Extratropical transition in the western North Pacific: Demonstration of the importance of phasing with the mid-latitude circulation pattern during the re-intensification stage

In the western North Pacific, tropical cyclones (TCs) that complete extratropical transition (ET) can become powerful, mid-latitude storms capable of causing significant damage to coastal and maritime interests. Klein et al. (1999) defined ET in terms of two stages: transformation, where the TC evolves into a baroclinic cyclone, and re-intensification, where the transformed storm then re-deepens. Previous studies (DiMego and Bosart 1982, Sinclair 1993, Foley and Hanstrum 1994, Klein 1997, Harr et al. 1999) have suggested that superposition of regions of upper-tropospheric divergence and positive vorticity advection (PVA) in the mid-latitude circulation pattern over lower-tropospheric baroclinity and cyclonic vorticity associated with the transformed TC triggers Petterssen "Type-B" extratropical cyclogenesis during the re-intensification stage of ET.

It is hypothesized that the final central sea-level pressure (SLP) observed at the end of the re-intensification stage will depend only on the phasing of the poleward-moving TC with the critical region in the mid-latitude synoptic circulation pattern that supports deep and/or rapid Petterssen "Type-B" extratropical cyclogenesis (that is, where the largest values of upper-tropospheric divergence and positive vorticity advection are found). In this study, the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) will be used to study several ET cases that occurred in the western North Pacific during June through October of 1994-99. Navy Operational Global Atmospheric Prediction System (NOGAPS) analyses of these cases at 1 degree lat./long. resolution will be used to initialize a 72-h COAMPS forecast of ET that will serve as the control forecast. The TC vortex will then be removed from the NOGAPS analyses using the method developed by Kurihara et al. (1993, 1995) for use with the Geophysical Fluid Dynamics Laboratory (GFDL) model, and a 72-h COAMPS forecast of the evolution of the environmental analysis field without a TC vortex will be completed. The initial position of the removed TC vortex will then be varied in the analysis field to produce a series of COAMPS 72-h forecasts where cases of deep re-intensification (final central SLP below 980 mb) achieve only moderate re-intensification (final central SLP above 980 mb but below 1000 mb), or do not re-intensify at all, if they fail to arrive in the mid-latitudes in phase with the critical region of support for deep and/or explosive Petterssen "Type-B"extratropical cyclogenesis. Likewise, deep re-intensification can be simulated in cases where the TC either achieved moderate re-intensification, or failed to re-intensify, by translating the initial TC vortex position such that the transformed TC arrives in the mid-latitudes in phase with the critical region of support for deep and/or explosive Petterssen "Type-B" extratropical cyclogenesis. If successful, these simulations will demonstrate that strong (weak) re-intensifications can be made weaker (stronger) by less (more) favorable phasing with the mid-latitude circulation.