Ocean-atmosphere processes determining the onset of Indian summer monsoon: differential influence of synoptic to sub-seasonal scale drivers

Abstract

The present study investigates the ocean–atmosphere processes associated with the onset of Indian Summer Monsoon rainfall by classifying into six categories: too early (before 20 May), very early (21–24 May), early (25–29 May), normal (30 May–2 June), late (3–7 June), and very late (after 7 June) onset years. This classification helps to identify interactions among processes operating at different temporal and spatial scales that influence the timing of monsoon onset, thereby improving the understanding of its underlying mechanisms and predictability. The analysis shows that the too-early onset (before 20 May) is mainly controlled by large-scale climatic patterns. These include La Niña conditions in the Pacific Ocean, cooling in the Indian Ocean during April–May, and strong transfer of energy and momentum through the upper-level wave activity over the Indian landmass. These factors enhance northward movement of convection and facilitate early monsoon onset, while the Madden–Julian Oscillation (MJO) or intraseasonal oscillation (ISO) signal remains weak. Thus, the anomalous basic state for the onset is largely determined by large-scale forcing. During very early-onset years, La Niña conditions and upper-level wave activity are present but weaker than in the too-early-onset cases. Instead, warm sea surface temperature (SST) anomalies over the equatorial and northern Indian Ocean, and their northward propagation, play an important role in triggering convection and initiating monsoon onset. In early and normal onset years, cool SST anomalies over the equatorial and northern Indian Ocean persist until the onset pentad, supporting the northward propagation of convection. In these cases, MJO-related anomalies help propagate convective signals from the equator to about 35°N, contributing to the onset between 25 May and 2 June. In contrast, late onset years are influenced by El Niño-related warming in the tropical Pacific and subsidence over the Indian Ocean, which suppress convection and delay the monsoon onset.