Self-regulation of ice flow varies across the ablation area in south-west Greenland

Abstract

Abstract. The concept of a positive feedback between ice flow and enhanced melt rates in a warmer climate fuelled the debate regarding the temporal and spatial controls on seasonal ice acceleration. Here we combine melt, basal water pressure and ice velocity data. Using 20 years of data covering the whole ablation area, we show that there is not a strong positive correlation between annual ice velocities and melt rates. Annual velocities even slightly decreased with increasing melt. Results also indicate that melt variations are most important for velocity variations in the upper ablation zone up to the equilibrium line altitude. During the extreme melt in 2012, a large velocity response near the equilibrium line was observed, highlighting the possibility of meltwater to have an impact even high on the ice sheet. This may lead to an increase of the annual ice velocity in the region above S9 and requires further monitoring.

Influence of meltwater on Greenland Ice Sheet dynamics

“…The persistent presence of a late-summer velocity minimum (Figures 2-1 b-e, 2-4) supports the theory that the local subglacial hydrologic network undergoes seasonal reorganization, with increasing channelization throughout the summer resulting in increased frictional coupling that promotes this slowdown [Bartholomew et al, 2010;Colgan et al, 2011;Hoffman et al, 2011;Sundal et al, 2011;Sole et al, 2013;van de Wal et al, 2015]. Thus, larger |: ;<= | has been proposed to represent more extensive subglacial channelization over the runoff season under higher melt conditions [Sundal et al, 2011] (Figure 2-11 a-c).…”

“…Our records exhibit similar trends, though neither of these trends are statistically significant given the high inter-annual variability in annual velocities and runoff over the 7-year record (Figure 2-3 g,h). While there is no correlation between velocity and runoff on annual timescales [Zwally et el., 2002;van de Wal et al, 2008van de Wal et al, , 2015Sole et al, 2013;Tedstone et al, 2015], the negative relationship between past mean runoff magnitude and annual velocity has been invoked in previous studies [Tedstone et al, 2015] to suggest a causal relationship where increased runoff drives slower velocities on multiyear timescales. Indeed, all three records depict a similar negative relationship between past mean runoff magnitude and annual velocity that strengthens with increasing values of = (Figures 2-3 e, 2-13, 2-14).…”

“…Further exploration of the K-transect GPS observations has detailed the elevation-dependent annual velocity curve structure ( Figure 1-4 a) [Bartholomew et al, 2010;van de Wal et al, 2015], characterized by slow flow through the spring; a sharp spike in velocities following surface melt onset; sustained fast flow during the first half of the summer with high daily-to multi-day velocity variations; overall decreasing but still variable velocities over the second half of the melt season; an annual minimum velocity following melt cessation; and a steady increase in velocities from the annual minimum over the fall and winter. This characteristic annual velocity curve is supported by observations in the Swiss Camp [Zwally et al, 2002;Colgan et al, 2011;Hoffman et al, 2011] and North Lake regions of the Greenland Ice Sheet, leading to further investigations on the relationship between the evolution of subglacial drainage system components ( Figure 1-3) and basal sliding that could be producing this annual velocity curve.…”

“…The K-transect time series extends from 1990-2011 (n=20). Annual velocity data for the K-transect are published in van de Wal et al [2015]. Local annual runoff estimates used to calculate 8 ;][ *j, *9] are derived from RACMO2.3 downscaled daily runoff v0.2 at North Lake and K-transect station S7.…”

“…Outside of the fast-flowing trunks of outlet glaciers, seasonal accelerations of ice sheet ablation zones are observed across the Greenland Ice Sheet in concert with surface melting, though the amount of net annual ice motion is largely insensitive to (or in some cases inversely related to) the magnitude of annual melt [Zwally et al, 2002;Joughin et al, 2008Joughin et al, , 2013van de Wal et al, 2008van de Wal et al, , 2015Sole et al, 2013;Tedstone et al, 2015;Stevens et al, 2016a]. The spatiotemporal variability of surface meltwater reaching and traveling along the ice-bed interface is frequently hypothesized to play a major role in controlling ice sheet velocities through its ability to "lubricate" the ice-bed interface Shepherd et al, 2009;Schoof, 2010;Hoffman et al, 2011;Smith et al, 2015;Stevens et al, 2015].…”