Detection and global climatology of two types of cyclone clustering

Abstract. Cyclone clustering, 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, most focus on a regional impact. We introduce a novel global detection for cyclone clustering, 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 'Bjerknes' 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 these cyclones do not move much in space.

We find that 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. Bjerknes 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 Bjerknes type does not decrease during a cluster, while in contrast secondary cyclones of the stagnant type are significantly weaker than primary cyclones. This suggests that these two types of cyclone clustering are dynamically different.