What are supraglacial lakes?
Supraglacial lakes form on the surface of glaciers. Meltwater accumulates in depressions or hollows on the ice surface. We think of the East Antarctic Ice Sheet as being very cold and dry, with temperatures well below 0°C, but in some places, surface melting does occur in the summer melt season. In Antarctica, this occurs primarily from November to February1. Supraglacial lakes are critically important for the stability of ice sheets, glaciers and ice shelves, and can help drive ice-shelf collapse.
Meltwater accumulates on the glacier or ice-shelf surface when firn (snow that has not yet been compressed into ice) becomes sufficiently saturated and can no longer absorb meltwater2,3.
Supraglacial lakes commonly refreeze at the end of the melt season, but some have been observed to drain into the ice6,14,16.
Why are supraglacial lakes important in Antarctica?
Supraglacial lakes are important because they can influence ice flow and mass balance in three ways. Firstly, because the ponded meltwater is darker than the surrounding white-grey ice, lakes have a lower albedo. More energy from the sun’s rays is absorbed rather than reflected,meaning the lakes can locally increase surface melting in a positive feedback effect4,5.
Secondly, supraglacial lakes can cause short-term ice flow accelerations when they drain through the ice sheet6. The meltwater can reach the bed of the ice sheet, and encourage basal sliding and thus faster ice flow.
Thirdly, supraglacial lakes have been linked to ice shelf collapse on the Antarctic Peninsula7,8. Ice shelves are the floating extensions of land ice; they receive ice from ice-flow from inland tributary glaciers and lose mass by calving icebergs. Ice shelf removal increases the flow of inland ice into the ocean and its contribution to sea-level rise9,10.
It is therefore important to understand where supraglacial lakes form and how they influence the behaviour and stability of the ice sheet. We need these data so that we can identify which parts of Antarctica may be more sensitive to climate warming and how the Antarctic Ice Sheet may respond under a future warming climate11.
Where are supraglacial lakes found in Antarctica?
Satellite remote sensing since the 1970s has allowed researchers to map supraglacial lakes over the whole Antarctic continent12,13. This has shown that thousands of supraglacial lakes are widespread around the margins of Antarctica during the summer melt season, including the periphery of the East Antarctic Ice Sheet12-16. A recent assessment found lakes up to 500 km inland, and at up to >1500 m above sea level13.
This is not a new phenomenon; satellite imagery records supraglacial lakes persistently forming annually for decades on Antarctic ice shelves. For example, lakes have been recorded on the Amery Ice Shelf in East Antarctica since at least 1974, where the largest recorded supraglacial lake (~80 km long) lake has been observed12,16-20.
This interactive Google Map below allows you to explore supraglacial lakes on the floating ice tongue of Sørsdal Glacier, Princess Elizabeth Land, East Antarctica. You can clearly see meltwater filling depressions and crevasses on the ice surface and lakes being fed by meltwater from upstream.
What controls supraglacial lake distributions in Antarctica?
Most supraglacial lakes in Antarctica form on ice shelves, where the long, flat profile of the floating ice shelf and low surface elevations encourage meltwater ponding13. Supraglacial lakes typically cluster a few kilometres down-ice from the grounding line12,13,21,31.
The figure below shows how supraglacial lake distributions around the continent are controlled by interactions between local and regional wind patterns, ice surface topography and albedo21.
On the Antarctic Peninsula, such as the Larsen C Ice Shelf, strong down-slope winds called föhn winds cause intense melting (>400 mm w.e. yr –1) and localised ponding, even in Antarctic winter22-26.
In coastal East Antarctica, supraglacial lakes are clustered near ice-shelf grounding lines, but are often absent further downstream towards calving fronts because higher snowfall here means surface meltwater can be absorbed into the porous snowpack before it can pond on the ice surface15.
Persistent katabatic winds induce vertical air mixing andcan scour snow surfaces to expose lower-albedo blue ice, which intensifies melting and ponding15. Lakes often form close to exposed rock, which has a low albedo and locally increases surface melting12.
Supraglacial lakes are sensitive to katabatic winds, which can cause intense short-lived snowmelt events31.
Links to ice-shelf collapse
Supraglacial lakes can flex and fracture ice shelves when they grow and drain7,8,27. A good example is the near-synchronous drainage of over 2750 lakes (up to 6.8 metres deep) on Larsen B Ice Shelf in the days before it collapsed in February-March 200227.
Lakes are thought to have drained through a process called hydrofracturing, where meltwater triggers fracturing by filling and expanding vertical crevasses7, 27. This is thought to have triggered a ‘chain reaction’ that caused widespread fracturing and lake drainage8.
Supraglacial lakes have been observed on other ice shelves before they collapsed or partially disintegrated, including Larsen A and Prince Gustav in 19957,28. They could therefore be important indicators of future ice shelf instability and collapse29,30.
The likelihood of lakes triggering ice shelf collapse in other locations is lower on thicker ice shelves confined within narrow embayments, such as the George VI Ice Shelf (between the mainland Antarctic Peninsula and Alexander Island), or those that are ‘pinned’ (locally in contact with the bedrock). This is because ice-shelf flow here is stabilised and so the ice shelf can support large numbers of lakes on its surface.
Observing Antarctic supraglacial lakes from space
The inaccessibility of lakes in Antarctica makes obtaining field measurements difficult, but satellite remote sensing can be used to map supraglacial lakes at sub-metre resolution on weekly timescales, and to measure seasonal changes in lake depth and volume (16,31).
Improvements in satellite remote sensing over time
The figure below shows how the ability to resolve individual supraglacial lakes has vastly improved since coarser sensors like AVHRR and MODIS, which were unable to distinguish these features.
Higher resolution (>10m) satellites such as Sentinel-2 and Worldview-3 can resolve lake bathymetry and surface features such as partial lake ice coverage.
Examples of the evolution of satellite image sensor resolution and detection of supraglacial lakes on Beaver Lake, adjacent to Amery Ice Shelf, East Antarctica. Panels moving from top left to bottom right represent satellite sensors in order of increasing spatial resolution. Smaller insets show the same supraglacial lake to demonstrate the improvement in detail. Imagery: Scambos et al. (1996); United States Geological Survey; Google, Maxar Technologies. Figure credit: Arthur et al. (2020).
Recent assessments have used imagery from multiple optical satellite sensors (such as Landsat-8 and Sentinel-2) to maximise the amount of cloud-free imagery for mapping supraglacial lake distributions at the continent scale12,13.
Measuring changes in supraglacial lakes
Changes in the extent, area and volume of individual supraglacial lakes can be tracked through multiple melt seasons using satellites with a short revisit time (time elapsed between observations of the same point on earth)16,31.
For example, the 16-day revisit of Landsat-8 and the 5-day revisit of the Sentinel-2A/B satellite constellation have been used to calculate seasonal fluctuations in supraglacial lake area, depth and volume through multiple melt seasons16,31. This is important for estimating the volume of meltwater stored in lakes on the surface of the Antarctic Ice Sheet and the potential for hydrofracturing (where the lake helps to break up an ice shelf).
Satellite imagery recently recorded an extreme, potentially unprecedented, melt event in January 2020 around Antarctica during unprecedented warm air temperatures, which caused widespread surface melting and lake growth.
This post draws from the review of recent progress made in understanding Antarctic supraglacial lakes using satellite remote sensing by Arthur et al. (2020) (ref. 35).
About the Author
This article was contributed by Jennifer Arthur from Durham University. Jennifer is a PhD candidate specialising in Antarctic surface hydrology and ice shelf dynamics. She’s interested in the distribution and seasonal evolution of supraglacial lakes and the potential influence of surface meltwater on outlet glacier and ice shelf stability. Jennifer’s PhD research uses satellite remote sensing to investigate this on the East Antarctic Ice Sheet. You can follow Jennifer on Twitter @AntarcticJenny and she is part of the @EGU_CR Cryosphere blog team.
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