Changing trends in the tropical Indian Ocean SST during La Nina years

The Tropical Indian Ocean ( TIO) was characterized by anomalous sea surface cooling during the La Nina years ( winter season) before 1976. Analysis of Hadley center Ice Sea Surface Temperature ( HadISST) showed that the cooling ( SST anomalies < - 0.1 degrees C) started during the boreal summer over most of the TIO ( 69% area) in the La Nina composites of 1958 - 1976, and which enhanced by the following winter, resulting in basin- wide cooling ( 98%). The upwelling biennial Rossby wave propagation from the east to the western Indian Ocean enhanced the cooling during boreal summer to the following winter. However, the La Nina composites of 1976 - 2001 showed strong warming over the TIO in boreal summer, which even though weakened in the following winter, did not cause any basin- wide cooling. The variability in the latent heat flux anomalies after 1976 was found to be consistent with the SST anomalies.