Characterising sub-seasonal weather persistence -- an illustration on summer European circulation

The long-lasting hot and dry conditions in summer 2022 in Europe are an example of episodic persistence of surface weather on a sub-seasonal timescale that resulted in high impacts. Similarly, the series of atmospheric rivers that reached the US west coast in winter and spring 2023 are an example of recurrent circulation features which also resulted in persistent surface weather on a sub-seasonal timescale. We will first present a short overview and a classification of methodological (statistical and machine learning) approaches that capture and quantify these two flavours of persistence – stationarity and recurrence. We will illustrate the application of some methods to the European summer circulation and discuss their strengths and limitations.

We will then discuss the large-scale circulation features associated with persistent hot spells and recurrent heavy precipitation for selected areas in Europe using the ERA-5 data set.  The first example are three-week hot spells over both northern Europe and western Europe. They are associated with a change in the jet latitude upstream over the Atlantic prior to the events from a northward shifted to a strong zonal jet at the beginning of the events. The second example are recurrent heavy precipitation events in summer over the Western Atlantic. They occur in the left exit region of the jet and are associated with a positive cyclone frequency anomaly, a trough over the western Atlantic and significant positive IVT anomalies. Both examples illustrate an important role of the jet stream (and hence the circulation) over the North Atlantic for sub-seasonal persistence in summer in Europe prior to and during the events.