MONSOONAL INTERACTIONS LEADING TO SUDDEN TROPICAL CYCLONE TRACK CHANGES

Sudden poleward track changes of tropical cyclones embedded in monsoon gyres in the western North Pacific are documented. During these track changes, which are generally not well forecast, the cyclones are ofte n accompanied by a separate comma-shaped area of gale-force winds and deep convection along the eastern periphery. This monsoon surge is dis tinct from the tropical cyclone. Synoptic analyses often reveal a buil ding anticyclone to the east or southeast of the monsoon gyre. The hyp othesis that the sudden track change is initiated by a binary interact ion of the tropical cyclone and monsoon gyre is tested with a nondiver gent barotropic model. Tropical cyclone-scale vortices with initial po sitions within the eastern semicircle of a larger monsoon gyre-scale v ortex initially coalesce with the monsoon gyre and then exhibit sudden poleward track changes that are similar to the observations. During t he coalescence phase, the large and relatively weak monsoon gyre under goes a beta-induced dispersion in which nonlinear vorticity advection also plays an important role. This dispersion process produces strong ridging to the east and southeast of the coalesced tropical cyclone an d monsoon gyre. An intermediate region of high winds that resembles th e observed monsoon surge develops between the monsoon gyre and the per ipheral ridging. A southerly steering develops across the coalescing t ropical cyclone and monsoon gyre and causes the poleward acceleration. Key features of the simulated streamfunction and isotach patterns ass ociated with the sudden track changes are substantiated with synoptic analyses of observed cases with similar track changes. Thus, it is con cluded that the tropical cyclone-monsoon gyre interactions are a likel y explanation for monsoon surge track changes and that the observed ph enomena may be explained to first order by conservation of absolute vo rticity on a beta plane.