Impact of East Asian Summer Monsoon on the Interannual Variation of South Asian High

Abstract:

Occupying the upper troposphere over the Eurasia during boreal summer, the South Asian High (SAH) is thought to be a critical regulator of East Asian Summer Monsoon (EASM), a system of particular important for regional climate and regional water cycle over the East Asia. In this study, the mechanisms responsible for the interannual variation of SAH intensity are investigated. When the SAH is strong, the 200 hPa geopotential height in the high center increases and the high system expands. Significant anomalies of the geopotential height were found over the Middle East, and to the east of the Tibetan Plateau (TP). The upper tropospheric temperature increases, while the precipitation increases over the Yangtze River Valley but decreases over the Middle East. The diagnosis of atmospheric dynamics and the numerical experiments consistently shown that the interannual variation of the SAH intensity is strongly influenced by summer precipitation over the Yangtze River Valley associated with EASM.The mechanisms are three-fold. First, the excessive EASM precipitation excites a local anticyclone in the upper troposphere according to Sverdrup Vorticity Balance, which leads to the eastward extension of the eastern edge of the SAH. In addition, condensational heating related to EASM warms the upper troposphere (200-400 hPa), reinforcing the geopotential height anomalies over East TP and East China. Furthermore, the monsoonal heating excites a westward propagating Rossby wave train, increasing geopotential height over the Middle East and inhibits the local precipitation. In conclusion, our study results in a new mechanistic paradigm in which the EASM is driving the SAH variation rather than passive response as traditionally thought.