The changes of basal conditions on Fleming Glacier, Antarctic Peninsula, between 2008 and 2021

The Wordie Ice Shelf (WIS) in the Antarctic Peninsula (AP) has continued to retreat since 1966, and it almost completely disintegrated in the late 1990s. Although the main supply glacier of the WIS, the Fleming Glacier (FG), did not respond immediately, increases in the glacier velocity and dynamic thinning have been observed over the past two decades, especially after 2008 when only a small ice shelf remained at the Fleming Glacier front. As FG is now the fastest flowing outlet glaciers in the west Antarctic Peninsula, ice dynamics is the primary cause of mass loss. Basal sliding is the key mechanism for glacier acceleration and as it responds to thinning and changes in basal conditions. Furthermore, changes in ice-ocean interaction, such as changes in buttressing of ice streams and outlet glaciers like Fleming Glacier, are also leading to acceleration.

Here, we use the Shallow Shelf Approximation (SSA) implementation of the Ice-sheet and Sea-level System Model (ISSM) simulating the basal shear stress distribution of FG in the years 2008, 2011, 2014, 2017, 2019 and 2021 using inverse modelling. To better regularize the glaciological inverse problem, we adopt the latest published L-curve analysis to select the optimal regularization level. Considering Fleming Glacier has a relatively small drainage basin, high resolution geometric data is necessary to obtain better constrained information of the basal conditions. We use TanDEM-X DEMs acquired in austral winter of 2011, 2014, 2017, 2019, and 2021 to provide accurate glacier surface elevations. These DEMs were generated from bi-static InSAR data acquired by the TanDEM-X mission and are with the most complete time series and the best quality that can be obtained in this area at present.  We evaluate the existing ice velocity products and performed a spatio-temporal interpolation to obtain the average velocity of the year corresponding to the elevation data. We use the higher Antarctic ice sheet surface mass balance data RACM2.3p2 at 2 km resolution as a boundary condition. Regarding the bedrock topography, one of the main factors restricting the inversion accuracy, we evaluated all the existing subglacial topography data products within our inversions. To more accurately represent friction at the bed, we also tested Budd’s, Weertman’s and Schoof’s sliding laws, with different friction exponents and variable geometric data.

Comparison of simulated basal shear stresses for 2008 and 2021 suggests the migration of the grounding line 8~9 km upstream by 2021 from the 2008 ice front/grounding line positions. This migration is consistent with the change in floating areas deduced from the calculated height above buoyancy. Our results indicate that the reducing basal shear stress may be directly related to the subglacial hydrologic system and lead to rapid increases in basal sliding and ongoing ungrounding. It will further promote the dynamic loss of glaciers when coupled with ocean forcing and retrograde bedrock.