bnormal Characteristics of the Highest Daily Temperature in Midsummer in Hunan Region and Its Possible Mechanisms

This study focuses on abnormal high temperature (AHT) events during midsummer in Hunan Province. By employing statistical and dynamic diagnostic methods, it aims to investigate the characteristics of these abnormal heat events and explore the underlying mechanisms responsible for their formation. The main findings show that the abnormal high temperature events in Hunan exhibit a significant upward trend. The abnormal high temperatures in Hunan are closely related to the strengthening, eastward expansion of the South Asia High, and the northward and westward extension of the Western Pacific Subtropical High. The coupling of these two high-pressure systems forms a stable high-pressure circulation, which hinders the southward movement of cold air, enhances subsiding airflow, reduces cloud and rainfall, and promotes abnormal surface warming. The descending airflow in the high-pressure region leads to strong adiabatic heating, which becomes the dominant factor for regional high temperatures. Further analysis reveals the presence of two wave trains, one originating from the mid- to high-latitudes of Northwest Asia and the other from the low-latitudes of the southeast. These wave trains propagate toward the north of Hunan and over the region, producing wave vector convergence, which favors the development and intensification of the abnormal low-pressure disturbance north of the region and the abnormal high-pressure disturbance over Hunan. The combined effect of the mid- to high-latitude systems and low-latitude wave trains leads to the strengthening of the abnormal high pressure over Hunan, enhancing the subsiding motion. The abnormal downward warming causes the occurrence of regional high-temperature weather events.