Role Of The Low Frequency Oscillations In The Heavy Rainfall Processes In The Yangtze-Huaihe River Valley During Boreal Summer

Using the NCEP/NCAR reanalysis and in situ precipitation daily data in 2003 and 2001, thedifferent roles of low frequency oscillations in anomalous flood/drought years in theYangtze-Huaihe River Valley during Jun to August have been studied in the present paper. Theresults are summarized as follows:(1) Both 30～70d (intraseasonal) and 8～15d (bi-weekly) oscillations have importantcontributions to the total precipitation variations in the Yangtze-Huaihe River Valley in thesummer of 2003. These two kinds of oscillations seem to be localized in a complementary way;the place where the 30～70d oscillation is stronger shows the weaker 8～15d oscillation, andvise versa. The low frequency circulations on these two time scales are of baroclinic structure;they have the positive vorticitiy in the lower troposphere, coupled with the negative one in theupper troposphere during the torrential rain periods. The positive anomalies of the precipitation inthe Yangtze-Huaihe River valley are strongly influenced by the 30～70d low frequency wavepropagations (P—J like) over the west and northwest Pacific regions at 850hPa, whereas at200hPa are by the 8～15d low frequency cyclones/anticyclones that move zonally eastward inthe mid-latitude, showing the eastward propagations of Rossby-wave energy. Fluxes of the watervapor integrated vertically from the earth surface to the top of the troposphere on 30～70d and8～15d time scales indicate that southwestly monsoonal winds along the edge of the westernPacific subtropical anticyclone are responsible for the water vapor transport. The moisture on30～70d time scale is found to be transported from the South China Sea. That on another timescale is from both the South China Sea and the west Pacific.(2) The 30～70d oscillation is observed clearly and localized in the Yangtze-Huaihe RiverValley in the summer of drought year 2001. In this year, the 30～70d low frequency oscillation asindicated by the vorticitiy anomalied in the lower and upper troposphere displays a feature of, tosome extent, barotropic structure, which leads to much tess rainfall in 2001. The western Pacificsubtropical anticyclone is more northern and eastern than normal, relating to the abnormal lowfrequency wave propagations (P—J) over the west and northwest Pacific regions. Interestingly, the ridge of high pressure at 500hPa appears in the middle and high latitudes very often,preventing the cold air from moving southward, resulting in the less than normal rainfall in theYangtze-Huaihe River Valley, although the moisture is rich in this region. In virtue of theanomalies of low frequency circulation, the moisture for the weak precipitation is from the Bohaibay and the west Pacific, which is quite different from the moisture source in the flood year 2003.