Mid-Holocene changes in warm-season heatwave characteristics and mechanisms underlying intensity variations over eastern China: Insights from PMIP4 simulations

Although the mid-Holocene mean climate state over China has been extensively studied, changes in heat extremes and related mechanisms remain insufficiently understood. Using multiple models from the Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4), we investigated changes in heatwave characteristics over eastern China in the boreal warm season (May-September) during the mid-Holocene, utilizing intensity-related metrics (heatwave magnitude and amplitude) and time-related metrics (heatwave number, duration, and total heatwave days), and examined mechanisms underlying heatwave intensity variations. Multimodel means reveal a “north-enhanced and south-weakened” dipolar pattern in heatwave intensity over eastern China. In contrast, heatwave duration and total heatwave days show an overall tripolar distribution, with decreases in areas south of the Yangtze River and northern Northeast China. Changes in heatwave number are comparatively weaker and spatially patchier. The dipolar intensity structures are primarily driven by orbitally induced large-scale circulation reorganizations. Increased boreal summer insolation during the mid-Holocene enhances the land-sea thermal contrast and the East Asian summer monsoon, thereby shifting the East Asian upper-level westerly jet poleward. This shift promotes positive geopotential height anomalies over midlatitude East Asia, located downstream of an eastward-propagating Rossby wave train. Consequently, reduced convection and cloud cover increase surface net shortwave radiation and sensible heat flux in the northern part of eastern China, thereby intensifying heatwaves, with broadly opposite changes in the south, consistent with weaker heatwave intensities. These findings highlight spatially heterogeneous heatwave responses to orbital forcing and provide a paleoclimate benchmark for understanding regional heat extremes under different warming backgrounds.