Study Of The Influences Of Monsoon Gyres On Formation And Sudden Track Change Of Typhoons

The summertime monsoon circulation over the tropical western North Pacific (WNP) and South China Sea (SCS) is usually characterized with a low-level monsoon trough. The monsoon trough is closely associated with most of tropical cyclone (TC) formation in the WNP and SCS and undergoes substantial migration and structural change with time. Sometimes the monsoon trough is replaced by a large-scale monsoon depression or gyre, which is a nearly circular cyclonic vortex with a diameter of about 2500 km. Studies suggest that such a monsoon gyre has important implications for TC formation, structure and motion in the WNP and SCS. While the majority of previous studies focused on the statistical analysis and barotropic numerical simulations, the mechanisms of the formation and sudden track change of typhoons are not well understood, due mainly to the sparsity of the observational data over the open oceans. By using WRF model, this study investigates the Rossby energy dispersion associated with monsoon gyres, and its influences on the typhoon formation and sudden track changes.hi baroclinic and diabatic conditions, an idealized axisymmetric monsoon gyre evolves to a similar structure to an observed composite. Beta-induced energy dispersion associated with monsoon gyre induces a Rossby wave train in the northwest-southeast direction to its southeast. This Rossby wave train develops and stretches upward to 400 hPa with well organization in the middle and lower troposphere, which agrees well with the observed.The initial environmental humidity and the size of monsoon gyre have influences on the energy dispersion.The Rossby wave energy dispersion associated with monsoon gyre provides the favorable conditions and initial disturbances for the formation of typhoons. The simulated typhoon forms near the center of the monsoon gyre, where the mesoscale convective systems merge at the northeast end of the enhanced southwesterly flows with maximum mid-level relative vorticity. Sudden northward track changes occur when the typhoon moves close to the center of monsoon gyre. The formation of the second typhoon is associated with the mesoscale convective systems with maximum low-level relative vorticity in the northwest quadrant of the monsoon gyre.The size of the monsoon gyre may be one of the important parameters controlling typhoon formation. The interaction of low-frequency monsoon gyres with typhoons can lead to sudden track changes. This study examines the influence of the monsoon gyres on the abnormal activities of Typhoon Morakot (2009). The results show that the twice coalescences of the low-frequency monsoon gyres with Typhoon Morakot lead to the northward shifts in its track and the slow-down in its translation in the vicinity of Taiwan, while the westward movement prior to the landfall on Taiwan is closely associated with a nearly zonal synoptic-scale wave train-like pattern.