Tibetan Plateau Westerly Forcing on the Cloud Amount over Sichuan Basin and the Early Asian Summer Monsoon
Chi-Hua Wu1 and Ming-Dah Chou2
1Research center for Environmental Changes, Academia Sinica, Taipei, Taiwan
2Department of Atmospheric Sciences, National Central University, Chung-Li, Taiwan
Data show that a sizeable mechanical effect of the Tibetan Plateau (TP) on the westerly, referred to as TP westerly forcing, is related to a sizeable mid-level cloud amount over Sichuan Basin (SB; centered near 30°N and 105°E) to the east of the TP. An investigation of the interannual variability of clouds over SB demonstrated that the TP westerly forcing closely correlates to the early Asian summer monsoon (ASM). When the monsoon commences in mid-May, the East Asian upper tropospheric jet stream moves northward with a southwest-northeast tilt; the tilt of the jet considerably influences the TP westerly forcing. Because of the stationary wave pattern superimposed on the westerly jet, the early ASM is substantially distinct by year. In the years the mid-level cloud amount in SB is sizeable (which corresponds to a strong TP westerly forcing), the tilt of the upper-level jet increases, and the ascending southerly east of SB in 110°E−125°E strengthens. The warming of the troposphere in 110°E−125°E centered near 35°N corresponds to the westerly and southerly changes. Further downstream in 125°E−140°E region, the ascending southerly weakens. Correspondingly, an anomalous anticyclonic circulation occurs between SB and Japan, leading to a westward extension of the North Pacific high pressure in the lower-troposphere and a reduction in precipitation of the early Mei-Yu season. Coincidentally, the mid-tropospheric monsoon trough south of the TP weakens, implying a weak monsoon in both East Asia and South Asia. Furthermore, the weak monsoon continues through the early summer.
Wu, C. H., and M. D. Chou, 2013: Tibetan Plateau westerly forcing on the cloud amount over Sichuan Basin and the Early Asian summer monsoon. J. Geophys. Res. Atmos., 118, 7558-7568. DOI: 10.1002/jgrd.50580
The schematic of years with a strong TP westerly forcing (M+ years). When the mid-level cloud amount in SB east of the TP is large, corresponding to a strong TP westerly forcing, the southwest-northeast tilt of the upper-level jet stream increases and the ascending southerly east of SB between 110°E and 125°E strengthens. Further downstream to the region between 125°E and 140°E, a strong TP westerly forcing is related to a weak ascending southerly. Correspondingly, an anomalous anticyclonic circulation between SB and Japan leads to a strong western North Pacific high pressure in the lower-troposphere. The monsoons in East Asia and South Asia are both weak, corresponding to a strong TP westerly forcing.