Mechanisms for regional compound hot extremes in the mid-lower reaches of the Yangtze River

Hot temperature extremes, particularly during summer, impose severe adverse effects on human health and natural systems. Although considerable attention has been paid to individual hot days and nights, little is known about mechanisms leading to compound hot extremes that combine daytime and nighttime heat. This important type of hot extreme has occurred frequently in the populated mid-lower reaches of the Yangtze River during the past three to four decades. Through composite analysis, dynamic-thermodynamic processes leading to the occurrence of compound hot extremes are investigated. Results show that an anomalous anticyclone, exhibiting a quasi-barotropic structure and persisting through day and night, is a prerequisite for the occurrence of compound hot extremes in the mid-lower reaches of the Yangtze River. The anomalous anticyclone reduces cloud cover and increases solar radiation, causing extreme daytime high temperature. Simultaneously, a descending branch within the column of the anomalous anticyclone enhances local adiabatic heating. This descending branch and resultant adiabatic heat are sustained and further strengthened by anomalous descending motions as part of vertical cells associated with positive precipitation anomalies to the north and south of the anticyclone. During compound hot extremes, intensified southerlies transport additional water vapour toward the study area, helping to establish a stable atmospheric stratification in the low troposphere. This low-level stable moist layer plays a particularly important role to offset nighttime radiative cooling and, therefore, elevates nighttime low temperatures to form compound hot extremes. Consequently, compound hot extremes in the study area are hot-humid in nature.