Jet Speed Variability Obscures Euro‐Atlantic Regime Structure

Abstract

Euro‐Atlantic regimes are typically identified using either the latitude of the North Atlantic jet or clustering algorithms in the phase space of 500‐hPa geopotential (Z500). However, while robust trimodality is visibly apparent in jet latitude indices, Z500 clusters require highly sensitive significance tests to distinguish them from autocorrelated noise. This leads to considerable decadal variability in regime patterns, confounding many potential applications. A clear‐cut choice of the optimal number of regimes is also hard to justify. We argue that the jet speed, a near‐Gaussian distribution projecting strongly onto the Z500 field, is the source of these difficulties. Once its influence is removed, the phase space becomes visibly non‐Gaussian, and clustering algorithms easily recover three regimes, closely corresponding to the jet latitude modes. Further analysis supports the existence of two additional blocking regimes, corresponding to a tilted and split jet. All five regimes are approximately stationary across the twentieth century.

A Topological Perspective on Weather Regimes

“…While the failure to detect the Southern jet mode should probably be viewed as a shortcoming, we would also suggest that this failure may shed some light on a few curious features in the literature. Firstly, many studies have tried to diagnose regimes in the Euro-Atlantic sector, and, depending on the choice of input data, pre-processing steps and diagnostics, these studies have suggested there may be anywhere between 2 and 6 regimes (see [66], [22], [48], [50] and [19] respectively for examples of each number). While the ambiguity between the choices 3, 4 and 5 is at least in part due to the confounding influence of the jet speed [22], and the choice of 2 regimes usually corresponds to the North Atlantic Oscillation dipole [67,66], the striking divergence in the number of regimes across studies using similar techniques is still somewhat puzzling.…”

“…Firstly, many studies have tried to diagnose regimes in the Euro-Atlantic sector, and, depending on the choice of input data, pre-processing steps and diagnostics, these studies have suggested there may be anywhere between 2 and 6 regimes (see [66], [22], [48], [50] and [19] respectively for examples of each number). While the ambiguity between the choices 3, 4 and 5 is at least in part due to the confounding influence of the jet speed [22], and the choice of 2 regimes usually corresponds to the North Atlantic Oscillation dipole [67,66], the striking divergence in the number of regimes across studies using similar techniques is still somewhat puzzling. Our results suggest that one possible reason for this is that,…”

“…The existence and properties of regimes in the wintertime Euro-Atlantic circulation has been extensively studied, either through the prism of pressure fields, typically geopotential height at 500hPa, or winds, in the form of zonal winds at 850hPa (hereafter ua850). Studies based on pressure data [6,7,48,22,19] typically use clustering algorithms to classify distinct regimes. On the other hand, wind data is usually processed more directly in order to capture the variability of the North Atlantic eddy-driven jetstream, a relatively coherent stream of zonal winds.…”

“…Firstly, the algorithms involved often require essentially ad-hoc choices up front, such as the choice of number of clusters in K-means clustering algorithms, or temporal persistence thresholds, which directly influence the output regimes. This adds an additional layer of complexity to analysis, as it can be hard to motivate the choice of one parameter over another [20,22]. Secondly, and more seriously, as we will show, any simplistic definition based on density and temporal persistence invariably fails to account for one or more classical regime systems in the literature.…”

Reconstructing winter climate anomalies in the Euro-Atlantic sector using circulation patterns

“…Cluster analysis applied to SLP or geopotential height is commonly used to classify circulation patterns into so-called weather regimes. In the North Atlantic during winter, four classical regimes are identified (e.g Cassou, 2008;Michelangeli et al, 1995;Vautard, 1990), although the optimal number of clusters is not clear-cut and depends somewhat on the algorithm, atmospheric field, domain, and considered temporal period (Christiansen, 2007;Dorrington and Strommen, 2020;Falkena et al, 2020;Woollings et al, 2010). Madonna et al (2017) showed that applying a cluster analysis to the low-level jet leads to four jet configurations that correspond to the four classical weather regimes in the Euro-Atlantic sector: the northern jet resembles the Atlantic Ridge regime, the central jet resembles the zonal/NAO+ regime, the mixed jet resembles the Scandinavian blocking regime, and the southern jet resembles the Greenland anticyclone/NAO− regime (see Fig.…”