Unraveling the nonstationary effect of climate change and urbanization on summer drought-heatwave coupling degree in Beijing-Tianjin-Hebei Area, China

Beijing-Tianjin-Hebei (BTH) urban agglomeration area of China Hot extremes may adversely impact human health and agricultural production. Owing to rapid urbanization and climate change, the dynamic interaction between drought and hot extremes in most areas of China need to be revisited from the perspective of nonstationarity. Utilizing daily precipitation and temperature data from Beijing-Tianjin-Hebei (BTH) urban agglomeration area of China for a period of 1970–2018, this study develops a dynamic Copula model incorporating the large-scale oscillation indexes and urbanization (denoted by impervious surface area: PISA) factor as parameter covariates to investigate the summer precipitation-temperature coupling. A general statistical inspection procedure which was composed of maximum likelihood (ML)-based estimation, nonstationary Goodness-of-fit (GOF) tests, the log likelihood ratio (LR) tests and minimum Akaike information criterion-based selection was promoted to select the best-fitted nonstationary models efficiently. Using 3-month Standardized Precipitation Index (SPI) values as proxies for drought, our analysis reveals a progressively intensifying relationship between summer drought and heatwave events under combined climate change and urbanization pressures. Using hierarchical nonstationary modeling, we examined three pathways: (1) large-scale climatic indexes-induced nonstationarity, (2) urbanization quantified by the percentage of impervious surface area from Landsat, and (3) compound influences: interaction between climate change-based drivers and urbanization. To quantitatively assess these drivers' relative contributions, we developed a novel Conditional Risk Sensitivity Indicator (CRSI)-based Sensitivity Enhancement Factor (SEF). Benefiting from the CRSI-based SEF analytical framework, the conditional risk of extreme heat events has intensified at most monitoring stations, primarily as a result of a markedly strengthened negative feedback between precipitation deficit and elevated temperatures. • A new Copula-based method analyzes how precipitation deficits and high temperatures interact. • In the past 50 years, precipitation deficits have intensified the risk of extreme heat events. • El Niño and urbanization (PISA) strengthen BTH's summer precipitation deficit-heat dependence. • A new metric reveals that climate change influences more than urbanization on precipitation-temperature interactions.