Detection and global climatology of two types of spatio-temporal clustering of extratropical cyclones

Abstract

The swift succession of multiple extratropical cyclones during a short period of time is often associated with weather extremes and characterised by a strong atmospheric jet and enhanced baroclinicity. While several diagnostics exist to detect cyclone clustering, they mostly focus on regional assessments or rely on statistical measures that do not allow for a direct association with individual storms. Hence, we introduce a global detection for spatio-temporal clustering of extratropical cyclones, inspired by the original idea of cyclone families by Bjerknes and Solberg, in which individual cyclones follow a similar track. We further subdivide cyclone clusters into two types, a sequential type and a stagnant type. The former is associated with cyclones that follow each other over a minimum distance, whereas the stagnant type requires a proximity over time, while not moving much in space. We find that spatio-temporal cyclone clustering is most frequent along the storm tracks, with more cyclone clustering during winter compared to summer. The majority of cyclone clustering occurs just south of the main storm tracks in the Atlantic and Pacific basins. In the Southern Hemisphere, most cyclone clustering is found in the South-Indian Ocean. Sequential type cyclone clustering is associated with stronger cyclones compared to non-clustered cyclones, while for the stagnant type this intensity difference is less pronounced. This effect is strongest for the North Atlantic and North Pacific, while clustered cyclones in the South Indian Ocean are generally not much stronger. The cyclone intensity within the sequential type does not decrease during a cluster, while in contrast ensuing cyclones of the stagnant type are significantly weaker than the respective primary cyclone. This suggests that these two types of spatio-temporal cyclone clustering are dynamically different.