Contrasting thinning patterns between lake- and land-terminating glaciers in the Bhutan Himalaya

Abstract. Despite the importance of glacial lake development in ice dynamics and glacier thinning, in situ and satellite based measurements from lake-terminating glaciers are sparse in the Bhutan Himalaya, where a number of supraglacial lakes exist. To better understand the influences of glacial lake formation and expansion on ice dynamics and glacier thinning, we acquired in situ and satellite based observations across lake- and land-terminating debris-covered glaciers in the Lunana region, Bhutan Himalaya. A repeat differential GPS survey reveals that thinning of the debris-covered ablation area of the lake-terminating Lugge Glacier for the 2004–2011 period (−4.67 ± 0.02 m a−1) is more than three times greater than that of the land-terminating Thorthormi Glacier (−1.40 ± 0.02 m a−1). The surface flow velocity decreases down-glacier along Thorthormi Glacier, whereas it increases from the upper part of ablation area to the terminus of Lugge Glacier. Numerical experiments with a two-dimensional ice flow model demonstrate that the rapid thinning of Lugge Glacier is driven primarily by a negative surface mass balance and that the dynamically induced change in ice thickness is small. However, the thinning of Thorthormi Glacier is suppressed by a longitudinally compressive flow regime. The magnitude of dynamic ice thickening more than offsets the glacier thinning, suggesting that over half of the negative surface mass balance is counterbalanced by the ice dynamics of Thorthormi Glacier. Multiple ponds on Thorthormi Glacier have been expanding since 2000 and merged into a single proglacial lake, with the glacier terminus detaching from its terminal moraine in 2011. Numerical experiments suggest that the speed up and thinning of Thorthormi Glacier will be accelerated with continued proglacial lake development.