Investigating the Influence of Monsoon Seasonality on Compound Flooding in Southeast Asia

Compound floods result from the interaction of multiple flood drivers occurring concurrently or successively, and their combined impacts can be disproportionate and wide-ranging. There is a significant gap in the literature with regards to compound flood events in Southeast Asia, despite the sizeable coastal population in the region. In this study, extreme value theory was used to explore the joint probability of extreme sea level and rainfall events in Southeast Asia and the South China Sea in the context of the East Asian and South China Sea monsoon systems. This research was carried out using observational tide gauge data and total precipitation data obtained from climate reanalysis, with the peaks-over-threshold (POT) method applied to select extreme values. Thresholds were selected using two methods: employing the empirical mean residual life plot and calculating the 99.5th percentile values. The underlying distribution was then approximated using a Generalised Pareto Distribution and a bivariate logistic threshold excess model subsequently fitted to obtain the joint probability of exceedances. Preliminary results from this investigation correlate with previous studies showing seasonality of dependance in various countries by demonstrating that most joint exceedances (in excess of 70% for all sites and thresholds) occur during the wet monsoon months in Hong Kong, Singapore, Kota Kinabalu and Manila. This suggests that monsoons play a vital role in impacting the co-occurrence of high sea level and heavy rainfall in the region, and underscores the importance of considering seasonality when studying compound flooding events to improve the assessment and management of coastal risk.