Abstract
Cyclone clusters are a frequent synoptic feature in the Euro-Atlantic area. Recent studies have shown that serial clustering of cyclones generally occurs on both flanks and downstream regions of the North Atlantic storm track, while cyclones tend to occur more regulary on the western side of the North Atlantic basin near Newfoundland. This study explores the sensitivity of serial clustering to the choice of cyclone tracking method using cyclone track data from 15 methods derived from ERA-Interim data (1979–2010). Clustering is estimated by the dispersion (ratio of variance to mean) of winter [December – February (DJF)] cyclone passages near each grid point over the Euro-Atlantic area. The mean number of cyclone counts and their variance are compared between methods, revealing considerable differences, particularly for the latter. Results show that all different tracking methods qualitatively capture similar large-scale spatial patterns of underdispersion and overdispersion over the study region. The quantitative differences can primarily be attributed to the differences in the variance of cyclone counts between the methods. Nevertheless, overdispersion is statistically significant for almost all methods over parts of the eastern North Atlantic and Western Europe, and is therefore considered as a robust feature. The influence of the North Atlantic Oscillation (NAO) on cyclone clustering displays a similar pattern for all tracking methods, with one maximum near Iceland and another between the Azores and Iberia. The differences in variance between methods are not related with different sensitivities to the NAO, which can account to over 50% of the clustering in some regions. We conclude that the general features of underdispersion and overdispersion of extratropical cyclones over the North Atlantic and Western Europe are robust to the choice of tracking method. The same is true for the influence of the NAO on cyclone dispersion.
Highlights
Extra-tropical cyclones over the North Atlantic play a key role in determining the weather and climate of Western Europe
Similar results are found for other grid points over the Eastern North Atlantic and Western Europe, revealing the robustness of overdispersion of cyclone counts for this area
The differences in φ can be primarily attributed to the differences in the variance of cyclone counts between the methods
Summary
The common result from these publications is that cyclone clustering (overdispersion) occurs on both flanks and downstream of the North Atlantic storm track (Mailier et al, 2006, their Fig. 6), while regularity (underdispersion) is found near the core of the storm track by Newfoundland This pattern is a robust feature in different reanalysis datasets (Pinto et al, 2013, their Fig. 3). The main question explored in this study is how robust the general features of underdispersion / overdispersion over the study area are to the choice of cyclone tracking method With this aim, we perform for the first time a multi-tracking approach analysis of clustering over the North Atlantic and Europe.
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