Comparison of 2013 and 2012 Greenland Ice Sheet Surface Melt and Associated Mechanisms

In this study, we describe the characteristics of the Greenland Ice Sheet surface melt occurring in summer 2013 and compare the results with anomalies of the record surface melt observed in summer 2012. We then investigate the dynamic and thermodynamic differences between these two cases and their impacts on the Greenland Ice Sheet surface melt. Results show that the maximum surface melt extent was only 44% in 2013, which is far less than 97% as observed in 2012. The averaged extent and duration of the surface melt in 2013 were close to the climatology and 20 days shorter than that of 2012. The summer atmospheric circulation anomalies in 2013 were nearly opposite those observed in 2012. Greenland and the surrounding areas showed anomalously low pressure, particularly a negative height at 500 hPa, which led to a more zonal pattern than those observed in 2012. The corresponding anomalous cyclonic circulation in the north and south areas of Greenland was favorable for advecting cold Arctic air to Greenland. This phenomenon decreased the mean surface temperature and the frequency of extreme heat event over Greenland. Moreover, the surface downward radiation pattern in 2013 was oriented in a southwest-northeast direction over Greenland, which differs from the north-south distribution in 2012. The net downward radiation was small due to the cancellation of downward shortwave and longwave components in 2013. However, downward shortwave dominated from the distribution perspective, leading to an insignificant impact on surface temperatures over Greenland. The combination of dynamic effects of atmospheric circulation change and thermodynamic effects of the radiation budget contributed to the surface melt of the Greenland Ice Sheet with values close to climatology.