Impacts of Extreme Blocking over Greenland

Blocked atmospheric flows over Greenland and the North Atlantic Arctic (NAA) have been studied for several decades and are an important part of the climatology of the region. Atmospheric blocking occurs when there is a breaking of synoptic Rossby waves resulting in a quasi-stationary high pressure system that disrupts synoptic scale circulation. Blocking typically results in a large-scale reversal of the meridional geopotential height gradient. The Greenland Blocking Index (GBI) is defined as the mean 500-hPa geopotential height for the 60°–80° N, 20°–80° W region. A positive (negative) GBI is generally associated with an increase (decrease) in 500-hPa heights to the north and a decrease (increase) in 500-hPa heights to the south. One characteristic of blocking is a high-amplitude barotropic anticyclone located to the north of an anomalous upper-troposphere easterlies. Persistent flow meandering around the anticyclone and slows the propagation of transient middle- and upper-troposphere waves. Blocking not only impacts weather patterns in the high latitudes and the NAA, but can also impact the lower latitudes in the Northern Hemisphere. Blocking can amplify persistent wind patterns that impact moisture transport and climate.

In this study, daily surface pressure, wind, geopotential height, and specific humidity fields from ERA-Interim and MERRA2 reanalyses were composited for extreme GBI occurrances, specifically the upper 90^th percentile and lower 10^th percentile for the years 1980-2017. Anomalies of each variable were examined to better understand atmospheric conditions under unusual blocked and unblocked flows. Finally, variability of integrated moisture transport over Greenland and the NAA was examined during extreme GBI occurrences.