Towards a Reliable Outlook of the Seasonal Evolution of the Southeast Asian Monsoon Rainy Season

This study uses an objective definition for the onset and demise date for the seasonal evolution of the rainy season over the Southeast Asia (SEA) region. The onset or the demise date is defined as the first or the last date of the year when the daily rain rate exceeds the annual mean climatological rainfall. This gives rise to a varying length of the season that has a significant impact on the seasonal rainfall. It is observed that the inter-annual variability of the seasonal rainfall is significantly affected by the variations of the onset and demise dates. But the influence of the onset and demise date appears to be independent of each other. Using the Integrated Multi-Satellite Retrievals for Global Precipitation Mission version 6 (IMERG) rainfall analysis available on 0.1°grid we have analyzed the variations of the rainy season over SEA. We generate an ensemble of 1001 members by slightly perturbing the original time-series to account for uncertainties in the rainfall analyses. This also ensures that the diagnoses of the onset/demise dates are not sensitive to sporadic synoptic/mesoscale rain events that may be unconnected to the seasonal evolution of rainfall in SEA. Using these ensemble of 1000 members per season, we find that we can provide reliable probabilistic outlook of the forthcoming rainy season over the SEA region.