Evolution of driving and resistive stresses throughout the recent surge cycle of Sít' Kusá, Lingít Aaní, from surface velocity and elevation observations

We present a comprehensive record of glacier motion observations during the quiescent and active phases of the most recent 2013-2021 surge cycle of Sít' Kusá (also known as Turner Glacier), located along the southeast Alaskan coast. Records of Sít' Kusá's previous surges indicate that it has the shortest observed surge repeat interval in the world, approximately 6 years. Time series of glacier surface elevation, derived from WorldView DSMs and airborne LiDAR, and surface velocity, from offset-tracking on Sentinel-1, Sentinel-2, Landsat 8, and PlanetScope image pairs, indicate that the northern tributary of the glacier remains active throughout the quiescent period, rapidly transferring mass over a steep icefall into the reservoir zone in seasonal pulses (Fig. 1). During quiescence, the reservoir zone (~7-24 km from the terminus) thickens at rates of up to 100 m a-1 while the receiving zone (0-7 km from the terminus) thins at rates of up to 25 m a-1, rapidly steepening the surface slope of the glacier along the longitudinal profile in these two zones. The most recent surge initiated in the winter of 2019-2020 in the reservoir zone and the surge front propagated down-glacier at a rate of ~50 km a-1 (137 m/d) until it reached ~7 km from the terminus, after which down-glacier propagation slowed to ~25 km a-1 (69 m/d). Here we present an analysis of the evolution of driving and resistive stresses and their influence on the surge initiation, surge propagation, and the change in surge propagation speed. The increase in the longitudinal surface elevation gradient during quiescence produces an increase in driving stress, promoting seasonal pulses of mass movement down-glacier that extend further into the reservoir zone as the glacier nears surge initiation. The pre-surge pulses start in late fall and reach their most down-glacier extent in late spring to summer, typically lasting more than 6 months (Fig. 1B). The seasonality in the pulse initiation and peak suggest that they are triggered by seasonal reductions in basal resistance related to seasonal change in basal water pressure. The surge initiates in winter 2019-2020 after a critical increase in the ratio between driving stress and resistive stress. Then, the surge front propagates down-glacier due to increased compressive longitudinal stress behind it. The change in surge propagation speed ~7 km from the terminus aligns with the inferred along-flow thinning from 7 km to the terminus. The analysis of detailed time series of stress and strain throughout Sít' Kusá's surge cycle will improve the understanding of the interplay of forces that trigger and propagate flow instabilities on glaciers.