Multiscale Nonlinear Response of Extreme Meteorological–Hydrothermal Events in the Upper Reaches of the Yangtze River

Under the backdrop of climate warming, the outbreak of short-term extreme heat events can easily lead to irreversible changes in aquatic ecosystems. Delving into their intrinsic driving mechanisms and nonlinear characteristics is key to preventing natural disasters. This study, focusing on the upper Yangtze River as the research area, constructs a joint copula function model to analyze the joint occurrence probability and return period of extreme meteorological heat events. Through the bivariate cross–wavelet transform method, the study explores the multiscale dynamic response relationships and phase characteristics of extreme meteorological–hydrothermal events in the upper Yangtze River. Furthermore, a joint multifractal model of nonlinear responses for extreme meteorological–hydrothermal events was established. The results indicate that extreme high-heat meteorological events in the upper Yangtze River tend to occur more frequently and severely, with duration–kurtosis events likely to coincide within a 2-year return period, as well as high-intensity low-frequency high-heat duration–severity events occurring simultaneously. Overall, before 2005, extreme high-hydrothermal events exhibited phase characteristics lagging behind meteorological changes, which then shifted from lag to lead. Under three scenarios of change, meteorological–hydrothermal events exhibit a clear nonlinear response relationship. Apart from duration, severity and kurtosis all show significant nonlinear relationships.