Seasonal and Intraseasonal Modulation of Tropical CyclogenesisTI Seasonal and Intraseasonal Modulation of Tropical Cyclogenesis Environment over the Bay of Bengal during the Extended Summer Monsoon

摘要

The environmental field of tropical cyclogenesis over the Bay of Bengal is analyzed for the extended summer monsoon season (approximately May-November) using best-track and reanalysis data. Genesis potential index (GPI) is used to assess four possible environmental factors responsible for tropical cyclogenesis: lower-tropospheric absolute vorticity, vertical shear, potential intensity, and midtropospheric relative humidity. The climatological cyclogenesis is active within high GPI in the premonsoon (similar to May) and postmonsoon seasons (approximately October-November), which is attributed to weak vertical shear. The genesis of intense tropical cyclone is suppressed within the low GPI in the mature monsoon (approximately June-September), which is due to the strong vertical shear. In addition to the climatological seasonal transition, the authors' composite analysis based on tropical cyclogenesis identified a high GPI signal moving northward with a periodicity of approximately 30-40 days, which is associated with boreal summer intraseasonal oscillation (BSISO). In a composite analysis based on the BSISO phase, the active cyclogenesis occurs in the high GPI phase of BSISO. It is revealed that the high GPI of BSISO is attributed to high relative humidity and large absolute vorticity. Furthermore, in the mature monsoon season, when the vertical shear is climatologically strong, tropical cyclogenesis particularly favors the phase of BSISO that reduces vertical shear effectively. Thus, the combination of seasonal and intraseasonal effects is important for the tropical cyclogenesis, rather than the independent effects.