Observational Analysis Of The Inlfluences Of Monsoon Troughs And Monsoon Gyres On Tropical Cyclone Formation Over The Western North Pacific

The monsoon circulation is the most remarkable feature over the western North Pacific in summer, which is usually characterized with a monsoon trough or a monsoon gyre. Previous studies show that they play an important role in the tropical cyclone (TC) formation over the western North Pacific. To further understand the influences of the two systems on TC formation and improve our understandings on TC formation, the TC formation events associated with monsoon troughs or monsoon gyres or synoptic-scale disturbances are reexamined through the combination of the TC removal and Lanczos filtering methods on isolating monsoon troughs or monsoon gyres from the synoptic-scale systems based on the National Centers for Environmental Prediction Final global analysis data with a 1.0°×1.0° resolution.It is found that the percentage of TC formation events associated with monsoon troughs and monsoon gyres to the total of TC formation events during May-October from 2000 to 2010 is about 62.9%.43.1% TCs formed in monsoon troughs while 19.8% TCs in monsoon gyres. The percentage of TC formation events in monsoon troughs is lower than those reported in previous studies due to the inclusion of TC formation events in monsoon gyres in some previous studies. The composite analysis shows that nearly all the TCs that formed in monsoon troughs formed in the monsoon shear region. This differs from the previous studies claiming that more TCs formed in the monsoon confluence zone. Compared to the confluence zone, the shear region has stronger low-level relative vorticity and the prevailing easterly vertical wind shear, which are favorable for TC formation. The occurrence of the monsoon gyre reach a peak in August-October while monsoon gyres during this period are located on the poleward side of the composited monsoon trough. The cyclonic circulation of the composited monsoon gyre reaches its maximum at 850 hPa and weakens with the height, and is replaced by negative relative vorticity above 200 hPa. Most of the TCs associated with monsoon gyres formed near the centers of monsoon gyres and the northeast end of the enhanced monsoonal southwesterlies, where the vertical wind shear is relative weak.Based on the cases of TC formation within monsoon troughs, the synoptic-scale influences on TC formation are investigated. The presence of the synoptic-scale disturbances can enhance the mid- and low-level relative vorticity and convergence, but contribute little to the vertical wind shear. In addition, the important role of the TC-precursor disturbance in TC formation is the establishment of a limited rotation-dominant region for TC formation, where the cyclonic rotation enhances with the time. Nearly all the particles keep recirculating in the core region with a radius of 220 km. The case of Typhoon Bilis (2006) suggests that the vorticity may increase through the merger of the mesoscale convective systems and the enhancing rotation in the core region may lead to a more efficient conversion from diabatic heating into kinetic energy for TC formation.