Extreme Greenland Blocking Events and Their Impact on Summer Runoff Across the Greenland Ice Sheet

Abstract:

A climatology of extreme Greenland blocks from 1958–2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years and accelerating Greenland Ice Sheet surface melting. The Greenland Blocking Index is used to quantify 500 hPa geopotential height anomalies for the identification of extreme blocking episodes. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. Over the western and eastern sectors of Greenland, surface energy budget components and meltwater production simulated by the Modèle Atmosphérique Régional (MAR) regional model are examined during extreme blocks, particularly those accompanied by precursor cyclones. Based on an analysis of backward trajectories and North Atlantic SST anomalies, enhanced surface melting during the summer, particularly over southern and western Greenland, is strongly linked to the combination of vigorous tropospheric warming generated by the blocking anticyclones and anomalous warm air advection supplied by the precursor cyclones. In addition to vigorous adiabatic warming and maximum insolation associated with the blocking high, diabatic sources of heating linked to precursor cyclones are also critical for enhancing runoff. Long-term increases in GrIS surface melting can be partially attributed to the interaction of these atmospheric and oceanic processes.