Variability of storm activity over the north Indian Ocean during the pre-monsoon (March to May), the post-monsoon (October to December; OND) and the monsoon seasons (June to September; JJAS) has been analysed for a period of 113 years (1891 to 2003). Two kinds of period are identified: a high-frequency period (HFP), when storm activity is enhanced; a low-frequency period (LFP), when storm activity is reduced. Seasonal conditions of sea-surface temperature (SST), low-level wind, vertical wind shear, horizontal wind shear and middle tropospheric humidity are analysed during the monsoon season to discuss the variability of storm activity during the both kinds of periods.
It is found that the frequency of storms during OND shows an increasing trend, whereas during JJAS the number of storms shows a decreasing trend. The trend is statistically significant at the 99.9% level during OND and at the 99% level during JJAS. The corresponding SSTs over the Bay of Bengal region also show a significant increasing trend during all three seasons. Analysis of both oceanic and atmospheric parameters during the HFP and LFP of storms shows contrasting features during the monsoon season. It is also seen that, in spite of higher SSTs, conditions like higher vertical wind shear, lower horizontal wind shear and a lower value of middle tropospheric humidity are associated with decreasing storm frequency during the LFP compared with that of the HFP. Thus, the observations suggest that variability in the large-scale atmospheric circulation is the main cause of the interdecadal variability of storm activity over the Indian region rather than the variability of SSTs over the region.