Thermodynamic characteristics of Extreme Heat Waves over the middle and lower reaches of the Yangtze River Basin

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Abstract
In August 2022, an exceptionally long-lasting heat wave (HW) affected the middle and lower reaches of the Yangtze River basin. This study uses the JRA55 daily reanalysis datasets to elucidate the daily heat budget of historical extreme HWs in this region. The HWs are generally characterized by the occurrence of anticyclonic circulation anomalies, which affect air temperature through both adiabatic and diabatic processes. On one hand, they induce evident descent motion in the entire troposphere, and the produced compression heating becomes the primary contributor. Simultaneously, the eastward warm advection is evident in the lower troposphere. On the other hand, more shortwave radiation reaches the ground surface, and the sensible heat flux becomes the important source of the diabatic heating before the onset of the HWs. After the peak day of the HWs, the accumulated excessive heat is primarily offset through the emission of longwave radiation and meridional thermal advection. The prolonged adiabatic heating processes and weakened diabatic cooling effect are the main factors contributing to the extreme persistence of the HWs in August 2022. The upper level portion of the anticyclonic circulation anomalies is linked to the strengthened South Asia High. After applying the state-of-art dynamic metric, i.e., local finite wave activity, we reveal that the formation of the anomalous South Asia High in August 2022 is associated with the Stokes drift flux, rather than the dispersion of Rossby wave energy. This characteristic sets it apart from other extreme HWs.

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