Entire-Basin Heat Wave in the Yangtze River Is Becoming the New Normal

Abstract Since the twenty-first century, the Yangtze River basin has experienced a number of extreme heat wave events. The basin exhibits two distinct high-frequency centers of heat waves, the east of the upper reaches and the mid–lower reaches and the south. In comparison to the period before 2000, the number of upper-reaches type and entire-basin type heat wave days has significantly increased in the twenty-first century, while the mid–lower-reaches type heat wave has increased slightly. The westward extension and the strengthening of the western Pacific subtropical high (WPSH) and the eastward extension of the South Asian high (SAH) over the Yangtze River favor extreme heat waves in the basin. This circulation pattern leads to strong descending motion through the troposphere along the basin. Sinking airs causes the diabatic heating effect and warms the surface air, and meanwhile reduces the cloud cover and the rainy weather, and enhances the radiation. In addition, observations reveal that the anomalous circulations in Europe, which cause heat waves in the local region, may also contribute to the heat waves in the upper reaches of the Yangtze River basin through the influence of the midlatitude circulations especially the double westerly jet. Projections of 13 phase 6 of the Coupled Model Intercomparison Project (CMIP6) models indicate that the temperature in the entire Yangtze River basin and the western Europe will increase remarkably in the near, mid, and long terms under different greenhouse gas emission scenarios [shared socioeconomic pathway (SSP) 2–4.5, SSP3–7.0, and SSP5–8.5]. After 2050, the WPSH and the SAH will be persistently located over the Yangtze River basin, indicating that heat waves will become a normal state in this region. In addition to the intensification of the high ridge over western Europe, these will result in the frequent occurrence of heat waves in the Yangtze River basin in the future.