Seasonal and intraseasonal modulations of environmental field for tropical cyclogenesis over the Bay of Bengal

In May 2008, Cyclone Nargis caused catastrophic destruction in Myanmar. From late April to early May including the period of Nargis's life-cycle, the large-scale westerly wind in the lower troposphere over the Bay of Bengal shifted from the equator to ~10N. The shift of westerly flow had longer time scale and larger horizontal scale than those of a tropical cyclone, which seems to be related to the seasonal transition and boreal summer intra-seasonal oscillation (BSISO) in the Asian monsoon. During the northward shift of the westerly flow, based on the genesis potential analysis (Emanuel and Nolan, 2004), the Bay of Bengal was characterized by an environment favorable for the tropical cyclogenesis. Therefore, in the present study, the climatological relation between environmental field and tropical cyclogeneses over the Bay of Bengal between 1982 and 2008 was analyzed using best-track and JCDAS reanalysis datasets.

The composite analysis showed that signal of high genesis potential shifted northward from the equator to the Bay of Bengal with time-scale of ~40 days during the period including tropical cyclogeneses, which resembles the characteristic of the BSISO. The high genesis potential was attributed to high vorticity in the lower troposphere and high relative humidity in the middle troposphere, and partially to weak vertical shear and large potential intensity. Thus, the modulation of large-scale environmental field influenced the tropical cyclogeneses over the Bay of Bengal.

In additional analysis based on the the phase of BSISO, the frequency of tropical cyclogenesis over the Bay of Bengal was apparently modulated by the BSISO. These results are important for the probabilistic prediction of tropical cyclogeneses in the region.