A Study Of The Characteristics Of The Low-Frequency Circulation On Tibetan Plateau And Its Association With Precipitation Over The Yangtze Valley

Based on the NCEP reanalysis wind fields and the 730-station precipitation over China from 1971 to 2000, and the TRMM rainfall in 1998, a study is investigated about the significant low-frequency periods of Meiyu rainfall over the mid-lower reaches of the Yangtze River, the propagation characteristics around the Tibetan Plateau from troposphere to stratosphere in flood years with significant low-frequency oscillation (LFO) and the relationship between them. The main results are as follows:(1) Low-frequency oscillation (LFO) widely exists in the summer rainfall over the mid-lower reaches of the Yangtze River, the periods of which can be divided into four belts——10—20 day, 20—35 day, 30—60 day and 60—100 day. Meiyu rainfall mainly has the 10—20-day or 20—35-day significant low-frequency oscillation when less than normal, while prodominately the 20—35-day or 30—60-day significant low-frequency oscillation when more than normal. Besides the much more rainfall over the mid-lower reaches of the Yangtze River, the Meiyu in 1991 and 1998 presents the 20—35-day and the 30—60-day significant low-frequency oscillation respectively.(2) The zonal propagation direction of the 30—60-day LFO around the Tibetan Plateau varies with seasons in troposphere in the summer of 1998: the eastward LFO before the rainy season of the Tibetan Plateau is remarkable, while the westward after. The northward LFO from the south of the plateau and the southward from the north is the notable whatever before or after the rainy season. The plateau is the sink in meridinal LFO propagation and the trigger area for the continual westward LFO in zonal direction only after the rainy season in May-August in 1998. The similar phenomena are also found in the 20—35-day oscillation in the summer of 1991.(3) The propagation characteristics around the Tibetan Plateau are quite similar at the middle and high levels in troposphere, but a little different from the lower level.The zonal propagation direction at stratosphere especially 50 hPa is similar with that at troposphere, and the meridinal propagation is basically characterized by the standing-waves. The strongest LFO occurs at the tropopause layer (100 hPa).(4) The Meiyu over the mid-lower reaches of the Yangtze River with 30—60-day oscillation has the notable out-of-phase relationship with the 30—60-day low-frequency vorticity at 100 hPa over the east of the Tibetan Plateau in 1998. The southward LFO anticyclones from the north of the plateau and the northward from the south meet and convergence each other over the east of the plateau, which results in the intensification of the local ascending motion. The convergent LFO anticyclone over the east of the plateau cooperated with the LFO cyclone from Kamchatka Peninsula, then via Japan and arriving at the east of the continent and another LFO cyclones over Afghanistan forms the distribution during the peak period of the Meiyu, vise versa.(5) Different from the 30—60-day oscillation in 1998, the Meiyu over the mid-lower reaches of the Yangtze River with 20—35-day oscillation has the notable in-phase relationship with the 20—35-day low-frequency vorticity at 100 hPa over the the east of the Tibetan Plateau in 1991. The southward LFO cyclone over the northeast of the plateau from the north and the northward LFO anticyclone over the mid-lower reaches of the Yangtze River from the South China Sea (SCS) lead to the strong ascending motion and the persistence of the heavy rainfall over the mid-lower reaches of the Yangtze River, vise versa.