Abstract
From spring to summer, the East Asian summer monsoon (EASM) rainband migrates northwestward. During summer, East Asian countries experience extensive precipitation due to the EASM rainband, but the springtime monsoon rainband lies over the Pacific. The seasonal evolution of the EASM rainband is influenced by the mechanical effect of the Tibetan Plateau, and seasonal changes in the westerly wind speeds impinging on the Tibetan Plateau are a key driver of this process. In this study, using interannual variability of the upstream zonal wind speed, the dynamical mechanism for the interannual variations of the EASM precipitation is revealed based on the topographically forced stationary Rossby wave theory. The dynamical mechanism regulating interannual variability in the EASM rainband is essentially the same mechanism that drives the seasonal evolution of the climatological EASM rainband. If the westerly winds impinging on the Tibetan Plateau are stronger (weaker) than average, then the EASM rainband shifts eastward (westward). Large variations in the upstream westerly wind during May induced considerable interannual variation in the zonal location of the rainband (up to a 20°–30° shift). The westerly wind speed exhibited less variations in June and July, resulting in a smaller zonal shift of approximately 10°.
Highlights
Variations in weather and climate are known as one of the most important factors affecting human activities, including the economy, industry, agriculture, environment, and recreation
Rossby wave response induced by the air uplift effect of the Tibetan Plateau causes a geopotential height anomaly, and the southerly wind shifts east-westward according to the change in the upstream wind speed
The eastward shift of the precipitation region in the monthly dataset is caused by the slower northwestward migration of the monsoon rainband. This means that the onset and withdrawal dates of the East Asian summer monsoon (EASM) are partly controlled by the westerly wind speed impinging on the Tibetan
Summary
Variations in weather and climate are known as one of the most important factors affecting human activities, including the economy, industry, agriculture, environment, and recreation. Son et al (2020) showed that the climatological seasonal evolution of the EASM rainband is controlled by variations in the upstream zonal wind speed impinging on the Tibetan. The importance of the dynamical effect (i.e., circulation) in determining EASM precipitation was demonstrated using a series of sensitivity experiments with a simplified general circulation model (Son et al 2019). Motivated by these previous studies, in this study, rather than using the aforementioned continental, oceanic and atmospheric factors responsible for the variation of EASM precipitation on the interannual time scale, we use the year-to-year variation of the westerly wind forcing upstream of Tibet to examine the precipitation variation. (2020), it is expected that the upstream mechanical forcing with a stronger wind speeds induce a more eastward formation in the area of the major northward moisture flux and rainband
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