Dissertation Defense: Ratnakar Gadi

Title: Comprehensive analysis of the melt regime in Ice Grounding Zones (IGZ)

Abstract: This dissertation presents a comprehensive study of the melt regime within the Ice Grounding Zone (IGZ) of two different glaciers: Petermann Gletscher, a marine-terminating glacier in Northwest Greenland, and Thwaites Glacier, a glacier in West Antarctica. We use a combination of modeled melt rates, satellite data-derived melt maps, and in-situ data wherever available to examine the melt regime within the IGZs. The findings highlight the importance of seawater intrusions within the IGZ, challenging the traditional modeling approach of no melt at the grounding line and semi-fixed grounding line. On Petermann Gletscher, we observe grounding line migrations of several kilometers during the tidal cycle from satellite radar interferometry data, regularly bringing pressurized, subsurface, warm ocean waters in contact with grounded ice. Using the Massachusetts Institute of Technology general circulation model in two dimensions, we estimate the ice melt rates as a function of ice grounding zone length and ocean thermal forcing. The model estimates higher ice melt rates in the grounding zone cavity than anywhere else in the ice shelf cavity. The modeled melt rates increase sub-linearly with the ice grounding zone length and ocean thermal forcing. Our model results agree well with remote sensing estimates of ice melt. The high basal ice melt rates in tidally-flushed ice grounding zones imply that marine-terminating glaciers are more sensitive to ocean thermal forcing than anticipated, increasing their projected contribution to sea level rise. At Thwaites Glacier, Antarctica, we also observe kilometer-scale seawater intrusions underneath the grounded ice using satellite data, where the magnitude of ice basal melt is not known. We present high-resolution simulations of its melt regime using the MITgcm ocean model. The model predicts low melt rates beneath Thwaites Eastern Ice Shelf (~10m/yr), consistent with observational data, but high melt rates (~100m/yr) beneath unexplored Thwaites Glacier Tongue responsible for 80\% of ice flux. Our modeled melt rates agree well with remote sensing estimates. The findings call for a major revision of the representation of IGZ in ice sheet models, which will increase their sensitivity to ocean warming and possibly double the risk of rapid sea level rise from Thwaites. The research presented here contributes to a deeper understanding of the melting regime in the IGZ and emphasizes the necessity for refining modeling approaches that account for the complex ice-ocean interaction within the IGZ. These insights are crucial for improving sea level rise projections and assessing the broader impacts of climate change on polar ice sheets.