Three different glacier surges at a spot: What satellites observe and what not

In the Karakoram, dozens of glacier surges occurred in the past two decades, making the region one of its global hot spots. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over one or two years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same region but differ strongly in surge behaviour. A full suite of satellite sensors and digital elevation models (DEMs) from different sources are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 km y−1, velocities up to 30 m d−1 and surface elevations increased by 200 m. In contrast, the neighbouring and three times smaller North Chongtar Glacier had a slow and near linear increase of advance rates (up to 500 m y−1), flow velocities below 1 m d−1 and elevation increases up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 km y−1. It seems that, despite a similar climatic setting, different surge mechanisms are at play and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges and elevations from ICESat-2 ATL06 data fit neatly. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes.

Brief communication: Detection of glacier surge activity using cloud computing of Sentinel-1 radar data

For studying the flow of glaciers and their response to climate change it is important to detect glacier surges. Here, we compute within Google Earth Engine the normalized differences between winter maxima of Sentinel-1 C-band radar backscatter image stacks over subsequent years. We arrive at a global map of annual backscatter changes, which are for glaciers in most cases related to changed crevassing associated with surge-type activity. For our demonstration period 2018–2019 we detected 69 surging glaciers, with many of them not classified so far as surge type. Comparison with glacier surface velocities shows that we reliably find known surge activities. Our method can support operational monitoring of glacier surges and some other special events such as large rock and snow avalanches.

A regionally resolved inventory of High Mountain Asia surge-type glaciers, derived from a multi-factor remote sensing approach

Knowledge about the occurrence and characteristics of surge-type glaciers is crucial due to the impact of surging on glacier melt and glacier related hazards. One of the "super-clusters" of surge-type glaciers is the mountains of Asia. However, no consistent region-wide inventory of surge-type glaciers in High Mountain Asia exists. We present a regionally resolved inventory of surge-type glaciers based on their behaviour across High Mountain Asia between 2000 and 2018. We identify surge-type behaviour from surface velocity, elevation and feature change patterns using a multi-factor remote sensing approach that combines yearly ITS_LIVE velocity data, DEM differences and very-high resolution imagery (Bing Maps, Google Earth). Out of the ≈ 95000 glaciers in HMA, we identified 666 that show diagnostic surge-type glacier behaviour between 2000 and 2018, which are mainly found in the Karakoram (223) and the Pamir regions (223). The total area covered by the 666 surge-type glaciers represents 19.5 % of the glacierized area in Randolph Glacier Inventory (RGI) V6.0 polygons in HMA. Across all regions of HMA, the surge-affected area within glacier complexes displays a significant power law dependency with glacier length. We validate 107 previously identified glaciers as surge-type and newly identify 491 glaciers. We finally discuss the possibility of self-organized criticality in glacier surges.

Complementary Approaches Towards a Universal Model of Glacier Surges

Although many convincing, diverse, and sometimes competing models of glacier surging have been proposed, the observed behavior of surging glaciers does not fit into distinct categories, and suggests the presence of a universal mechanism driving all surges. On the one hand, recent simulations of oscillatory flow behavior through the description of transient basal drag hint at a fundamental underlying process. On the other hand, the proposition of a unified model of oscillatory flow through the concept of enthalpy adopts a systems based view, in an attempt to rather unify different mechanisms through a single universal measure. While these two general approaches differ in perspective, they are not mutually exclusive, and seem likely to complement each other. A framework incorporating both approaches would see the mechanics of basal drag describing ice flow velocity and surge propagation as a function of forcing by conditions at the glacier bed, in turn modulated through the unified measure of enthalpy.

Remote Sensing Based Study of Surge Characterization of Klutlan and Fisher Glaciers, St. Elias Mountains, North America

The detailed study of glacier surges in St. Elias Mountains is very scarce.Robust and repeat observation of surface displacement, elevation changes and surge reoccurrence intervalsare limited to few surge-type glaciers (e.g., Variegated, Bearing, Lowell and Donjek). Therefore, this study presents the first detailed surge dynamics of Klutlan (1990-2019) and Fisher (1984-2019) glaciers in the St. Elias Mountains, North America. Surface displacement estimation using optical imagery (Landsat TM, ETM+, OLI and Sentinel 2) and surface elevation changes derived from ASTER DEMs were used to understand the surge dynamics. Klutlan Glacier lackspre-surge acceleration and had six years longactive phase (2013-2019). The surge of Klutlan Glacier showed two surface flow maxima(6.2 ± 0.2 m d-1and ~5 ± 0.2 m d-1),in summer of 2016 and2018 respectively.During 2019-2020, in the reservoir zone, maximum surface lowering of -65± 33m was observed whereas, in the receiving zone, maximum ice thickness increased by +31 ± 40 m. The dynamic balance line (DBL)on Klutlan Glaciermoved ~16 km down-glacier during 2019 (788masl.) in comparison with 2004 (1998 masl.). The Fisher Glacier exhibitssix years (2007-2013) long pre-surge acceleration, three years (2013-2016) long active phase and surge terminated gradually. The peak surge displacement ~7 ± 0.4 m d-1was observed in summer of 2015. The reservoir zone experienced a maximum lowering of -60 ± 22 mfrom 2019 to 2003 while lower receiving zone maximum thickened by +80± 22 m. The DBL shifted ~3 km down-glacier during 2019 (959 masl.) as compared to 2016 (1006 masl.). This study assumes that the surge of Fisher Glacier is partially matched with a thermally controlled surge. However, the surge characteristics of Klutlan Glacier doesnot corroborate with globally recognised hydrological or thermally controlled surge mechanism.

A REVIEW OF SURGE-TYPE GLACIERS

The study of glacier surges is meaningful for the understanding of glacier dynamics and the mechanisms of surge-type glaciers. Here, we briefly introduce the distribution of surge-type glaciers and the reason of their tendency to cluster within particular regions. Then we make a review of optical and SAR remote sensing methods for the spaceborne measurement of the surface velocity about surge-type glaciers and show the details of characteristics of Surging Glaciers including the acceleration and deceleration of the velocity of the glacier surface, the terminus may move forward a few kilometers in just a few months, then we give a specific example of surge-type glacier, Sortebræ. Finally, we summarize the existing mechanism of the glaciers surging.

Brief Communication: Detection of glacier surge activity using cloud computing of Sentinel-1 radar data

For studying the flow of glaciers and their response to climate change it is important to detect glacier surges. Here, we compute within Google Earth Engine the normalized differences between winter maxima of Sentinel-1 C-band radar backscatter image stacks over subsequent years. We arrive at a global map of annual backscatter changes, which are for glaciers in most cases related to changed crevassing associated with surge-type activity. For our demonstration period 2018–2019 we detected 69 surging glaciers, with many of them not classified so far as surge type. Comparison with glacier surface velocities shows that we reliably find known surge activities. Our method can support operational monitoring of glacier surges, and some other special events such as large rock and snow avalanches.

Kinematics of the exceptionally-short surge cycles of Sít’ Kusá (Turner Glacier), Alaska, from 1983 to 2013

Glacier surges are periodic episodes of mass redistribution characterized by dramatic increases in ice flow velocity and, sometimes, terminus advance. We use optical satellite imagery to document five previously unexamined surge events of Sít’ Kusá (Turner Glacier) in the St. Elias Mountains of Alaska from 1983 to 2013. Surge events had an average recurrence interval of ~5 years, making it the shortest known regular recurrence interval in the world. Surge events appear to initiate in the winter, with speeds reaching up to ~25 m d−1. The surges propagate down-glacier over ~2 years, resulting in maximum thinning of ~100 m in the reservoir zone and comparable thickening at the terminus. Collectively, the rapid recurrence interval, winter initiation and down-glacier propagation suggest Sít’ Kusá's surges are driven by periodic changes in subglacial hydrology and glacier sliding. Elevation change observations from the northern tributary show a kinematic disconnect above and below an icefall located 23 km from the terminus. We suggest the kinematic disconnect inhibits drawdown from the accumulation zone above the icefall, which leads to a steady flux of ice into the reservoir zone, and contributes to the glacier's exceptionally short recurrence interval.