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

Shun Tsutaki,Koji Fujita,Jiro Komori,Phuntsho Tshering,Akiko Sakai,Takayuki Nuimura,Shin Sugiyama

doi:10.5194/tc-13-2733-2019

Abstract

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 Bhutanese Himalaya, where a number of proglacial lakes exist. We acquired in situ and satellite-based observations across lake- and land-terminating debris-covered glaciers in the Lunana region, Bhutanese Himalaya. A repeated differential global positioning system survey reveals that thickness change of the debris-covered ablation area of the lake-terminating Lugge Glacier (-4.67±0.07 m a−1) is more than 3 times more negative than that of the land-terminating Thorthormi Glacier (-1.40±0.07 m a−1) for the 2004–2011 period. The surface flow velocities decrease down-glacier along Thorthormi Glacier, whereas they increase from the upper part of the ablation area to the terminus of Lugge Glacier. Numerical experiments using a two-dimensional ice flow model demonstrate that the rapid thinning of Lugge Glacier is driven by both a negative surface mass balance and dynamically induced ice thinning. However, the thinning of Thorthormi Glacier is minimised by a longitudinally compressive flow regime. Multiple supraglacial ponds on Thorthormi Glacier have been expanding since 2000 and have merged into a single proglacial lake, with the glacier terminus detaching from its terminal moraine in 2011. Numerical experiments suggest that the thinning of Thorthormi Glacier will accelerate with continued proglacial lake development.

Highlights

The spatially heterogeneous shrinkage of Himalayan glaciers has been revealed by in situ measurements (Yao et al, 2012; Azam et al, 2018), satellite-based observations (Bolch et al, 2012; Kääb et al, 2012; Brun et al, 2017), mass balance and climate models (Fujita and Nuimura, 2011; Mölg et al, 2014), and a compilation of multiple methods (Cogley, 2016)
The mean zs/ t values of Thorthormi Glacier derived from the differential GPS (DGPS)-digital elevation models (DEMs) (−1.40± 0.27 m a−1) and ASTER-DEMs (−1.61 ± 2.75 m a−1) from 2004 to 2011 are comparable with previous measurements, which range from −3 to 0 m a−1 for the 2002–2004 period (Naito et al, 2012)
We performed numerical experiments to investigate the contributions of surface mass balance (SMB) and ice dynamics in relation to the observed ice thinning

Summary

Introduction

The spatially heterogeneous shrinkage of Himalayan glaciers has been revealed by in situ measurements (Yao et al, 2012; Azam et al, 2018), satellite-based observations (Bolch et al, 2012; Kääb et al, 2012; Brun et al, 2017), mass balance and climate models (Fujita and Nuimura, 2011; Mölg et al, 2014), and a compilation of multiple methods (Cogley, 2016). The glacier area loss in Bhutan was 13.3 ± 0.1 % between 1990 and 2010, based on repeated decadal glacier inventories (Bajracharya et al, 2014). Multitemporal digital elevation models (DEMs) revealed that the glacier-wide mass balance of Bhutanese glaciers was −0.17 ± 0.05 m w.e. a−1 during 1974–2006 (Maurer et al, 2016) and −0.22 ± 0.12 m w.e. a−1 during 1999–2010 (Gardelle et al, 2013).

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