Persistent anomalies of the North Atlantic jet stream and associated surface extremes over Europe

Abstract

Unusual, persistent configurations of the North Atlantic jet stream affect the weather and climate over Europe. We focus on winter and on intraseasonal and seasonal time scales, and study persistent jet anomalies through the lens of large deviation theory using Coupled Model Intercomparison Project (CMIP6) simulations of the MPI-ESM-LR model and ERA5 reanalysis data. The configurations of interest are defined as long-lasting anomalies of a few months in jet latitude, speed or zonality. Our results show that persistent temperature and precipitation extremes over large European regions are anomalously frequent during the unusual, persistent jet configurations we identify. Furthermore, the relative increase in frequency of surface extremes is larger for more intense surface extremes and/or more extreme jet anomalies. This is relevant in the context of the predictability of these extremes. The highest extreme event frequencies at the surface are observed in case of precipitation over the Mediterranean and Western Europe during anomalously zonal and/or fast jet events, pointing to these jet anomalies matching rather homogeneous large scale atmospheric configurations with a clear surface footprint. Additionally, our results emphasise the usefulness of large deviation rate functions to estimate the frequency of occurrence of persistent jet anomalies. They therefore provide a tool to statistically describe long-lasting anomalies, much like extreme value theory may be used to investigate shorter-lived extreme events.