A Climatology of Indirect Tropical Cyclone Interactions

While it is understood that a recurving tropical cyclone which interacts with the midlatitude flow can cause large downstream midlatitude forecast errors, it is much less well-understood how and whether such events can affect the predictability and evolution of a downstream tropical cyclone equatorward of the midlatitude waveguide. For this study, an indirect tropical-cyclone interaction is defined as one in which an initial tropical cyclone perturbs the downstream midlatitude waveguide within one synoptic-scale wavelength of a subsequent tropical cyclone. To achieve a better understanding of the mechanics behind these interactions, the ERA-Interim reanalysis is used to generate a climatology of indirect interactions between two (or more) tropical and/or subtropical cyclones in the North Atlantic and western North Pacific basins from 2000-2017. A metric for the interaction of a tropical cyclone with the midlatitude flow is used to identify indirect-interaction events and to ascribe interaction timing and location. For each cyclone in the identified interaction events, track and intensity forecasts from operational warning centers and selected numerical guidance are verified to quantify the extent to which these cyclones are associated with enhanced or degraded predictability relative to climatology. Statistical significance testing is also conducted for relevant composite fields to help elucidate the impact the interaction of the initial cyclone with the midlatitude waveguide on the synoptic-scale environments in which the subsequent cyclones are located. Future research directions include numerical simulations of selected cases to quantify the extent to which the initial tropical cyclone impacts the midlatitude waveguide and ensemble-based studies to more rigorously evaluate the subsequent tropical cyclone’s evolution and predictability.