Decadal variability of tropical Indian Ocean sea surface temperature and its impact on the Indian summer monsoon

Abstract

Decadal variability of climate over the Pacific and Atlantic Ocean is well studied. However, the decadal climate variability over the Indian Ocean and its possible impact on the summer monsoon received relatively less attention. The present study aims to explore the decadal variability of the Tropical Indian Ocean (TIO) sea surface temperature (SST) and its associations with the Indian summer monsoon rainfall (ISMR) variability. More than a hundred years of observed monthly SST data from Extended Reconstructed sea surface temperature and rainfall data from India Meteorological Department are used for the analysis. In addition to these, century reanalysis fields of winds, moisture, vertical velocity, tropospheric temperature, and sea level pressure are used for diagnosing different processes. Time series and wavelet analysis confirmed the presence of decadal variability (~ 9 to 30 years) in the TIO SST. The decadal variance of TIO SST is maximum in the eastern equatorial Indian Ocean, followed by the north Arabian Sea. Decadal EOF of TIO SST shows a dominant basin-wide mode explaining about 50% of total variance and has robust decadal variability during 1940 to 1952; wavelet analysis supported this robust signal statistically. Similar analysis for the ISMR reveals that the decadal variance of rainfall has significant strength over monsoon core zone and Western Ghats. The EOF analysis further confirms this spatial pattern of rainfall decadal variability over India. Correlation between of decadal TIO SST and over monsoon core zone (MCZ) rainfall is significant with 2 years lag. To understand how the decadal variability of TIO SST influences the ISMR, monsoon features during strong warm and cold phase are studied. During the warm phase, MCZ and Western Ghats receive more rain than normal and vice versa for cold phase. Which is consistent with strong southwesterly winds, strong pressure gradient, and strong convergence over the MCZ for the warm phase. Also, during the warm phase, positive anomaly of mid-troposphere temperature, vertical velocity, and moist static energy are found to be associated with excess convective activity. Apart from this, larger scale zonal (Walker) and meridional (Hadley) circulation fields are also in phase with the TIO SST and rainfall variability. Our study advocates that decadal variability in TIO SST influences the monsoon dynamics and moist thermodynamics leading to near in-phase changes in the rainfall over the MCZ and Western Ghats region.