A Study of the Characteristics of the Low-Frequency Circulation over the Tibetan Plateau and its Association with Precipitation in the Yangtze River Valley in 1998

The propagation characteristics of the atmospheric low frequency (LF, 30--60 days) oscillation (LFO) around the Tibetan Plateau from troposphere to stratosphere and its relationship with the floods over the mid-lower reaches of the Yangtze River in the summer of 1998 are studied, based on the GAME dataset from Meteorological Research Institute (MRI)/Japan Meteorological Agency, the TRMM satellite rainfall and the 730-station precipitation over China. The results show that the zonal propagation direction of LFOs in horizontal winds varies with seasons in the troposphere during May to August in 1998. The eastward propagation of LFOs is remarkable before the start of the rainy season in the Tibetan Plateau and the eastern Asian continent, while the westward propagation is significant after the start date. The northward LFOs from the south side of the plateau and the southward LFOs from the north are both significant before and after the start date. The plateau is a LFO sink in the meridional and zonal directions, but the west part of it is an intensifying area for the continual westward LFOs only after the start of the rainy season. Besides, the strongest LFOs occur at the tropopause (100 hPa) and rapidly decay after entering the stratosphere. The rainfall over the mid-low reaches of Yangtze River in the summer of 1998 exhibits two LFO cycles. According to the phases of the two rainfall LFO cycles, the composite analysesof precipitation distribution, LF circulations at 500 and 100 hPa,and LF vertical motion along 30°N are performed. It is the joint effect of the mid-upper tropospheric strong 30--60-day filtered cyclone (anticyclone) over the eastern plateau and the LFO anticyclone (cyclone) over the west subtropical Pacific that induces the whole layer LF descending (ascending) motion over the mid-lower reaches of Yangtze River, which provides the favorable condition for the break (maintenance) of precipitation.