Monsoonal Influence on the Track Change of Typhoon Morakot (2009)

Typhoon Morakot (2009) formed in the tropical western North Pacific on 3 August 2009 and made landfall over Taiwan late on 7 August 2009, leading to the worst flooding over the past 50 years in southern Taiwan. While previous studies emphasized the influence of southwesterly winds associated with intraseasonal oscillations and monsoon surges on moisture supply, the interaction between Morakot and low-frequency monsoon flows and the resulting influence on the slow movement and northward shift of the typhoon were examined through observational analysis and numerical simulation. Morakot generally moved westward prior to its landfall on Taiwan and underwent a coalescence process first with a cyclonic gyre on the quasi-biweekly oscillation (QBW) time scale and then with a cyclonic gyre on the Madden-Julian oscillation (MJO) time scale. The coalescence enhanced the synoptic-scale southwesterly winds of Morakot and thus decreased its westward movement and turned the track northward, leading to an unusually long residence time in the vicinity of Taiwan. The numerical experiments were conducted to further confirm the influences of multi-time scale monsoonal flows on the track change of Morakot. The control simulation captured the slowing and northward deflections in the vicinity of the Taiwan Island, the highly asymmetric rainfall structure, and the associated rainfall pattern. The sensitivity experiments suggested that Morakot moved westward directly towards Taiwan due to a synoptic wavetrain-like pattern, which was consisted of Goni over mainland China, Morakot, and a cyclone over the western North Pacific with an anticyclone to the west of Morakot. Then the northward track shifts that occurred in the vicinity of the Taiwan Island were a result of the coalescences of Morakot with a QBW-scale gyre prior to the landfall on Taiwan and a MJO-scale gyre in the Taiwan Strait. In agreement with the previous study, the interaction between tropical cyclones and low-frequency monsoon gyres can cause sudden changes in tropical cyclone tracks.