Assessing the role of precursor cyclones on the formation of extreme Greenland blocks and their impact on summer melting across the Greenland ice sheet

Interannual variability in the frequency of Greenland blocks, persistent areas of high pressure that commonly develop over the northwestern Atlantic Ocean, has been linked to the recent increase in summer melting over the Greenland ice sheet (GrIS). However, the role of other atmospheric and oceanic processes, such as North Atlantic cyclone activity and sea surface temperatures, on the evolution of Greenland blocks has remained relatively unexplored. In this study, a 30-year climatology of North Atlantic cyclones from 1979–2008 is examined within the context of extreme Greenland blocking and accelerating GrIS surface melting. The Greenland Blocking Index (GBI), defined as the mean 500 hPa geopotential height over the Greenland region spanning from 60°–80°N latitude and 20°–80°W longitude, is used to identify extreme Greenland blocks within daily ERA-Interim reanalysis fields. A distinct class of extratropical cyclones, known as precursor cyclones, contributed to a significant strengthening of extreme Greenland blocks through the process of upper-level wave amplification. Across all seasons, most extreme blocks are associated with multiple precursor cyclones prior to peaking in intensity, and a majority of these cyclones have continental rather than oceanic origins. On average, precursor cyclones are both slower moving and more persistent when compared to the climatological distribution of North Atlantic cyclones from 1979–2008, and these attributes may be more important for the formation and maintenance of Greenland blocks rather than cyclone intensification. Over the western and eastern sectors of Greenland, daily meltwater production simulated by the MAR regional climate model is greater during extreme blocks accompanied by precursor cyclones compared to blocking events lacking a precursor cyclone. Based on an analysis of HYSPLIT 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. Long-term increases in GrIS surface melting can be partially attributed to the interaction of these atmospheric and oceanic processes.