Greenland ice sheet motion insensitive to exceptional meltwater forcing

Abstract

Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt-induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt-induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ~3.9 σ above the 1958-2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt-induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.

Keywords: global positioning systems; ice sheet dynamics; ice sheet hydrology; ice sheet melt.

Fig. 1.

Location of the transect on the western margin of the GrIS. Stars indicate sites where ice motion, temperature, and seasonal melting were measured. The triangle indicates where proglacial discharge was measured and the GPS base station located. Circles indicate locations of ‘KAN’ PROMICE/GAP weather stations along the K transect. Contours (in meters) are from a digital elevation model (DEM) of the ice sheet surface produced from interferometric synthetic aperture radar (InSAR) (29). The inferred hydrological catchment of Leverett Glacier, delineated in light gray, was calculated from the ice sheet surface DEM. Inset shows surface and bed elevation along our transect as measured by IceBridge ATM (ILATM2) and MCoRDS (IRMCR2) in 2010 and 2011, respectively (30).

Fig. 2.

Transect observations during 2012. (A–E). Daily (24 h) along-track ice velocities (stepped black lines) and positive degree days (gray bars) for each transect site at which daily measurements were made. (F) Discharge hydrograph for Leverett Glacier (in cubic meters per second), with cumulative discharge between May 7 and August 27 (marked by gray box). The associated catchment is shown on Fig. 1. (A–F) Gray shading defines peak velocity response to July 12 and July 29 melt events (see text).

Fig. 3.

Observations around July 12 melt event. (A–E) Near-surface air temperatures (dashed lines), daily (24 h) along-track ice velocities (stepped black lines) and short-term along-track ice velocities (gray lines) for each site at which daily measurements were made. Periods with inadequate quality observations removed. (F) Discharge hydrograph for Leverett Glacier (in cubic meters per second).(A–F) Gray shading defines the peak velocity response to the melt event (see text).

Fig. 4.

Observations around July 29 melt event. See A–F in Fig. 3 for details.

Fig. 5.

(A) Annual (May 1–April 30) ablation in water equivalent meters for sites 2–7 in 2009 and 2012. (B) Summer (Sum, May 1–August 31), winter (Win, September 1–April 30), and annual (Ann, May 1–April 30) velocities for each site in 2009 and 2012.