Abstract
Rapid changes in thickness and velocity have been observed at many marine-terminating glaciers in Greenland, impacting the volume of ice they export, or discharge, from the ice sheet. While annual estimates of ice-sheet wide discharge have been previously derived, higher-resolution records are required to fully constrain the temporal response of these glaciers to various climatic and mechanical drivers that vary in sub-annual scales. Here we sample outlet glaciers wider than 1 km (N = 230) to derive the first continuous, ice-sheet wide record of total ice sheet discharge for the 2000-2016 period, resolving a seasonal variability of 6 %. The amplitude of seasonality varies spatially across the ice sheet from 5 % in the southeastern region to 9 % in the northwest region. We analyze seasonal to annual variability in the discharge time series with respect to both modelled meltwater runoff, obtained from RACMO2.3p2, and glacier front position changes over the same period. We find that year-to-year changes in total ice sheet discharge are related to annual front changes (r 2 = 0.59, p = 10-4) and that the annual magnitude of discharge is closely related to cumulative front position changes (r 2 = 0.79), which show a net retreat of > 400 km, or an average retreat of > 2 km at each surveyed glacier. Neither maximum seasonal runoff or annual runoff totals are correlated to annual discharge, which suggests that larger annual quantities of runoff do not relate to increased annual discharge. Discharge and runoff, however, follow similar patterns of seasonal variability with near-coincident periods of acceleration and seasonal maxima. These results suggest that changes in glacier front position drive secular trends in discharge, whereas the impact of runoff is likely limited to the summer months when observed seasonal variations are substantially controlled by the timing of meltwater input.
Highlights
Mass loss from the Greenland Ice Sheet (GrIS) is the single largest cause of sea level rise (Vaughan et al, 2013; Box and Sharp, 2017), contributing approximately 1 mm a−1 of global water equivalent over the 2010–2015 period (van den Broeke et al, 2016)
Beginning from a mean annual discharge of 440 ± 8 Gt a−1 in 2000, D increases to a maximum of 524 ± 9 Gt a−1 in late June 2005, primarily due to the accelerations of the Kangerdlugssuaq and Helheim glaciers in the east (Howat et al, 2007; Joughin et al, 2008)
In the following 2 years, the rapid decrease in D from these two glaciers resulted in the greatest seasonal decrease in GrIS D in 2006, declining to a minimum of 461 ± 9 Gt a−1 by January 2008
Summary
Mass loss from the Greenland Ice Sheet (GrIS) is the single largest cause of sea level rise (Vaughan et al, 2013; Box and Sharp, 2017), contributing approximately 1 mm a−1 of global water equivalent over the 2010–2015 period (van den Broeke et al, 2016). Since the mid 1990s, the GrIS has been losing ice at an increasing rate (Rignot et al, 2011; Sasgen et al, 2012; Hanna et al, 2013; Enderlin et al, 2014) due in part to increased discharge from marine-terminating outlet glaciers (Rignot and Kanagaratnam, 2006; Rignot et al, 2008; Enderlin et al, 2014; Andersen et al, 2015). Substantial increases in ice discharge are observed at large outlet glaciers over periods of months or less King et al.: Seasonal variability of Greenland glaciers understanding the dynamics of these glaciers requires measurements with a high temporal resolution
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