Contribution of tropical waves to the formation of Supertyphoon Megi (2010)

Through observational analysis and numerical simulations, this study examines the roles of the Madden-Julian oscillation (MJO) and tropical waves in the three-stage formation of Supertyphoon Megi (2010) including 1) convective bursts followed by vorticity aggregation, 2) vortex rearrangement during decaying convection, and 3) convective reinvigoration and vortex intensification. The MJO was responsible for preconditioning the large-scale circulation and low-level moisture favorable for convection during all stages, while the counterpropagating Kelvin and equatorial Rossby (ER) waves brought low-level convergence and cyclonic vorticity anomalies to enhance massive convection in the western tropical Pacific in stage 1. Convection strengthened the vorticity anomalies nearby, which subsequently developed into Megi's embryo by the end of stage 1 through merging with the positive vorticity anomaly carried by a westward-propagating mixed Rossby-gravity and tropical depression (MRG-TD)-type wave. The ER- and MRG-TD-type waves might also contribute to Megi's formation through increasing low-level southwesterlies to the southwest of the precursor during stages 2 and 3. These tropical waves also indirectly affect Megi's genesis through modulating surroundings near the precursor. Without the MJO, the low-level vorticity anomaly to the near west of the precursor would intensify more effectively and develop into a tropical cyclone instead of the observed Megi. Removing the Kelvin or ER wave would enhance convection to the far west of Megi's precursor, which was less favorable for low-level convergence in the region of the precursor, and thus the formation of Megi.