Genesis of Typhoon Chanchu (2006) during the MJO: Formation and synoptic evolution of a tilted vortex

While the development of tropical cyclone (TC) precursors during the MJO is common, the ability to distinguish between the few that develop into TCs and the many that fail to intensify remains poor. In this study, the early life cycle of a developing West Pacific tropical disturbance is investigated from its initial formation during the active MJO up to tropical cyclogenesis (TCG). Results of this case study of Typhoon Chanchu (2006), based on observational analyses and a high-resolution WRF simulation, call attention to the criticality of the vertical structure of the incipient cyclonic disturbance. The coherent large-scale vertical tilt appears to play a nontrivial role in TCG.

The pre-Chanchu disturbance develops in late April in association with the active phase of the MJO and in the vicinity of an eastward-surging westerly wind burst (WWB). Observations show that the WWB exhibits a westward vertical tilt during its eastward transit into the Pacific. The cyclonic pre-Chanchu vortex similarly exhibits a westward tilt at the time of initial formation.

A WRF simulation captures many salient features of the disturbance, including initial formation in the zone of horizontal shear where the WWB impinges on the easterly trades. As observed, the simulated vortex maintains a westward component to its tilt throughout the week prior to TCG. Previous research dictates that the balanced response of a tilted vortex is forced ascent (descent) on the downtilt-right (-left) half of the vortex. In the simulation, the forced downtilt-right isentropic ascent appears sufficient to maintain an area of precipitation to the north of the pre-Chanchu vortex.

Downtilt-right precipitation persists during the week-long pre-genesis, during which time the vertical tilt of the vortex diminishes. A synthesis of the present results and those established in the literature regarding tilted vortices suggests a possible mechanism by which westward-tilted vortices that develop during the MJO could deviate poleward and intensify due to the positive feedback between the persistent westward tilted vortex and downtilt-right precipitation processes.