Thwaites Eastern Ice Shelf Cavity Observations Reveal Multi-year Sea Ice Dynamics and Deep-Water Warming in Pine Island Bay, West Antarctica [Preprint]

Pine Island Bay, situated in the Amundsen Sea, is renowned for its retreating ice shelves and sea ice variability. Brine rejection from sea ice formation and glacial meltwater exported from ice-shelf cavities impact seawater density and thus regional ocean circulation. While the effects of brine rejection on the continental shelf are relatively well documented, little is known about its effects on water subsequently circulating beneath floating ice shelves. Here, we present insights from oceanographic instruments deployed via boreholes into the ocean cavity beneath the Thwaites Eastern Ice Shelf (TEIS) from 2020 to 2023. These observations reveal warming and thickening of the modified Circumpolar Deep Water (mCDW) layer near the seabed since January 2020. Concurrently, multi-year sea ice anchored along the coastline has retreated over 150 km to the calving fronts of Pine Island and Thwaites Glaciers, leading to increased Winter Water advection and a cooling of over 1˚C in the upper 250 m below TEIS between July 2021 and January 2023. The causal link between sea ice dynamics and changing hydrographic properties in the subshelf cavity is supported by distinct events lasting several weeks during periods of mobile sea ice coverage. During these events, mid-depth waters temporarily warm and increase in salinity, leading to an increase in density, while deeper mCDW simultaneously cools and becomes fresher, reducing its density. These observations are important for refining process models and enhancing the accuracy of basal melt-rate parametrizations for coupled iceocean modelling.

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