the short record of observed wind and ocean conditions in this region leaves our knowledge of the mechanisms driving glacier retreat highly uncertain (i.e., what wind/ocean changes occurred, and whether they are characteristic of natural variability or a response to anthropogenic forcing). Here, I use proxy data to reconstruct atmospheric circulation around Antarctica over the full 20th century, revealing century-scale trends and a large westerly event in the 1940s; both are candidates for initiating the current stage of retreat. I investigate the potential for the 1940s westerly event to induce substantial changes in ocean conditions near West Antarctica by using numerical modeling to constrain the sensitivity of ocean circulation to various local wind events. I find evidence that: (1) the circumpolar westerly winds have strengthened over the 20th century, in a zonally asymmetric pattern with the strongest wind trends occurring in the mid-latitude Pacific at approximately 55°S; (2) an unusually large and persistent westerly event occurred near West Antarctica from approximately 1938 to 1942, coinciding with the estimated start of glacier retreat. The results from my numerical ocean modeling experiments show that 5-year wind events with a similar pattern are indeed capable of enhancing the transport of warm circumpolar deep water toward the glaciers, but that the ocean is highly sensitive to the pattern of local wind forcing. The studies in this dissertation demonstrate that using nontraditional data sources from proxy records can provide novel constraints and useful insight on the complex atmosphere-ocean-ice processes occurring in this region. The results reveal several unexpected nuances to relationships widely accepted by the scientific community and highlight several specific lines of research that can further advance our understanding of the historical drivers of glacier retreat in West Antarctica