Tidewater glacier dynamics and the mass budget of the Northwest Greenland ice sheet

Abstract:

The rate of mass loss from the Greenland ice sheet continues to increase in response to increased surface melt and the retreat, acceleration and thinning of marine-terminating outlet glaciers. Marginal thinning is concentrated on the southeastern and western coasts; areas drained by numerous large, fast-flowing marine-terminating glaciers. Considerable temporal variability exists in the timing of regional mass loss, with an emerging picture of a clockwise progression of mass loss spreading from the southeast to the west of the ice sheet. The partitioning of regional mass loss into surface mass balance and glacier dynamic driven components is a question of considerable scientific interest.

We present a mass budget for the Northwest Greenland ice sheet, along with long term, high temporal resolution records of glacier flow velocity and calving front position. We feature track optical and Synthetic Aperture Radar (SAR) imagery from Landsat-5 TM, Landsat-7 ETM+ (slc-on), Landsat-8 OLI, ERS-1 SAR, ERS-2 SAR and Envisat ASAR data covering the period 1985-2014. It has been suggested that some Northwest Greenland glaciers have undergone two periods of dynamic mass loss over this time period. Our records span 1985-2014 for these glaciers and 2000-2014 for other large outlets. Velocity records were converted into ice discharge estimates using bedrock and surface Digital Elevation Models and assumptions about depth integrated velocity and ice density. Surface Mass Balance (SMB) model output was used to complete the mass budget.

The 30 year observational record shows that the 21^st century period of glacier dynamic change in Northwest Greenland is exceptional and ongoing. Our results do not support the assertion of an earlier period of dynamic mass loss in the late 1980s and early 1990s. However, many of the observed dynamic changes initiated substantially prior to the gravimetric observation of increased regional mass loss from summer 2005 onwards. Modelled SMB exhibits a strong negative anomaly between 2006 and 2009, caused by increased run-off. The satellite gravimetric observation of increased regional mass loss likely captures a decrease in surface mass balance superimposed on ongoing dynamic mass loss.