Changes in the midsummer extreme high-temperature events over the Yangtze River Valley associated with the thermal effect of the Tibetan Plateau and Arctic Oscillation

By using the observational and reanalysis datasets during the period of 1979-2018, the role of the thermal effect of the Tibetan Plateau (TP) and Artic Oscillation (AO) in the midsummer (July-August) extreme hightemperature events (EHEs) over the Yangtze River Valley (YRV) is investigated. The primary mode of the EHEs (EHEs_EOF1) over the YRV is characterized by an increased frequency of the EHEs over the YRV, with two centers located in the upper and lower reaches of the YRV, respectively. The results indicate that the higher YRV EHEs frequency is associated with anticyclonic anomalies over northern China, which leads to the descending motion over the YRV, further resulting in the reduced middle to low clouds and enhanced solar radiation. In addition, warmer surface ground will heat the atmosphere through the sensible heat and upward longwave radiation. Moreover, less soil moisture also favors the occurrence of the EHEs. Further analysis demonstrates that the EHEs_EOF1 is related to the midsummer surface air temperature over the TP (TP-SAT) and the AO. The anticyclonic anomalies over northern China are considered to be the key factors favoring the YRV EHEs, which is associated with the TP-SAT and AO, of which TP-SAT dominates. TP warming can induce anticyclonic anomalies over northern China, thus favoring the YRV EHEs. AO can also affect the anticyclonic anomalies over northern China through the Rossby wave train originating from the northwestern Atlantic, which is largely modulated by the TP thermal forcing. In addition, the model experiment with the linear baroclinic model (LBM) further provides direct evidence to illustrate the impact of the thermal condition of the TP on generating circulation anomalies that conducive to the EHEs over the YRV.