A study of the characteristics of the low-frequency circulation on Tibetan plateau and its association with precipitation over the Yangtze valley in 1998

The propagation characteristics of the atmospheric low frequency(LF, 30-60 days) oscillations(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 of horizontal winds varied with seasons in troposphere during May to August in 1998. The eastward propagation of LFOs was remarkable before the rainy season of the Tibetan Plateau and the eastern Asian continent, while the westward propagation was significant after the start of the rainy season. The northward LFOs from the south side of the plateau and the southward LFOs from the north side were both significant whatever before or after the start of the rainy season. The plateau was a LFO sink in the meridional direction, and in the zonal direction, the west part of the plateau was an intensifying area for the continual westward LFOs over the plateau only after the start of the rainy season. Besides, the strongest LFOs occurred at the tropopause (100 hPa), and rapidly decayed after entering the stratosphere. The rainfall presented two LFO cycles over the mid-low reaches of Yangtze River in the summer of 1998. According to the phases of the two rainfall LFO cycles, the composite analyses of precipitation distribution, LF circulations at 500 hPa and 100 hPa, and LF vertical circulation along 30°N are performed. It is the joint effect of the midupper tropospheric strong 30-60 days filtered cyclone (anticyclone) over the eastern plateau and the LF anticyclone (cyclone) over the western subtropical Pacific that induced the whole layer LF subsidence (ascending) motion over the midlower reaches of Yangtze River, which provided the favorable condition for the break (maintenance) of precipitation.