A preliminary study on the variability of post monsoon Tropical cyclone activity over the north Indian Ocean

Abstract

By the composite method, the long period seasonal mean charts of flow features at 850 hPa and 200 hPa, vertical wind shear and monthly (September) mean charts of SST anomaly for periods with large term activity and small storm activity were constructed to examine the circulation features associated with inter-annual variability of post monsoon (October-December) tropical cyclone activity over the north Indian Ocean. It was found that the prominent circulation features for the periods with small storm activity are: Absence of ITCZ at 850 hPa north of 5°N, weaker upper level subtropical westerlies, southward shift of 200 hPa ridge and relatively larger vertical wind shear over most of the oceanic area. Negative SST anomalies during the month of September also tend to be unfavorable for the formation of storms.

A hypothesis is made for the causes of these circulation changes and SST anomalies. Changes in intensity of monsoon circulation. Cause to develop SST anomalies. These SST anomalies affect the atmosphere by large scale ocean-atmospheric interaction and thus creates an environment favorable or unfavorable for storm development.