Study On The Features Of Precipitation Low Frequency Oscillation Over The Southeast Tibetan Plateau

The characteristics of precipitation low-frequency oscillation (LFO) over the southeast Tibetan Plateau (TP) in the recent30years (1979—2008a) is investigated using daily NCEP/NCAR reanalysis and the observed daily rainfall data of730stations in China by Meteorological information center of China and high resolution gridded precipitation data over China provided by Chinese Academy of Meteorological Sciences. On this basis, the characteristics of rainfall LFO and its related element fields are analyzed over the southeast TP in the typical flood and drought years. At last, the relationship between rainfall LFO over the southeast TP and that of the middle and lower reaches of Yangtze River is investigated in the summer of flood year1998. The main results are shown as follows:(1) The southeast TP is the main precipitation area and sensitive area of precipitation. The10-20days (Quasi-biweekly Oscillation, QBWO for short) and30～60days (Intra-seasonal Oscillation, ISO for short) are the main periods of oscillation, especially the10-20days. The QBWO appear the most times, and its variance contribution is the largest.(2) There is obvious difference between the flood and drought years on the rainfall QBWO and ISO. The QBWO is the main type of oscillation in the drought years, while the ISO in the flood years. The variance contribution of QBWO is more significant in drought years, while the variance contribution of ISO is more significant in flood years. The100hPa divergence ISO and QBWO are mainly standing wave oscillation, and secondly propagate from west to east, but there are also some oscillation propagate from east to west. The standing wave and westward propagating are the main type of ISO, and the standing wave is the main type of QBWO in flood years. The standing wave and eastward propagating are the main type of QBWO in drought years.(3) The most significant flood year over the southeast TP is1998in the last30years, and the ISO variance contribution is the largest in the flood years. During the positive (negative) phase, there is a low-frequency cyclone (anticyclone) to the south side of the TP, and a low-frequency anticyclone (cyclone) in the Sea of Japan. The southeast TP is moisture convergence (divergence) LFO areas, heat source (sink) LFO areas and active (inactive) convection areas.(4) The most significant drought year over the southeast TP is2006, and the QBWO variance contribution is the largest in the drought years. The rainfall is more (less) when the QBWO phase and the ISO phase are in a superimposition. During the positive (negative) phase, there is a low-frequency cyclone (anticyclone) over the TP, and the southeast TP is moisture convergence (divergence) LFO areas, heat source (sink) LFO areas and active (inactive) convection areas.(5) The precipitation ISO (QBWO) exists both over the southeast and northeast TP in1998(2006), but the positive and negative phase don’t in accord with each other. Therefore, it’s not suitable to make the entire plateau averaged when we have an analysis of precipitation of LFO.(6) The southeast TP and the middle and lower reaches of Yangtze River have significant resonance period in the30-60d band in the summer of1998. A westward propagating ISO from the West Pacific passes the middle and lower reaches of Yangtze River and then to the southeast TP. It indicates that the rainfall ISO over the southeast TP may be influenced by the ISO westward propagating from the downstream areas.