Weakened Flow, Persistent Circulation, and Prolonged Weather Extremes in Boreal Summer

Changes in atmospheric circulation can strongly alter the frequency and/or magnitude of high‐impact extreme weather events. Here we address the link between circulation changes and the occurrence of long‐lasting heat waves in the Northern Hemisphere summer. We show that boreal summer circulation has seen pronounced changes in circulation over the last decades, possibly related to rapid warming of the Arctic. Generally, the mid-latitude zonal mean flow has weakened and also the kinetic energy associated with transient synoptic eddies has reduced. At the same time, for some wave numbers, we see an increased occurrence‐frequency of high‐amplitude quasi‐stationary waves. We argue that this increase in frequency is associated with a recent cluster of resonance events that can create such high‐amplitude waves. The reduction in amplitude of fast‐moving transient waves and the more‐frequent occurrence of high‐amplitude quasi‐stationary waves both favor more persistent weather conditions. We present statistical evidence of links between such persistent upper‐level flow and the occurrence of heat extremes at the surface.