Nonlinear and regime-dependent extreme drought in humid regions revealed by a standardized water cycle index

Drought in humid regions is increasingly influenced by interactions among atmospheric demand, vegetation, and terrestrial water storage, whereas conventional univariate indices often overlook these coupled processes. Here we assess extreme drought in Guangdong Province during 1961–2100 using a multivariate Standardized Water Cycle Index (SWCI) based on D-vine copulas that integrates precipitation, evaporation, leaf area index, surface runoff, and subsurface runoff. Using bias-corrected CMIP6 simulations under three Shared Socioeconomic Pathways, we find that the response of future extreme drought to radiative forcing is nonlinear and region dependent. The intermediate forcing scenario produces the longest and most severe drought events in northern inland Guangdong, whereas the highest forcing scenario is associated with an eastward shift of drought hotspots. Driver attribution further indicates a transition from precipitation-dominated control toward stronger roles of evaporation demand and runoff redistribution, with distinct process-dominant regimes across sub-regions. These results show that multivariate drought metrics provide a more physically informative basis for drought-risk assessment in humid subtropical regions.