10-30-day Low-frequency Oscillation Characteristics Of 200 HPa Zonal Wind Over The Eastern Tibetan Plateau In Summer And Its Influence On Precipitation Over The Yangtze-Huaihe River Basin

Based on 753 stations daily precipitation data in China, the ERA-Interim reanalysis data of four times a day and Outing Longwave Radiation (OLR) data of NOAA, spanning from 1980 to 2013, using the wavelet analysis method, Lanczos filter and other statistical methods, the 10-30-day low-frequency oscillation characteristics of 200 hPa zonal wind over the Eastern Tibetan Plateau in summer and its influence on precipitation over the Yangtze-Huaihe River Basin are analyzed. The results show that:(1) The dominate period of 200 hPa zonal wind’s low-frequency oscillation over the Eastern Tibetan Plateau during summer is 10-30-day. Its spatial distribution and zonal propagation has significant differences when the intensity of 10-30-day low-frequency oscillation is different. In low-frequency oscillation strong (weak) years, its intensity is strong (weak), its duration is long (short) and it moves eastward (westward).(2)The 10-30-day low-frequency oscillation of 200 hPa zonal wind over the Eastern Tibetan Plateau has close relations with precipitation over the Yangtze-Huaihe River Basin. The precipitation lags behind the low-frequency oscillation over the Eastern Tibetan Plateau of a phase. Low-frequency system moves to the southeast from the Tibetan Plateau to impact the low-frequency precipitation over the Yangtze-Huaihe River Basin changing between the active and break stage. The 10-30-day low-frequency oscillation systems over the Eastern Tibetan Plateau reach the Yangtze-Huaihe River Basin after a phase and then affect the low-frequency precipitation here. At 200 hPa of the upper troposphere, there is a low-frequency anticyclone (cyclone) over the northeast Tibetan Plateau, then it moves southeast to the Yangtze-Huaihe River Basin. This will cause the westerly jet to move northward (southward), cooperating with high altitude ridge (trough). Thus, the Yangtze-Huaihe River Basin is influenced by low-frequency east (west) wind of the southern low-frequency anticyclone (cyclone). With the low-frequency convergence (divergence) at upper troposphere, this type of circulation is against (for) the low-frequency precipitation over the Yangtze-Huaihe River Basin. At 500 hPa of the middle troposphere, positive (negative) low-frequency potential height Center moves southeast to the Yangtze-Huaihe River Basin, and this will weaken (enhance) the trough over Northeast China, thus the Yangtze-Huaihe River Basin hasn’t (has) cold air transportation cooperation with the upper westerly jet. This type of circulation is against (for) the low-frequency upward vertical movement and the low-frequency precipitation over the Yangtze-Huaihe River Basin.(3) 10-30-day low-frequency oscillation over the Eastern Tibetan Plateau has a good effect in prediction of low-frequency precipitation process over the Yangtze-Huaihe River Basin, so taking the low- frequency oscillation system over the Eastern Tibetan Plateau as one of the prediction factors of the low-frequency precipitation process over the Yangtze-Huaihe River Basin will significantly improve the forecast accuracy. The low-frequency circulation center over the Eastern Tibetan Plateau, the Western Pacific Subtropical High at 500 hPa and convective activities in low latitude regions have significant differences before and after the low-frequency precipitation process over the Yangtze-Huaihe River Basin. Before the low-frequency precipitation process, the low-frequency systems of the above three areas have the intensity and position changes. Combined these three factors to build an integrated predictor, the forecast accuracy and false alarm rate are both improved comparing with forecasting by 2 or less factors, and the forecast accuracy rate reaches to 68.8%.