Indian Ocean SST and Indian summer rainfall : Predictive relationships and their decadal variability

We examine relationships between Indian Ocean sea surface temperature (SST) variability and the Indian monsoon, including analysis of potential long-lead predictions of Indian rainfall by regional SST and the influence of ENSO and decadal variability on the stability of the relationships. Using monthly gridded (4°x 4°) SST data from the Global Sea-Ice and Sea Surface Temperature (GISST) dataset that spans 1945-94, the correlation fields between the All-India Rainfall Index (AIRI) and SST fields over the tropical Indian Ocean are calculated. In the boreal fall and winter preceding the summer Indian monsoon, SST throughout the tropical Indian Ocean correlates positively with subsequent monsoon rainfall. Negative correlation occurs between SST and the AIRI in the subsequent autumn in the northern Indian Ocean only. A strong correlation (0.53) is found between the summer AIRI and the preceding December through February Arabian Sea SST. The correlation between the AIRI and the SST to the northwest of Australia for the same period is 0.58. The highest correlation (0.87) is found for the years following 1977 between the AIRI and the central Indian Ocean SST in the preceding September through November, but this relationship is much weaker in earlier years. Based upon these correlations, we define Arabian Sea (AS1), northwest Australia (NWA1), and central Indian Ocean (CIO1) SST indices.

The relationship of these indices to the AIRI and ENSO are examined. We find that the high correlation of the AS1 and NWA1 SST indices with the Indian summer rainfall are largely unaffected by the removal of the ENSO signal, whereas the correlation of the CIO1 index with the AIRI is reduced. We examine the interdecadal variability of the relationships between SST and the AIRI and show that the Indian Ocean has undergone significant secular variation associated with a climate shift in 1976. The possible mechanisms underlying the correlation patterns and the implications of the relationship to the biennial nature of the monsoon and predictability are discussed.