Multi-Decadal Variability in the Indian Summer Monsoon Rainfall: Evidences through Proxy Sources
Based on the analysis of the observed Indian summer monsoon rainfall (ISMR) for the period 1871-2000, a 60-year cycle appeared to be evident. However, the decreasing trend in the ISMR and the increase in the number of deficient monsoons during the last two decades suggests a re-examination of this 60-year periodicity. Since observed data are available for about 150 years since 1871, this issue is examined through proxy sources. Using three different proxies of monsoon rainfall for a duration of over 500 years, interesting results emerge. Results suggests that the Asian monsoon has a multidecadal oscillation that changes in time in an episodic manner. Using Empirical Mode Decomposition / Intrinsic Mode Function analysis, we show that the multi-decadal oscillation could be considered as a 'mode' of variability of the Asian monsoon. We further find that, Atlantic Multi-decadal Oscillation (AMO), El Niño Southern Oscillation (ENSO) as well as the Pacific Decadal Oscillation (PDO) also have such a multi-decadal variability. Coherence between Asian monsoon multi-decadal mode and that of AMO, ENSO and PDO shows that they all are likely to be integral part of a global multi-decadal mode. We speculate that such a multi-decadal mode of variability may arise from ocean-atmosphere-land interactions.