Contrasting Mechanisms of Summer Blocking Over Western Eurasia

Abstract

The formation of summer blocking events appears to have been mostly studied for a few individual events often associated with heat waves. Here we investigate summer blocking event dynamics in three areas over western Eurasia in order to draw some more general conclusions, mostly in terms of high- and low-frequency processes. A 2-D blocking event detection algorithm is applied to the 500-hPa-geopotential field from the ERA-40 and ERA-Interim reanalyses over the 1958–2017 period. It is shown that both high- and low-frequency processes are important to initiate blocking events over southern central Europe. Blocking events over western Russia are preceded by a significant low-frequency large-scale wave train, and their formation and maintenance are dominated by low-frequency processes only. Finally, it is shown that the risk of extreme seasons such as summer 2010 cannot be accurately estimated from the Poisson statistics of past events.