Extratropical Transition of Western North Pacific Tropical Cyclones: Midlatitude and Tropical Cyclone Contributions to Reintensification

Abstract

This study of extratropical transition of western North Pacific tropical cyclones (TCs) addresses the reintensification stage during which the TC remnants develop as an extratropical cyclone. The hypothesis examined here is that reintensification depends on the interaction between the midlatitude circulation contributions from mid- and upper-level dynamic processes, low-level thermal processes from the decaying TC, and upper-level outflow characteristics from the decaying TC. Reintensification occurs when the combination of the dynamic and thermodynamic processes define a region that is favorable for extratropical cyclone development. The midlatitude circulation contribution to reintensification is characterized by comparing a control forecast made with an atmosphere-only version of the Coupled Ocean?Atmosphere Mesoscale Prediction System with a simulation in which the TC has been removed (NOTC). The midlatitude contribution is favorable if a significant extratropical cyclone forms in the NOTC simulation. A neutral midlatitude contribution is defined to occur when weak or no extratropical cyclogenesis occurs in the NOTC simulation. Finally, an unfavorable midlatitude contribution occurs when anticyclogenesis is predicted in the NOTC simulation. The TC contribution to the reintensification is characterized in a similar manner by assessing the different reintensification characteristics between a control simulation and the NOTC simulation. Within favorable, neutral, and unfavorable midlatitude categories, the initial vortex is displaced to achieve increased (decreased) interactions between the TC remnants and midlatitude development region and, thus, more (less) reintensification. The displaced-vortex simulations indicate an interaction between the TC and the midlatitude circulation may shift the location of the development region and vary the relative contributions from various physical processes to the development. Reintensification is favored when the upper-level TC outflow enhances the equatorward entrance region of a downstream jet streak, and when the TC remnant circulation interacts with the lower-tropospheric baroclinic zone. Thus the interaction is not a static process, but a dynamic process in which both the TC and midlatitude circulation have a contribution.