The Tropical Indian Ocean (TIO) was characterized by anomalous sea surface cooling during the La Niña years (winter season) before 1976. Analysis of Hadley center Ice Sea Surface Temperature (HadISST) showed that the cooling (SST anomalies < −0.1°C) started during the boreal summer over most of the TIO (69% area) in the La Niña 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 Niña 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.
Using the National Oceanic and Atmospheric Administration (NOAA) sea surface temperature (SST) data, we found that the peak Indian summer monsoon season SST in the northern Bay of Bengal (NBOB) has been increased since late 1990s. Moreover, the NBOB SST reveals an increasing trend after 1999 (hereafter POST99), whereas prior to 1999 (PRE99) the trend is decreasing. During POST99, the NBOB SST facilitates a deep convection, while during PRE99 it does not seem so. A robust causality analysis also reveals a large impact on the convection and precipitation over the western Tibetan Plateau during POST99, whereas no significant causality is identified before 1999. Besides, regressions onto the NBOB SST result in a cyclonic circulation pattern and ascending winds on upper vertical levels over the western Tibetan Plateau during POST99, which are distinctly different from those during PRE99.
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