The spatial and temporal variability of extreme rainfall events over India and their long‐term trend have been analysed during the southwest monsoon season from June to September (JJAS) by taking daily rainfall data for 55 years (1951–2005). The contribution of extreme rainfall events (Category iii with rainfall > 124.4 mm day⁻¹) to the total seasonal rainfall is also analysed for the same period. The analysis shows that the occurrence of extreme rainfall events over India during JJAS shows spatial variations with preferred regions of occurrence over the west coast, central parts of India and northeast India. The average frequency of extreme rainfall events along with the contribution of extreme rainfall events to the seasonal rainfall shows a significant increasing trend (above the 98% confidence level) over the Indian region during JJAS, and also during June and July. It is also found that the increasing trend of contribution from extreme rainfall events during JJAS is balanced by a decreasing trend in Category i (rainfall ≤ 64.4 mm day⁻¹) rainfall events. Using the filtered data it is found that the increasing trend of extreme rainfall over central India is mainly contributed by synoptic scale systems (periodicity between 3 and 7 days). The seasonal mean moist convective instability (CI) during JJAS averaged over central India indicates a significant increasing trend (99.9% confidence level) and is basically due to the increasing trend of the number of days with greater degree of moist CI during JJAS, which may be one possible cause for the increasing trend in the frequency of extreme rainfall events over the region. Copyright © 2009 Royal Meteorological Society

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. Copyright © 2005 Royal Meteorological Society