Abstract
Abstract. Ice-shelf-like floating extensions at the termini of Greenland glaciers are undergoing rapid changes with potential implications for the stability of upstream glaciers and the ice sheet as a whole. While submarine melting is recognized as a major contributor to mass loss, the spatial distribution of submarine melting and its contribution to the total mass balance of these floating extensions is incompletely known and understood. Here, we use high-resolution WorldView satellite imagery collected between 2011 and 2015 to infer the magnitude and spatial variability of melt rates under Greenland's largest remaining ice tongues – Nioghalvfjerdsbræ (79 North Glacier, 79N), Ryder Glacier (RG), and Petermann Glacier (PG). Submarine melt rates under the ice tongues vary considerably, exceeding 50 m a−1 near the grounding zone and decaying rapidly downstream. Channels, likely originating from upstream subglacial channels, give rise to large melt variations across the ice tongues. We compare the total melt rates to the influx of ice to the ice tongue to assess their contribution to the current mass balance. At Petermann Glacier and Ryder Glacier, we find that the combined submarine and aerial melt approximately balances the ice flux from the grounded ice sheet. At Nioghalvfjerdsbræ the total melt flux (14.2 ± 0.96 km3 a−1 w.e., water equivalent) exceeds the inflow of ice (10.2 ± 0.59 km3 a−1 w.e.), indicating present thinning of the ice tongue.
Highlights
Mass loss from ice sheets is often greatest at the marine termini (Truffer and Motyka, 2016)
Melt rates for the three ice tongues indicate that the largest melt rates occur near the grounding line (Fig. 1) and decay within 10 km down-glacier from the upstream boundary of our analysis
Assuming that the ocean temperature at the grounding depth is equal to that observed in the fjords and equal to 1, 0.5, and 0.25 ◦C for 79N, Petermann Glacier (PG), and Ryder Glacier (RG), respectively, and grounding depths of 700, 500, and 450 m, we estimate the thermal forcing for these three systems to be about 3.4, 2.7, and 2.5 ◦C
Summary
Mass loss from ice sheets is often greatest at the marine termini (Truffer and Motyka, 2016). Dupont and Alley, 2005; Furst et al, 2016) For these reasons, monitoring and understanding processes at marine outlets is key to understanding past and predicting future ice sheet variability. In Greenland, floating ice tongues currently protrude, or have recently protruded, from the termini of several major outlet glacier systems. Warming air and ocean temperatures since the mid-1990s have accompanied the reduction or disappearance of most of Greenland’s floating ice tongues. This includes the rapid retreat and collapse of the Jakobshavn Glacier (Jakobshavn Isbræ) beginning in 1998 (Motyka et al, 2010). Changes in submarine melting have been identified as the likely driver of these ice tongue changes (e.g. Holland et al, 2008; Motyka et al, 2011; Münchow et al, 2016)
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