Seasonal to decadal variability in ice discharge from the Greenland Ice Sheet

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.

Figure 1:

Continuous estimates of discharge, D, for the GrIS for the 2000–2016 period, expressed as a rate of Gigatons per year (Gta⁻¹). Shading represents the 95 % confidence interval.

Figure 2:

Comparative cumulative GrIS mass change relative to 2003 between GRACE and monthly SMB-D. Cumulative SMB is also plotted, with cumulative differences between estimates plotted in the lower panel, associated with the right y-axis.

Figure 3:

Net regional D including (solid line) and excluding (dashed line) the dominant glaciers in each region, with shading representing the 95% confidence interval. From top to bottom these regions include: The northwest (a), plotted with and without Jakobshavn (JI), the southeast (b) with and without Helheim (HL), Kangerdlugssuaq (KQ), and the main trunk of Køge Bugt (KB), the northeast (c), with and without Zachariae Isstrøm (ZI) and 79 North (Fjorden) Glacier (79F), and the southwest (d) with and without Kangia Glacier.

Figure 4:

Normalized, detrended D time series for the total GrIS (top), NW (blue), SE (cyan), NE (green) and SW (red) regions. The normalized discharge within each panel spans from −1 to 1. Vertical black lines align with the annual maximum D of the GrIS-wide series.

Figure 5:

(a) Colored dots are GrIS-wide cumulative average front position change since 1 January 2000, with negative values indicating retreat, versus annual average discharge, D, for each year between 2000 and 2016. The black line is the linear best fit to the data points, with the variance (r²) and probability value (p) of the fit labelled. (b) The average rate of front position change, with negative values indicating retreat, for each year versus the change in average annual discharge between years. (c) Same relationship described in (a), but for glaciers in the NW region only. (d) Relationship between average front position change and the change in Annual D from the current to the following year (D_i+1 − D_i). Date color scale and statistics for (b), (c), and (d) are as described for (a).

Figure 6:

Cumulative GrIS D (black, left y-axis) plotted with raw daily runoff totals (gray bars, right y-axis). The timing of the seasonal maximum runoff is emphasized by vertical dotted lines. Regional runoff totals, smoothed by a 31-day running mean, are shown for the NW and SE region.