Interannual flow dynamics driven by frontal retreat of a lake-terminating glacier in the Chinese Central Himalaya

Abstract

Most lake-terminating glaciers in the Himalaya retreat rapidly due to periodic frontal ice loss at their terminus, but long-term observations are still limited regarding their flow dynamics, which is crucial for understanding the processes of ice mass loss and proglacial lake growth. We present multi-decadal surface velocity dynamics of the Longbasaba Glacier, a rapid retreating lake-terminating glacier in the Chinese Himalaya, using an image feature tracking method applied on optical satellite images between 1989 and 2018. We show that, in companion with rapid retreat (−51.7 m a−1), its lower 5 km tongue experienced high interannual fluctuations in velocity, comprising periodic acceleration and slowdown in 1989-1995 and 2001-2010 and a recent remarkable acceleration since 2012. The temporal variation of longitudinal velocity distribution indicates an upward propagation of the lake-ward acceleration (namely a downglacier inversion of strain from compression to extension). This propagation is coupled to the retreat of the glacier front and occurs along the lowermost 1∼1.5 km lake-adjacent section as the proglacial lake expands. The most recent acceleration of the near-lake section since 2012 has likely facilitated a dynamic thinning on its upper sections, where flow acceleration started two years later in 2014. This pattern contrasts markedly with a nearby decelerating land-terminating glacier, which has experienced a much slower retreat rate (−7.8 m a−1) and the same magnitude of mean thinning rate at its lower part since 2000. Our results confirm the strong influence of the proglacial lake on ice flow dynamics and suggest that lake-ice interactions are important to consider when analyzing, interpreting or modeling dynamics of rapidly retreating lake-terminating glaciers in the Himalayas as well as around the world.