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 often accompanied by a separate comma-shaped area of gale-force winds and deep convection along the eastern periphery. This monsoon surge is distinct from the tropical cyclone. Synoptic analyses often reveal a building anticyclone to the east or southeast of the monsoon gyre. The hypothesis that the sudden track change is initiated by a binary interaction of the tropical cyclone and monsoon gyre is tested with a nondivergent barotropic model. Tropical cyclone-scale vortices with initial positions within the eastern semicircle of a larger monsoon gyre-scale vortex initially coalesce with the monsoon gyre and then exhibit sudden poleward track changes that are similar to the observations. During the coalescence phase, the large and relatively weak monsoon gyre undergoes 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 and monsoon gyre. An intermediate region of high winds that resembles the observed monsoon surge develops between the monsoon gyre and the peripheral ridging. A southerly steering develops across the coalescing tropical cyclone and monsoon gyre and causes the poleward acceleration. Key features of the simulated streamfunction and isotach patterns associated with the sudden track changes are substantiated with synoptic analyses of observed cases with similar track changes. Thus, it is concluded that the tropical cyclone-monsoon gyre interactions are a likely explanation for monsoon surge track changes and that the observed phenomena may be explained to first order by conservation of absolute vorticity on a beta plane.