Winter motion mediates dynamic response of the Greenland Ice Sheet to warmer summers

Article · August 2013with57 Reads
DOI: 10.1002/grl.50764
We present ice velocities from a land-terminating transect extending >115km into the western Greenland Ice Sheet during three contrasting melt years (2009-2011) to determine whether enhanced melting accelerates dynamic mass loss. We find no significant correlation between surface melt and annual ice flow. There is however a positive correlation between melt and summer ice displacement, but a negative correlation with winter displacement. This response is consistent with hydro-dynamic coupling; enhanced summer ice flow results from longer periods of increasing surface melting and greater duration ice surface to bed connections, while reduced winter motion is explicable by drainage of high basal water pressure regions by larger more extensive subglacial channels. Despite mean interannual surface melt variability of up to 70%, mean annual ice velocities changed by <7.5%. Increased summer melting thereby preconditions the ice-bed interface for reduced winter motion resulting in limited dynamic sensitivity to interannual variations in surface melting.
  • ...However, increased ice motion early in the summer can be offset by an earlier onset of channelization and corresponding deceleration ( Sundal et al., 2011;van de Wal et al., 2015). Increases in channelization extent may also lead to slower mean winter flow, due 10 to more extensive drainage of the subglacial system leading to lower water pressures during winter ( Sole et al., 2013). As melt season intensity continues to increase, it remains unclear how ice velocities may be altered due to changing patterns of input, and input at higher elevations where ice thickness is greater. ...
  • ...A full steady state takes approximately two years (Hewitt, 2013). In contrast, observations suggest that summer melt has an impact on the state of the hydrological system during the subsequent winter (Chu et al., 2016;Sole et al., 2013). The model output of the model therefore can only be 5 considered an approximation to a generic hydrological state. ...
  • ...The higher surface lake elevations observed in our study compared toSundal et al. (2009)is in line with other work that indicates an increasing presence of water at higher elevations with increasing air temperatures (Bartholomew et al., 2010;Howat et al., 2013;Morriss et al., 2013;). If these higher elevation lakes increasingly drain rapidly rather than freeze-through, they could increase ice velocities in the short term if the water is accommodated in an enlarging subglacial cavity network (Zwally et al., 2002;Doyle et al., 2014), or decrease velocities in the long term if discharges are sufficient to cause channelization (Bartholomew et al., 2012;Sole et al., 2013). An improved understanding of the precise controls on hydrofracture would help to estimate whether surface lakes will be able to drain at these higher elevations in the future. ...
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