Environmental and External Factors in the Genesis of Tropical Cyclone Nargis in April 2008 over the Bay of Bengal

Tropical cyclone Nargis was generated over the Bay of Bengal (BoB) in late April 2008; and it devastated the coastal area of Myanmar. This study reveals the environmental and external factors in its genesis. It is found that large-scale environmental conditions around the genesis point were generally favorable for cyclogenesis. Positive lower-tropospheric relative vorticity and the weak vertical shear of horizontal wind were observed; which were distinctive features of April 2008 that are not seen in April climatology. Sea surface temperature and atmospheric thermodynamic profile were also favorable for cyclogenesis, as in climatology. Mid-tropospheric humidity was significantly low and unfavorable until six days before the genesis; however; it increased abruptly to become favorable just before the genesis. The atmospheric disturbance that seemed to trigger the genesis was lower-tropospheric easterly surge blowing into the genesis point. Horizontal convergence at the head of the surge associated with active convection was considered as an External Forced Convergence that triggered cyclogenesis. In addition, the active convection seemed to contribute to the preconditioning of mid-tropospheric humidity. The easterly surge was associated with a cold surge in the eastern coastal area of the Eurasian continent, which is a typical feature of cold surges in April.