Regional differentiation in multidecadal connections between Indian monsoon rainfall and solar activity

Abstract

The wavelet cross spectra of the yearly sunspot index and homogeneous-zone Indian monsoon rainfall time series are examined over the 120 a period 1871-1990 using Morlet continuous wavelets. The cross spectra of sunspot numbers with synthetic noise ensembles, including those matching the spectrum and probability distribution function of the rainfall time series, are used as reference to assess the significance of the cross spectrum between sunspot numbers and rainfall; their differences are studied using standard statistical tests. It is found particularly revealing to consider the two test periods 1878-1913 and 1933-1964, each comprising three complete solar cycles, which between them exhibit maximum contrast in solar activity since the beginning of accurate rainfall data collection in India [1871]. It is shown that the cumulative distribution of the 9-13 a band averaged cross power between sunspots and rainfall, derived from yearly values over either test period, differs from that between sunspots and spectrally matched noise in the same period at confidence levels of 99.5% or higher by the χ² test. Further, the cross power of the sunspot index with rainfall, averaged over either test period, exceeds that with synthetic noise at z test confidence levels exceeding 99.99% over scale bands covering the 11.6 a sunspot cycle. The results further show variations across the different homogeneous rainfall zones of India, northeast India exhibiting a dramatically different phase relationship compared to the western regions of the country. The strong connections demonstrated here between Indian rainfall and solar activity are found to be organized approximately along quasi-longitudinal bands tilting eastward at their northern end. This suggests that higher solar activity causes intensification of the Walker and Hadley circulations; the mechanisms that may be responsible and the evidence from models are briefly discussed.