Articles | Volume 11, issue 3
The Cryosphere, 11, 1371–1386, 2017

Special issue: Mass balance of the Greenland Ice Sheet

The Cryosphere, 11, 1371–1386, 2017

Research article 09 Jun 2017

Research article | 09 Jun 2017

Hypsometric amplification and routing moderation of Greenland ice sheet meltwater release

Dirk van As1, Andreas Bech Mikkelsen2, Morten Holtegaard Nielsen3, Jason E. Box1, Lillemor Claesson Liljedahl4, Katrin Lindbäck5, Lincoln Pitcher6, and Bent Hasholt2 Dirk van As et al.
  • 1Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark
  • 2Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark
  • 3Marine Science & Consulting, Peder Lykkes Vej 8, 4. th, 2300 Copenhagen, Denmark
  • 4Svensk Kärnbränslehantering AB, Research and Safety Assessment, Box 250, 101 24 Stockholm, Sweden
  • 5Norwegian Polar Institute, Framsentret, Postboks 6606, Langnes, 9296 Tromsø, Norway
  • 6Department of Geography, University of California Los Angeles, Los Angeles, California, 90095, USA

Abstract. Concurrent ice sheet surface runoff and proglacial discharge monitoring are essential for understanding Greenland ice sheet meltwater release. We use an updated, well-constrained river discharge time series from the Watson River in southwest Greenland, with an accurate, observation-based ice sheet surface mass balance model of the  ∼  12 000 km2 ice sheet area feeding the river. For the 2006–2015 decade, we find a large range of a factor of 3 in interannual variability in discharge. The amount of discharge is amplified  ∼  56 % by the ice sheet's hypsometry, i.e., area increase with elevation. A good match between river discharge and ice sheet surface meltwater production is found after introducing elevation-dependent transit delays that moderate diurnal variability in meltwater release by a factor of 10–20. The routing lag time increases with ice sheet elevation and attains values in excess of 1 week for the upper reaches of the runoff area at  ∼  1800 m above sea level. These multi-day routing delays ensure that the highest proglacial discharge levels and thus overbank flooding events are more likely to occur after multi-day melt episodes. Finally, for the Watson River ice sheet catchment, we find no evidence of meltwater storage in or release from the en- and subglacial environments in quantities exceeding our methodological uncertainty, based on the good match between ice sheet runoff and proglacial discharge.

Short summary
The Greenland ice sheet melts faster in a warmer climate. The ice sheet is flatter at high elevation, therefore atmospheric warming increases the melt area exponentially. For current climate conditions, we find that the ice sheet shape amplifies the total meltwater generation by roughly 60 %. Meltwater is not stored underneath the ice sheet, as previously found, but it does take multiple days for it to pass through the seasonally developing subglacial drainage channels, moderating discharge.