Intensity Variations of Summertime South Asia High and its Impacts on Tropical Cyclones over the Western North Pacific

Abstract
The South Asia high (SAH) is a prominent circulation system of the Asian summer monsoon, exerting profound influences on the weather and climate in China and surrounding regions. Its formation and maintenance is closely associated with strong summertime continental heating in the form of surface sensible heat flux mainly over the Tibetan Plateau. Previous studies investigated the intensity variations of the SAH by treating it as a large anticyclonic circulation system in the upper-level (e.g., 200 hPa), covering the entire Eastern Hemisphere. However, two anticyclonic centers can be identified in boreal summer after the removal of the zonal mean flow, which are associated with the Tibetan Plateau and Iranian Plateau. We investigated the intensity variations of the summertime SAH through an empirical orthogonal function (EOF) analysis. The EOF analysis indicates that the SAH intensity variations exhibit an anomalious anticyclone over the Tibetan Plateau and an anomalious cyclone over the Iranian Plateau on the climate change and interdecadal time scales. We will show that the intensity changes of the SAH are closely associated with changes of the Tibetan Plateau thermal forcing. It is interesting that the tropical cyclone (TC) activity over the western North Pacific shows similar changes during last 40 years. As the Tibetan Plateau thermal forcing weakens, an anomalous anticyclone can be induced to the east of 90ºE in the upper-level, leading to the change of the steering flow.