Wavelet analysis of the association between the Southern Oscillation and the Indian summer monsoon

A new aspect of the monsoon-Southern Oscillation (SO) link has been investigated. All India Summer Monsoon Rainfall (AISMR) and Southern Oscillation Index (SOI) data (for August-September-October months) for the period 1871-1998 have been processed for wavelet analysis. Using the Haar wavelet function, the data are decomposed into seven dyadic scales corresponding to periods of 2, 4, 8, 16, 32, 64 and 128 years. The time frequency localization in the wavelet analysis was used to study the temporal variability of modes in AISMR and SOI. The 2 and 8 year modes in both are found to exhibit low frequency modulation. The 4 year mode in both showed large intermittency. The periods of high/low activities of 2, 4 and 8 year modes were associated with a large/low number of deficient AISMR years. The SOI derived from 2, 4 and 8 year modes in the ENSO years, is found to be related to AISMR variability, at 1 level of significance. The 2, 4 and 8 year modes in AISMR and SOI are found to be correlated at a 5 level of significance. There is a large temporal variability in the correlations of these modes. The occurrences of maxima and minima in these correlations followed a sequence, first in the 8 year mode, then in the 4 year mode and in the end, in the 2 year mode. The reasons for de-association between AISMR activity and SOI in the last 8 years of the present decade have been attributed to (i) the negative contributions by 128, 64, 32 and 16 year modes, (ii) the low activity of 4 and 8 year modes and (iii) the weak correlation between AISMR and SOI in 4 and 8 year modes during this period.