An Analysis of Large-Scale Regime Transitions on the Spatiotemporal Distribution of Cyclone Clustering

Northern Hemisphere (NH) cool season atmospheric predictability for sub-seasonal time scales (1-4 weeks) is greatly affected by the structure and evolution of the North Pacific jet stream (NPJ). The NPJ can be impacted by tropical and polar disturbances such as longitudinally varying convection associated with the Madden-Julian Oscillation, re-curving tropical cyclones, especially those that undergo extratropical transition, and equatorward-moving arctic air masses that originate over the Asian continent. Perturbations of the NPJ on its equatorward and poleward sides by tropical systems and arctic disturbances, respectively, can create eastward-propagating Rossby wave trains (RWTs), subsequent anticyclonic and cyclonic wave breaking, and downstream wave amplification. This downstream RWT propagation, wave breaking, and wave amplification can enhance the probability of the occurrence of extreme weather events (EWEs) such as, for example, the sequential clustering of cyclogenesis events, and early season equatorward surges of arctic air over the CONUS. These EWE can have enormous societal impacts ranging from agricultural damage to public safety.

A seasonal cyclone clustering climatology over the Northern Hemisphere for the years 1979-2015 will be constructed, mapped, and displayed. This climatology will be obtained by utilizing cyclone tracking algorithms (Hodges and an ECMWF-produced) for the NCEP CFSR and the ERAI re-analysis datasets. Cyclone clustering and cyclone density maps will be used to establish whether certain large-scale flow regimes are more conducive than others to cyclone clustering and to illustrate the geographical dependence of cyclone clustering. An assessment of the three cyclone tracking datasets on the sensitivity and geographic distribution of cyclone clustering in the NH will be shown.