Impacts of regional dependence and cyclone systems on joint return periods of coastal compound flooding in China

Compound flooding arising from concurrent extreme precipitation and storm surge poses severe risks to coastal regions. Using a 40-year dataset from 28 estuaries along China's coast, we identified 337, 208, and 193 compound events in the northern, eastern, and southern coastal China, respectively, revealing strong spatial contrasts in event frequency. Tail dependence between precipitation and storm surge is strongest in the eastern and southern regions with Kendall's τ roughly 0.15–0.25 and weakest in the north with τ between −0.10 and 0.15. These contrasts substantially modify compound flood likelihood. Joint return periods (JRPs) are much shorter when dependence is accounted for than when tail independence is assumed, particularly in regions with stronger dependence. For example, at a representative eastern coastal site, a concurrent 1-in-5-year precipitation and surge event yields a JRP of about 13 years when dependence is considered, compared with 25 years under independence, indicating that neglecting positive dependence underestimates compound flood risk. However, this reduction is far less pronounced for the same combination in the northern region where dependence is weak. Cyclone-type contributions help explain these regional contrasts. Tropical cyclones (TCs) generate the most extreme compound events in the east and south and strongly enhance tail dependence, such that removing TC events markedly lengthens JRPs. In the north, extratropical cyclones (ETCs) dominate event frequency but exhibit weaker synchrony between extremes. Removing these frequent but weakly dependent events therefore shortens JRPs. These findings underscore the need to incorporate regional dependence structures and cyclone-type influences into coastal flood risk assessments.