Basal channel outflow inferred from persistent polynya variability at the Eastern Thwaites Ice Shelf

Enhanced localized basal melt rates can incise ice-shelf basal channels that structurally weaken ice shelves. Many of these channels convey water sourced from upstream locations or the grounding zone to the ice-shelf edge. This outflow often includes residually warm ocean water with enough remaining heat to maintain locally sea ice free ocean conditions, seasonally or year-round, for small areas near the ice front, which we term 'persistent polynyas'. The evolution of persistent polynyas provides insight into processes driving ice-shelf thinning and ice-shelf stability. However, direct in situ outflow measurements are sparse, and few studies to date have explored the use of remote sensing for this task. Here we used Landsat 8 optical and thermal infrared imagery from 2013 to 2021 to assess year-round characteristics and variability of the persistent polynya located along Thwaites Eastern Ice Shelf. We designed a semi-supervised image segmentation algorithm that automates the detection and characterization of the persistent polynya, capturing area, location, and sea surface temperature retrievals across the time series. Over the eight-year record, the polynya appeared every year, indicating active basal channel outflow. The polynya exhibited consistent seasonal variability, generally appearing in austral spring, expanding to its maximum extent in December or January, and disappearing in summer or fall. However, the timespan over which the polynya was open each year ranged from as little as 1 month to more than 4 months, and its maximum areal extent varied by more than two orders of magnitude. This wide range of variability suggests there may be large seasonal and interannual fluctuations in outflow heat content, ocean surface stratification, local calving, and/or sea ice thicknesses, which may all interact to moderate polynya formation. We suggest that years with greater polynya longevity and extent in the first half of the record may indicate enhanced ice-shelf basal melt rates, and therefore, greater surface outflow and heat transport to the ocean surface. Our findings highlight that these small windows into the sub-ice-shelf environment may provide rich and critical insight into ice-ocean interactions occurring within basal channel systems and impacting whole ice-shelf stability.