Long-term variability of Indian monsoon rainfall related to warming of Indian and Pacific oceans

Abstract

Summer monsoon rainfall over India, vital for the socio-economic welfare of more than a billion people, exhibits large interannual variability. The sea surface temperatures of the Pacific and Indian oceans have been identified as the two most dominant factors affecting the seasonal rainfall over India. However, the combined influence of these two oceans, especially during the last several warming decades, is not well understood. Without any pre-assumption or pre-filtering of daily rainfall for 121 years, we have extracted space-time modes of the Indian monsoon rainfall related to El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) using a data-adaptive method. We provide a unified framework to understand how ENSO and IOD of the warming oceans simultaneously influence the short-term and long-term variability of monsoon rainfall. We show that the seasonal mean rainfall for different years is determined by the strengths and phases of the ENSO and IOD modes and depends on either constructive or destructive interference of the phases of the two modes. The correlation between observed and combined ENSO and IOD mode rainfall is 0.83 for 1901–2021. The long-term variability of monsoon rainfall is largely related to the relative strengths of the two modes. An important conclusion of this study is that anomalous warming of the Indian ocean has significantly influenced the intensity and the phases of the IOD mode and the associated monsoon rainfall during the past 60 years. This study provides a framework for future research to understand and predict seasonal and long-term variability of monsoon rainfall.