The NCEP/NCAR pentad mean reanalysis data from 1980 to 1994 are employed to examine the activities of the 100 hPa South Asia High (SAH) during the boreal summer. The results show that there exists bimodality in the longitude location of the SAH. According to the two preferred regions for the SAH, the SAH is classified into the Tibetan Mode (TM) and the Iranian Mode (IM), respectively. The studies on the maintenance mechanism, both from circulation structure and thermal structure, manifest that the SAH has the feature of warm preference. The diagnosis based on the thermodynamic equation further reveals that the TM is closely related to the diabatic heating of the Tibetan Plateau, whereas the IM is more associated with the adiabatic heating in the free atmosphere, as well as the diabatic heating near the surface.
The statistical composites, and case studies, corresponding to the two modes show that the SAH bimodality is strongly related to the climate anomalies over East Asia. In the case of TM, the southerly airflow over the subtropics and the northerly airflow over the middle-high latitude at 850 hPa are enhanced, forming a convergence zone along 30°N and resulting in increased rainfall extending from the south Japan, Korea Peninsula, and Yangtze-Yellow river valley of China to the Tibetan Plateau. In the case of IM, at 850 hPa the mid-latitude East Asia is dominated by a huge anomalous anticyclone, thus results in the decreased rainfall over the area.