The Southern Oscillation and its relation to the monsoon rainfall

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Abstract
The interannual fluctuations in the Southern Oscillation indices (Wright, 1975) and their relations to the Indian monsoon (June‐September) rainfall have been examined for the period of 106 years from 1875 to 1980. The monsoon rainfall is significantly (99.9 per cent level) correlated with the Southern Oscillation indices for the seasons: MJJ (0.59), ASO (0.67), NDJ (0.53), and FMA of the following year (0.38). The fluctuations in the Southern Oscillation index for the ASO season appear strongly related to the nearly simultaneous monsoon rainfall of India. This implies that the large positive (negative) value of the Southern Oscillation index, signifying strengthening (weakening) of the Walker circulation coincides with large excess (deficient) monsoon rainfall over India. The coherence spectrum reveals that the Southern Oscillation index and the monsoon rainfall are highly correlated in the period range of about 2–2.5 years and 4–6 years. The first of these periods corresponds with the Quasi‐Biennial Oscillation and the latter agrees with the features of the Southern Oscillation, suggesting a strong link between Indian monsoon rainfall and these two phenomena. The striking feature of the composites of the Southern Oscillation index averaged for all the drought years and for all the flood years is the simultaneous occurrence of low (high) Southern Oscillation index and droughts (floods) in India. However, this association has limited use in long‐range prediction. A preliminary study suggests that a nearly simultaneous occurrence of major climatic anomalies of the tropics, such as droughts in India and El Niño off the coast of Peru, are linked to the Southern Oscillation, indicating some kind of time dependent zonal east‐west circulation, i.e. Walker circulation.