Significant longer-term periodicities in the proxy record of the Indian monsoon rainfall

Abstract

Fast Fourier transform (FFT) and the maximum likelihood analyses (MLA) of stable carbon and oxygen isotope records (δ¹⁸O and δ¹³C time series, proxies for monsoon rainfall) of the last 331 years from an annually laminated speleothem reveal significant power in several periods that have a likely solar origin, e.g. 132, 21, 18, and 2.4 years. These cycles are non-stationary in nature. Using wavelet analysis we find that the ~21-year period is strong during 1850-1920 A.D. Between 1780 and 1920 A.D., low rainfall intervals are concurrent with low solar activity. However, this behaviour breaks down for the older periods. In the δ¹³C periodogram, additional significant periods appear viz. ~59, ~8, ~6.5 and ~3 years: these could have originated from solar variations and/or changes in the biological degradation of soil carbon. Surprisingly, while the low power solar cycles (viz. ~22 year and ~2.4 year) are seen in the δ¹⁸O and δ¹³C spectra with the ~21 year cycle dominating, the stronger ~11 year cycle is only weakly represented in the proxy record, confirming earlier findings based on a more limited data set.