Aspects of North Atlantic jet stream persistence and impacts on the surface weather in Europe

Recent studies have highlighted the link between upper-level jet dynamics, especially the persistence of certain configurations, and extreme summer weather in Europe. Using our recently published toolbox for jet dynamics characterization, we use the various persistence metrics developed therein to find the most persistent episodes in recent data, as well as in a large ensemble containing future scenarios. We study precursors to these persistent episodes with potential for added predictability, as well as the surface weather extremes that can co-occur with these episodes.

First, we apply a jet axis detection and tracking algorithm in order to extract individual jets and classify them in the canonical categories of polar and subtropical jets. This allows us to measure the jets' instantaneous advection speed, as well as their lifetime, until they have weakened and cannot be extracted from the background wind anymore, or until they are advected out of the domain. These two metrics, advection speed and lifetime, provide measures of object persistence for each of the jets, that are, respectively, local and non-local in time.

Second, we apply the self-organizing map (SOM) clustering algorithm to the same data to create a distance-preserving, discrete, 2D phase space. The dynamics can then be described by the time series of visited SOM nodes, in which a long stay in a given node relates to a persistent state and a rapid transition between nodes that are far apart relates to a sudden dramatic shift in the configuration of upper-level flow. This allows us to quantify state persistence using the average length of a stay on a given SOM node.

Under these different views of persistence in the Euro-Atlantic sector, we establish certain jet properties as precursors for persistent episodes, study the role of Rossby wave breaking before and during these episodes, and explore the impacts of a persistent upper-level flow on surface weather and weather extremes in Europe.