Solar effect on rainfall in West Bengal

Abstract

The General characteristics of rainfall and it's correlation with sunspot during 1931-81 over 8 stations in West Bengal have been studied. An average dispersion of ~ 20% has been noticed with an increasing tendency for decreasing latitude. Three years moving average analysis reveals that whenever the sunspot number increases or decreases the rainfall does so almost simultaneously except during solar max. years. But average period of rainfall cycle is half of the period of solar cycle, i.e., 5.5 years, a period compatible with the southern oscillation. Moreover, in SS maximum years the rainfall decreases with the increase of SS number. Calculations of average rainfall during different sunspot phases indicate that the annual rate of rainfall is greater during solar maximum to minimum phase than that during minimum to maximum phase. However, the difference in the annual rates is not much, only 15% for two stations and much less for other stations. Using a growth rate model it is found that in 69% cases the rainfall decreases as sunspot number decreases. Further when the growth rate and other studies of rainfall in Sub Himalayan West Bengal and Gangetic West Bengal arc made separately it appears that the rainfall of these two regions are oppositely affected by solar activities. The correlation coefficients between rainfall and sunspot of same year, of previous year and of two years before have come out as <= + 0.35 and mostly it is positive. Further, sunspot's influence seems to be most effective on rainfall of the following year.