Identification and Variability Analysis of New Homogeneous Summer Monsoon Rainfall Regions Over India by Using K-Means Clustering Technique

Abstract

The identification of new homogeneous Indian summer monsoon rainfall regions is important for both research and operational forecasting purposes. The homogeneous rainfall regions used by the India Meteorological Department include areas that are actually dissimilar in terms of both the amount of rainfall and its variability. In the present study, we have identified three primary clusters of homogenous summer monsoon rainfall zones (low, medium and high rainfall) of India by using K-means clustering technique and Calinski–Harabasz (CH) index. The identified clusters are again subdivided based on the distance between the clusters into six homogenous rainfall zones such as Southeast India (SI), West India (WI), Central India (CI), West Coast of India (WC), Northeast India1&2 (NE1 & NE2). Summer monsoon rainfall exhibit a positive trend over the western and southern region of India (SI, WC, and WI) while the eastern part of India (CI, NE1 and NE2) exhibit a negative trend. The increased sea surface temperature of tropical north Atlantic during May is conducive for the enhanced summer monsoon rainfall activity over low rainfall zone (Southeast India and West India). Tropical North Atlantic SST during May is related with the subsequent warming of Indo Pacific Ocean and cooling of Central Pacific Ocean, which induces southeasterlies towards Indian region and strengthen the rainfall activity over SI and WI. The position and intensity of the upper-level circulations have profound impact on the interannual variability of rainfall over different homogenous regions. Above-normal rainfall over SI and WI (CI and WC) is enhanced by the upper-level anticyclonic (cyclonic) circulation in Bay of Bengal (West Asia), which extends north westward (south eastward) to the Indian landmass. Indian Ocean warming during May is associated with above (below) normal rainfall over NE2 region (the eastern part of CI). Southern Ocean SST during May is significantly correlated with the rainfall over southwest coast of India, which is also indicated by the positive correlation between April southern Annular Mode and south west coast rainfall. Sea surface temperature during May in different ocean basins can be used as the potential predictors for improving the long-range forecast of seasonal rainfall over newly identified homogenous rainfall zones.