Do tropical waves play a role in modulating monsoon onset?

Abstract

The onset of the Indian summer monsoon (ISM) marks a rapid transition from dry to wet conditions, yet the role of tropical equatorial waves in initiating this transition remains poorly understood. Here, we investigate this process in detail using observational and reanalysis datasets for the period 2001–2023. Our results demonstrate that convectively coupled equatorial Rossby (ER) waves play a pivotal role in modulating the onset and early evolution of the ISM over Kerala. Monsoon onset occurring during the cyclonic phase of ER waves is characterized by a strengthened low-level jet, enhanced moisture transport, organized deep convection, and widespread rainfall over the southwest coast of India. In contrast, onset during the anticyclonic phase is associated with suppressed convection, a 4–6-d delay in widespread rainfall, and weakened low-level circulation until the arrival of a cyclonic ER wave. The results show that the cyclonic phase of ER waves facilitates the northward propagation of monsoon convection, whereas such propagation is absent during the anticyclonic phase of ER waves. These findings identify ER waves as a key modulator of monsoon onset and imply that monitoring their phase could improve short-range predictions of onset timing and the subsequent evolution of monsoon rainfall.