Land- to lake-terminating transition triggers dynamic thinning of a Bhutanese glacier

Abstract. There have been rapid increases in both the number and expansion of the proglacial lakes across High Mountain Asia. However, the relationship between proglacial lakes and glacier dynamics remains unclear in the Himalayan region. Here we present the surface elevation, flow velocity changes, and proglacial lake expansion of Thorthormi and Lugge glaciers in the Lunana region, Bhutanese Himalaya, during the 2000–2018 period using photogrammetry and GPS survey data. The lake expansion and surface lowering rates, and flow velocity field of Lugge Glacier, a lake-terminating glacier, have remained approximately constant since 2000. Conversely, there has been accelerated proglacial lake expansion and a two-fold increase in the thinning rate of Thorthormi Glacier since 2011, as well as a considerable speed-up in the flow velocity field (> 150 m a−1). We reveal that the lake formation and transition of Thorthormi Glacier from a land- to lake-terminating glacier have triggered glacier speed-up and rapid thinning via a positive (compressive) to negative (extensional) change in the emergence velocities. This study provides the first evidence of dynamic glacier changes that are associated with proglacial lake formation across the Himalayan region.