Seasonal land ice-flow variability in the Antarctic Peninsula

Recent satellite-remote sensing studies have documented the multi-decadal acceleration of the Antarctic Ice Sheet in response to rapid rates of concurrent ice-sheet retreat and thinning. Unlike the Greenland Ice Sheet, where historical, high temporal resolution satellite and in situ observations have revealed distinct changes in land ice flow across intra-annual timescales, similar seasonal signals have not previously been observed in Antarctica. Here, we use high spatial and temporal resolution 10 Copernicus Sentinel-1A/B synthetic aperture radar observations acquired between 2014 and 2020 to provide the first evidence for seasonal flow variability of the land ice feeding George VI Ice Shelf (GVIIS), Antarctic Peninsula. Our observations reveal a distinct austral summertime (December – February) speedup of ~0.06 ± 0.005 m d (~22 ± 1.8 m yr) at, and immediately inland of, the grounding line of the glaciers nourishing the ice shelf, which constitutes a mean acceleration of ~15% relative to baseline (timeseries-averaged) rates of flow. These findings are corroborated by independent, optically derived velocity 15 observations. Regional contrasts in the onset of ice-flow acceleration and the overall timing of the speedup events across GVIIS fingerprint oceanic forcing as the primary control of this seasonality. Our findings imply that analogous ice-ocean interactions may be ongoing at the grounding lines of other ocean-vulnerable outlet glaciers around Antarctica. Assessing the degree of seasonal ice-flow variability at such locations is important for quantifying Antarctica’s future contribution to global sea-level rise. 20