Abstract
ABSTRACTWe show results from a positive degree-day (PDD) model of Greenland ice sheet (GrIS) surface mass balance (SMB), 1870–2012, forced with reanalysis data. The model includes an improved daily temperature parameterization as compared with a previous version and is run at 1 km rather than 5 km resolution. The improvements lead overall to higher SMB with the same forcing data. We also compare our model with results from two regional climate models (RCMs). While there is good qualitative agreement between our PDD model and the RCMs, it usually results in lower precipitation and lower runoff but approximately equivalent SMB: mean 1979–2012 SMB (± standard deviation), in Gt a−1, is 382 ± 78 in the PDD model, compared with 379 ± 101 and 425 ± 90 for the RCMs. Comparison with in situ SMB observations suggests that the RCMs may be more accurate than PDD at local level, in some areas, although the latter generally compares well. Dividing the GrIS into seven drainage basins we show that SMB has decreased sharply in all regions since 2000. Finally we show correlation between runoff close to two calving glaciers and either calving front retreat or calving flux, this being most noticeable from the mid-1990s.
Highlights
The surface mass balance (SMB) of an ice sheet measures its surface response to changes in climate (Hanna and others, 2013)
Our results could be an indication of a possible nonlinear link between runoff or SMB and Greenland ice sheet (GrIS) iceberg discharge, the nature of which may have changed over time, as found by Bigg and others (2014) and Zhao and others (2016), whereby SMB, or in our case runoff, has been a factor influencing GrIS calving throughout the 20th century but with varying importance, becoming more significant in recent years
We have shown that using a positive degree-day (PDD) model to calculate SMB of the GrIS at a high spatial resolution of 1 km × 1 km compared with a previous work at 5 km × 5 km spatial resolution leads to higher runoff and lower SMB due to lower-elevation areas on the edge of the ice sheet being better resolved
Summary
The surface mass balance (SMB) of an ice sheet measures its surface response to changes in climate (Hanna and others, 2013). Due to vast volumes of data and computational demands, previous studies of whole GrIS SMB changes have generally been restricted to using spatial resolutions of ≥5 km (e.g. Hanna and others, 2005, 2011; Box, 2013; Box and others, 2013) This has limitations in resolving details of the outer ice margins, around major outlet glaciers, where some notable changes in mass balance and glacier dynamics have recently been observed (Krabill and others, 2004; Rignot and Kanagaratnam, 2006; Kjeldsen and others, 2015). This is especially important when trying to understand links between SMB and dynamic changes (e.g. Sundal and others, 2011; Tedstone and others, 2015).
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