Interannual flow dynamics driven by frontal retreat of a lake-terminating glacier in the Chinese Central Himalaya
Introduction
The Central Himalaya holds the largest glacier area and ice volume compared to the western and eastern parts of the Himalayan arc (Bolch et al., 2012). Observations document a notable mass loss of glaciers here during past decades, even though they cover large areas in high altitudes (Kääb et al., 2012). Among all glaciers, lake-terminating glaciers, which are widely distributed in the Central Himalaya, have shown rapid retreat and thinning (Gardelle et al., 2013; Brun et al., 2019) combined with acceleration of ice transport (King et al., 2018; Zhang et al., 2019). This suggests that these glaciers show a peculiar behavior (King et al., 2018; Brun et al., 2019; Zhang et al., 2019) compared to the common thinning-induced slowdown of most land-terminating glaciers in High Mountain Asia (Dehecq et al., 2019). The rapid retreat of most lake-terminating glaciers is accompanied by the expansion of proglacial lakes (Nie et al., 2017), of which some pose notable risks to Himalayan mountain communities from glacier lake outburst floods (GLOFs) (Nie et al., 2018) and may lead to trans-boundary hazard impacts in certain regions of Nepal, Bhutan and Chinese Tibet (Richardson and Reynolds, 2000).
When a glacier terminates into a sufficiently deep-water body, ice flow velocity near the terminus will be increased due to enhanced sliding. This is caused by reduced effective pressure (defined as ice overburden minus subglacial water pressure), since the subglacial drainage system is connected to the lake hydraulics (Warren et al., 2001; Benn et al., 2007; Sugiyama et al., 2011). Terminus acceleration can weaken the usually compressive flow in the glacier tongue area and thus probably induces dynamic thinning of the glacier (Nuimura et al., 2012; King et al., 2018). The glacier tongue in contact with the lake is immediately subject to high basal melt rates during summer and therefore thinning (Truffer and Motyka, 2016), which in turn promotes buoyancy-driven destabilization and calving of the floating ice (Warren et al., 2001; Benn and Åström, 2018). Glaciers that terminate into lakes often experience mass loss by calving, which might be an important component of the overall mass loss. Recent studies have highlighted that glaciers with calving fronts have shown generally higher mass loss rates (Nuimura et al., 2012; Maurer et al., 2016; King et al., 2017). Mass balance modeling of a lake-terminating glacier in Canada (Chernos et al., 2016) showed that calving loss accounts for 10 to 25% of the total mass balance averaged over multiple summers and may increase up to 49% during some exceptional seasons.
The wide distribution and increasing number of lake-terminating glaciers concentrated in the Central Himalaya indicates that fast lake growth and high frontal calving loss not only intensify the mass loss from lake-terminating glaciers, but also increase the potential of serious risks related to GLOFs in this region due to higher lake volumes. Detailed information of frontal mass loss processes and involved ice flow dynamics are therefore important to accurately predict the responses of lake-terminating glaciers and the proglacial lakes to future climate forcing. Previous research investigated decadal or multi-decadal changes of area, ice thickness and velocity for some lake-terminating glaciers in the Himalaya (Kääb, 2005; Quincey et al., 2009; Basnett et al., 2013; King et al., 2017, King et al., 2018; Zhang et al., 2019). Some studies focused on the heterogenic geometry response to climatic change compared to land-terminating glaciers, or on the thermal conditions and evolution of proglacial lakes (Chikita et al., 2000; Fujita et al., 2009; Sakai et al., 2009; Somos-Valenzuela et al., 2014; Thakuri et al., 2016; Haritashya et al., 2018). More recently, Watson et al. (2020) documented recent calving events in a single year of three lake-terminating glaciers in the Himalaya in detail and estimated their mass loss contributions. However, so far continuous and long-term observations of interannual ice flow dynamics are still limited, even though such investigations are crucial for understanding the processes of lake-ice interaction and the subsequent ice mass loss.
In this study, we use multi-decadal remote sensing data to investigate the dynamics of Longbasaba Glacier (Randolph Glacier Inventory ID: RGI60-15.10434), a rapidly retreating lake-terminating glacier in Chinese Central Himalaya (north of Mt. Kanchenjunga). The study area is located at the triple junction border between Nepal, China and India (Sikkim) (Fig. 1), where hosts a high concentration of moraine-dammed proglacial lakes, most of which experienced rapid expansion during past decades (Wang et al., 2012; Racoviteanu et al., 2015; Nie et al., 2017). We present annual glacier length variations combined with biennially/annually surface velocities between 1989 and 2018. Together, and by comparing with a nearby unnamed land-terminating glacier (Randolph Glacier Inventory ID: RGI60-15.10437), we aim to assess the impact of the proglacial lake on the retreat and flow dynamics of this lake-terminating glacier, and their coupled processes.
Section snippets
Settings and methods
The Longbasaba Glacier (9 km long, ∼31 km2 in area in 2018) has experienced rapid retreat since the 1970s (Wang et al., 2008; Yao et al., 2012), and currently terminates into a moraine-dammed proglacial lake (also named Longbasaba Lake, 2.05 km2 in 2018; Fig. 1). The glacier is partly debris covered, with several moraine belts running parallel to flow down along the ice tongue. This lower part of the ice surface is characterized by numerous ice pinnacles and crevasses. During months with strong
Glacier recession and proglacial lake expansion
The terminus retreat history of the two neighboring glaciers shows distinct differences between 1988 and 2018 (Fig. 1). The lake terminating Longbasaba Glacier retreated about 1.5 km in total, accompanied by an area loss of 1.15 km2 and an equivalent proglacial lake expansion. In contrast, the land-terminating glacier retreated about 0.23 km in total during the same period. The mean annual retreat rate of the Longbasaba Glacier (−51.7 m a−1) considerably exceeds that of the nearby
Discussion
Although both glaciers (Longbasaba and the nearby land-terminating glacier) thinned at a comparable rate over the period 2000-2016, the periodic acceleration of the Longbasaba Glacier contrasts markedly with the deceleration of its land-terminating counterpart, suggesting that lake-ice interactions can greatly modify the lake-terminating glacier's ice flow regime. While basal sliding probably plays some roles to modulate the flow behavior of the land-terminating glacier, the larger surface
Conclusions
Using multi-decadal Landsat series of optical images acquired between 1989 and 2018, we investigated the flow dynamics of a rapidly retreating lake-terminating glacier in the Chinese Central Himalaya by extracting its biennial or annual velocities along the center flow line with the feature tracking method. During the past decades, the Longbasaba Glacier has undergone continuous thinning (−0.9 m a−1 during 1975-2000 and −1.7 m a−1 during 2000-2016), which has not led to a deceleration of ice
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
This work was funded by the National Natural Science Foundation of China (NSFC 41871069 and 41571104), NSFC Key Project for international cooperation (41761144077). This work was also supported by the Kathmandu Center for Research and Education, Chinese Academy of Sciences-Tribhuvan University, and the First-class University Construction Projects (C176240208003). The authors gratefully acknowledge the Planet for provision of PlanetScope and RapidEye imagery, and U.S. Geological Survey (USGS)
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