Mechanism of MJO-Modulated Triggering on the Rainy Season Onset over the Indian Subcontinent

As rainfed agriculture remains India's critical source of livelihood, improving our understanding of rainy season onset timing in the region is of great importance for a better prediction. Using a new gridded dataset of rainy season characteristics, we found a clear phase relationship between the Madden-Julian oscillation (MJO) and the onset timing of the rainy season over the Indian subcontinent. A significantly high probability of rainy season onset is observed when the MJO convection stays over the western-central Indian Ocean. On the other hand, the rainy season onset is infrequent when the MJO is over the Maritime Continent and western Pacific. The MJO-associated convective instability with anomalous warm and moist air in the lower troposphere appears and grows during the period 10 days prior to the onset of rainy season, and drops substantially after the start of rainy season, suggesting its role as a trigger of rainy season onset. In contrast, the low-frequency background state (LFBS) with a period > 90 days favors a convectively unstable stratification even after the onset of the rainy season, supporting the succeeding precipitation during the entire rainy season. Based on the scale-decomposed moisture budget diagnosis, we further found that the key processes inducing the abrupt transition from a dry to a wet condition come mainly from two processes: 1) convergence of LFBS moisture by MJO-related circulation perturbations and 2) advection of MJO moisture anomalies by the background cross-equatorial flow toward the Indian subcontinent. The results may help provide a better and longer lead-time prediction of the rainy season onset over the Indian subcontinent.

Automated summary

Understanding the relationship between the Madden-Julian oscillation (MJO) and the onset timing of the rainy season in India is important for predicting it better. The study found that the rainy season onset is more likely when the MJO convection is over the western-central Indian Ocean, and less likely when it is over the Maritime Continent and western Pacific. The MJO-associated convective instability plays a role in triggering the rainy season onset, while the low-frequency background state (LFBS) supports precipitation throughout the entire rainy season. Key processes leading to a sudden transition from dry to wet conditions are the convergence of LFBS moisture by MJO-related circulation perturbations and the advection of MJO moisture anomalies towards the Indian subcontinent by the background cross-equatorial flow.