Atmospheric Bottlenecks over the Arctic: A Climatological Investigation of Extreme Greenland Blocking Episodes and Their Impact on Melting across the Greenland Ice Sheet

Strong synoptic-scale ridges of high pressure, known as blocking anticyclones, develop frequently across the North Atlantic Ocean, with a preferred origination just to the south of Greenland. Since 2007, the frequency and intensity of these Greenland blocking episodes have been highly anomalous compared to the climatological record, leading to unprecedented melting of the Greenland ice sheet. Associated with persistent negative phases of the North Atlantic Oscillation, impacts of Greenland blocking are not limited to the Arctic region since the amplification of upper-tropospheric Rossby waves affects both the strength and orientation of the polar jet stream over the mid-latitudes. Climatological characteristics of extreme Greenland blocks and the role of North Atlantic cyclones on the development of these blocking anticyclones are examined. A circulation type classification of extreme Greenland blocks and a HYSPLIT trajectory analysis of air mass advections associated with these blocks are used to determine the synoptic patterns that are most conducive to highly anomalous melting across the Greenland ice sheet.

Based on the findings of previous research, the following summary provides some of the key expected results to be gleaned from this study. Extreme Greenland blocks occur most frequently during winter (DJF) and persist much longer on average than climatologically normal-to-anomalous blocks. In addition, these extreme blocks are characterized by hybrid Rex/omega flow patterns over the North Atlantic, similar to the block observed prior to Hurricane Sandy's landfall in October 2012, resulting in the formation of a “super blocking pattern”. Relatively weak but rapidly developing cyclones that track very close to the southern tip of Greenland do contribute to the development and maintenance of extreme Greenland blocks. Greenland blocks with precursor cyclones are significantly stronger, more persistent, and more predominant during winter than blocks lacking a precursor cyclone. The HYSPLIT back trajectory analysis will reveal that summer blocking events associated with a precursor cyclone correspond to periods of enhanced sensible and latent heat advection over warmer-than-normal North Atlantic SSTs, which induces highly above-normal melt extents across the western half of the ice sheet.