The Anomalous Characteristics of the South Asian High during Its Establishment and Withdrawal Period over the Tibetan Plateau

The position of the South Asian High (SAH) over the Tibetan Plateau (TP) exhibits significant interannual variability. In this study, the criteria for defining the establishment and withdrawal of the SAH，as well as their activities over the TP, have been re-evaluated. The SAH is classified as "established over the TP" when the 1660 dagpm geopotential height contour covers 25% or more of the TP, and "withdrawn from the TP" when this contour covers less than 25%. Statistical analysis shows that, on average, the SAH establishes over the TP by the 4th pentad of May (28th pentad) and withdraw from the TP around the 2nd pentad of October (56th pentad). Over the past 50 years (1974-2023), the SAH has tended to establish over the TP earlier and withdraw later, resulting in a longer duration of control over the TP, with the rate of increase accelerating since 2017. The SAH exhibits a long-term linear trend of eastward shift during both its establishment and withdrawal phases. The anomalous activity of the SAH over the TP is influenced by atmospheric heat sources. When there is anomalous warming (cooling) of atmospheric heating over the Bay of Bengal and the Indochina Peninsula, the SAH tends to establish and withdraw earlier and further westward (later and further eastward), with the westerly trough located over East Asia (the western part of the TP).