Persistent episodes of the Euro-Atlantic upper-level jets in summer: precursors, maintainers and impacts

Abstract. Recent studies have highlighted the link between upper-level jet stream dynamics, especially the persistence of certain jet configurations, and extreme summer weather in Europe. In this work, we use a recently developed toolbox for the detection and study of jet core features in wind speed fields to define persistent episodes of the subtropical or of the eddy-driven jet over the Euro-Atlantic sector and focusing on summer. During these events, we analyse the state of the jets themselves, as well as of the atmosphere directly surrounding the jet, using jet-centred composites. We study the role of these quantities as potential precursors to, or maintainers of persistent episodes. Fields used to quantify the state of the atmosphere before or during persistent episodes include events of high variability or high persistence like Rossby wave breaking or blocks, and potential sources or sinks of momentum such as temperature gradient, diabatic processes or eddy activity. We also systematically study the link between these episodes and severe weather events in Europe.

We show that the two jet categories have different persistence properties and persistent episode temporal distribution. The precursors, maintainers and impacts of these events are also vastly different, although commonalities exist. Both jets are, on average over their persistent episodes, stronger and shifted equatorward compared to their summer mean state. Looking into each individual episode reveals that each jet's persistence can be explained using a combination mechanisms, which include both remote, large-scale drivers as well as local effects. We a significant increase in hot and dry spells during persistent episodes of the STJ, and of wet spells during persistent episodes of the EDJ.