Early prediction of the Indian summer monsoon rainfall by the Atlantic Meridional Mode

Indian summer monsoon rainfall (ISMR) provides 80% of India’s total annual precipitation and thus has a tremendous impact on water resource management, agricultural yield and,consequently, India’s gross domestic product. Because of its importance, different studies have examined climatic controls on ISMR. Here we found a link between the spring (March–May; MAM) Atlantic meridional mode (AMM) and the ISMR, with the strengthening (weakening) of ISMR during the positive (negative) phase of the AMM. During the positive phase of the spring AMM, warmer sea surface temperatures (SSTs) persist over the tropical north Atlantic Ocean, strengthening anomalous cyclonic circulation and strengthening convection over the Sahel region; this process, in turn, modulates the winds over the Western Indian Ocean, cooling SST and strengthening monsoon circulation over India. However, during the negative phase of the spring AMM, we notice a weakening of the monsoon circulation over India. As a way of highlighting the importance of this climate connection, we compare the skill in forecasting ISMR using the information from the pre-monsoon AMM against the forecasts from the North American Multi-Model Ensemble (NMME). Over the years 1981–2017, our simple model produces more accurate results than the NMME models, and further highlights the crucial role played by pre-monsoon AMM in modulating the IMSR. This new finding will allow for Indian farmers and policy makers to better anticipate water availability from ISMR.