The bimodality of South Asia High and its relationship with Asian rainfall in September

Abstract

AbstractThis study examines daily longitudinal locations of South Asia High (SAH) in September and its connection with Asian September rainfall variation. Results suggest that daily longitudinal locations of September SAH exhibits as bimodality, which is featured by two preferable locations with one high‐frequency location lying over southern Tibetan Plateau and the other over southern Iranian Plateau. Referred to the two preferred locations, the SAH is classified into east and west SAH mode. The two modes may be attributed to a warm column over central eastern Tibetan or eastern Iranian Plateau, which warms the above troposphere and maintains a SAH at upper troposphere over corresponding location. In addition, bimodality of the SAH has a close relationship with Asian September rainfall. The east mode of SAH is associated with an increasing rainfall over western China and decreasing rainfall over northern Indian Peninsula, southwestern Tibetan Plateau, Indochina Peninsula, eastern China and Korea Peninsula, while the west mode of SAH is linked with the reverse pattern. Specifically, the atmospheric circulation at upper troposphere is featured by one large anticyclonic circulation above eastern Tibetan Plateau and two cyclonic circulations over northern Iranian Plateau and eastern Korea. In the lower troposphere, southerly and northerly anomalies form a convergence zone over western China. Meantime, a cyclonic moisture circulation with warm and wet southerly vapour flow results in above‐normal rainfall over western China. An anticyclone over northern Iranian Plateau at lower troposphere results in an anticyclonic moisture divergence and deficient rainfall over southwestern Tibetan Plateau and northern India. Moreover, a cyclonic circulation over eastern Korea Peninsula with marked northwesterly and moisture flux divergence results in below‐normal rainfall over eastern China and Korea Peninsula.