Evidence for Strong Links between Indian Rainfall and Solar Activity

Subarna Bhattacharyya [subarna@jncasr.ac.in] and Roddam Narasimha [roddam@jncasr.ac.in],Jawaharlal Nehru Center for Advanced Scientific Research, Jakkur Bangalore-560064, India.

            Over the 120 y period (1871--1990) both the Indian rainfall and solar activity parameters exhibit nonstationarity. Noting this fact, we present here an analysis of four solar activity indices and seven major Indian monsoon rainfall time series, over two distinct test periods respectively of low and high solar activity, each comprising three complete solar cycles. Higher average rainfall is observed during periods of greater solar activity for all cases, at confidence levels varying from 75% to 99%, being 95% or greater in three of them. Analysis using wavelet techniques reveals higher power in the 8--16 y band during the higher solar activity period, in 6 of the 7 rainfall time series, at confidence levels exceeding 99.99% (see, Bhattacharyya and Narasimha, 2005, GRL). Furthermore, the wavelet cross spectra of the yearly sunspot index and homogeneous Indian monsoon rainfall are examined using Morlet continuous wavelets. The cross spectra of solar activity index with noise ensembles, including those simulating  the spectrum and probability distribution function of the rainfall time series, are compared with the cross spectrum between rainfall and solar activity index; their differences are studied using standard statistical significance tests. It is found that over the two test-periods respectively of high and low solar activity, the average cross power of the solar activity index with rainfall exceeds that with the noise at z-test confidence levels exceeding 99.99% over period-bands covering the 11.6 y sunspot cycle. These results provide strong evidence for connections between Indian rainfall and solar activity.