Long range forecasting of summer monsoon rainfall from SST in the central equatorial Indian Ocean
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Several prediction techniques have been developed to predict all India summer monsoon rainfall by using different parameters like sea level pressure tendency during January-April at Darwin, 500 mb location along 75 degrees E during April; sea level pressure change at Bombay during Winter to pre-monsoon and Sea Surface Temperature (SST) over Pacific and Indian Ocean regions. The relationship between SST in the Central Equatorial Indian Ocean (CEIO) and all India Summer Monsoon Rainfall using the data during the period 1945-1999 is examined. The relationship with rainfall over homogeneous regions also has been examined. Online time series rainfall data created by IITM, Pune and MOHSST-6 SST data have been used to compute 21 and 31 year sliding window correlations. Very strong correlations (significant at 99% level) have been observed between average SST during September-November, (SON SST) of the preceding year in the region, 0-5 degrees N, 80-85 degrees E with the all India summer monsoon rainfall in the following year, during the recent decades. The Correlations arc insignificant before 1960 and stable and significant during the recent decades. The correlations for the recent periods, 1978-1999 and 1968-1999 are found to be 0.8 and 0.7 respectively which arc significant at greater than 99% level. To see impact of climate shift (warming of Indian Ocean after 1976) on SST - monsoon rainfall relationship, the correlations have been computed for the period, 1977-95. Very high correlation of 0.82 is noticed between SON SST and all India summer monsoon rainfall during the above period. The correlations with rainfall over homogeneous regions also arc significant at 99% level except North-Eastern and Central North Eastern regions. Study shows the possibility of predicting monsoon rainfall from SON SST of the preceding year in the region, 0-5 degrees N, 80-85degrees E. The mechanism behind these strong correlations could be due to the tropical biennial oscillation (TBO) as reported by Mcchl and Arblaster (2001b).
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