Enhanced covariability of Indian and Sahelian summer monsoons in recent decades

Abstract Monsoons, in spite of being parts of the global-scale Intertropical Convergence Zone (ITCZ), have well-defined regional characteristics. The Indian summer monsoon and the Sahelian summer monsoon are two important components of this global monsoon system that show substantial interannual variations impacting the livelihoods of hundreds of millions of people. Thus, knowledge of how these systems coevolve and interact would help us understand and predict them better. Here, using a century-long raingauge-based rainfall data set, we show that interannual co-variability between the Indian monsoon rainfall (IMR) and Sahelian monsoon rainfall (SMR) has increased in the recent decades (1985–2020; r = 0.58) as compared to the past decades (1950–1984; r = 0.33). We show that intraseasonal waves generated over central India propagate westward and reach West Africa about a couple of weeks later, triggering moist convection over the Sahel. The African Easterly Jet (AEJ), which peaks in the mid-troposphere, plays a crucial role in this wave propagation. A strong AEJ in recent decades, driven by the increased meridional gradient of tropospheric temperature, helps propagate the intraseasonal waves from the Indian to the Sahelian region. Further, an increase in the total column water vapour during the active phases of the Madden-Julian Oscillation over Africa and the Indian Ocean facilitates such propagation. Our results have important implications for understanding interannual variations of the Indian and Sahelian monsoons and should help interpret seasonal and intraseasonal predictions by numerical models.