An observational study of multiple tropical cyclone events in the western north Pacific

Best-track and NCEP/NCAR reanalysis data are used to study the statistics of multiple cyclone events (MCEs), in which one tropical cyclone (the ‘daughter’) forms to the east of another extant TC (the ‘mother’) during the mother’s lifetime, in the western north Pacific. It is found that approximately 30% of all tropical cyclones become mothers, and that MCEs occur relatively more frequently in the early and late season than the peak season. Composite differences in large-scale conditions between MCEs and events in which a daughter does not form show that MCEs are favoured by easterly vertical shear and cyclonic low-level horizontal shear. These findings are broadly consistent with (though they do not prove) the hypothesis that Rossby wave radiation is an important mechanism in a significant fraction of MCE events and that the radiation is governed by linear stationary wave dynamics.