Rain-induced speed up events of a temperate surging glacier, AK, inferred from GPS measurements

Abstract. Temperate surge-type glaciers undergo periodic order-of-magnitude increases in flow speeds generally attributed to drainage system evolution, making them ideal sites to explore how the kinematic response of glaciers to rain events varies with subglacial drainage system efficiency. Here, we compare ten GPS-derived glacier surface speed records with modeled rainfall to assess short-term evolutions of the drainage system at the end of a surge (2020–2021) and during quiescent flow (2022–2024) at Sít’ Kusá, Alaska. Speed anomalies were identified using a short-term to long-term average ratio approach and all speed anomalies that followed large rain events were characterized by their duration, magnitude, and immediacy. During the surge, 9 of 16 large rain events (56%) caused speed anomalies, peaking at 7.5 m/d (116% increase) and lasting 66.5 hours on average. During quiescence, rain-induced speed-ups were less frequent (17 of 37 rain events (46%)), smaller (~1.5 m/d), shorter (~33 hours), but greater in percent increase (251%). Although the differences in speed characteristics generally indicate that the drainage system is more efficient during quiescence, speed-ups throughout the surge cycle suggest there is a persistently inefficient component of the drainage system. We find that particularly large and prolonged rain events can cause temporary slow-downs during the surge, but that speeds return to pre-event levels in less than two weeks as the drainage system reverts to its inefficient state. Our work supports recent analyses of glacier instabilities suggesting there isn’t a binary switch in drainage system configuration during surges. More broadly, our findings expand our understanding of glacier kinematic responses to rainfall, demonstrating that rain events can modulate speeds of both fast- and slow-flowing glaciers.