The Unusual Track and Rapid Intensification of Cyclone Nargis in 2008 under a Characteristic Environmental Flow over the Bay of Bengal

抄録

Cyclone Nargis in 2008 was the worst natural disaster in the recorded history of Myanmar. Using observational and reanalysis datasets, this paper describes the features of Nargis’s track, intensity change, and environmental conditions. The cyclone track, recurving eastward over the Bay of Bengal and reaching Myanmar, is quite rare in the track record of recent 30 years. On the other hand, this record also shows that a limited number of cyclones with similar tracks formed in a concentrated period (i.e., April). Moreover, they are similar to Nargis in terms of undergoing rapid intensification before landfall. An analysis of microwave satellite data revealed a significant change in the structure of Nargis before and after the recurvature. In the analysis of environment, the relevance of a midtropospheric flow to the cyclone’s recurvature and structural change is focused. The flow in the north of the bay was characterized by a subtropical jet with low relative humidity in the middle troposphere due to subsidence along the southern slope of the Tibetan Plateau. A southward branch of the dry flow forced the cyclone’s movement to change from northward to eastward, while the intrusion of dry air into the cyclone center interrupted the convective development. After the recurvature, this dry flow meandered to the south of the cyclone, and this caused a decrease in vertical shear near the cyclone center, leading to the redevelopment of convection and intensification of the cyclone. The characteristic meander of a midtropospheric dry flow was also analyzed in the environment of the former cyclones that reached Myanmar, suggesting the applicability of a similar scenario to their intensification. These results suggest the importance of a midtropospheric flow as a determinant of cyclone track and intensification over the Bay of Bengal in spring.