A Monsoon Bridge Across Continents: Untangling the Strengthening Link Between Indian and Sahel Rainfall

Monsoons, although embedded within the large-scale Intertropical Convergence Zone, exhibit distinct regional dynamics. Two major components viz Indian Summer Monsoon and Sahelian Summer Monsoons display substantial interannual variability that affects a significant part of the world’s population. Thus, understanding how these systems interact is essential for the predictability both at the intraseasonal and interannual timescales. 

In this study, we combine observational datasets and reanalysis products to investigate a dynamical pathway that couples the two Monsoon systems. We also analyze the strength of this coupling in a changing climate. Our analysis suggests that the Indian Monsoon Rainfall (IMR) and Sahelian Monsoon Rainfall (SMR) have become coupled in recent decades (1985–2020), showing a much stronger interannual relationship than during 1950–1984. This enhanced coupling is closely linked to large-scale dynamical changes, particularly those associated with the African Easterly Jet (AEJ). 

The coupling between the two systems is governed by the intraseasonal convective disturbances that originate over Northern India and propagate westwards and reach Sahel roughly two weeks later, enhancing moist convection and rainfall anomalies. A defining feature of these westward-propagating intraseasonal disturbances is their coherent potential vorticity (PV) core in the mid-troposphere, which collocates with the core of the AEJ in the mid-troposphere. This alignment of the PV core with the AEJ core dynamically traps these waves along the AEJ and thus results in a coherent wave propagation.  

In the recent decades, the AEJ has strengthened due to an increased meridional temperature gradient, thus the propagation of these waves from Indian region to Sahel have become more effective thereby contributing to the observed strengthening of the two large scales Monsoons.