The Spatial And Temporal Distribution Of The Spring Persistent Rains And Its Relationship To Previous SST

By using the 753 stations daily precipitation data in China from 1979 to 2010, the spatial scale(23°-30°,110°-120°E) and time range(from 12 to 27 pentad) of the Spring Persistent Rains (SPR) are redefined, and the spatial and temporal distribution of SPR are analyzed by EOF(empirical orthogonal function) method. Two main modes of the region consistent type and south-north reverse type are obtained. On this basis, the atmospheric circulation background of different modes are discussed in terms of the NCEP/NCAR reanalysis dataset. The relationship between the spatial and temporal variability of the SPR and early SST is researched finally through correlation and synthesis method based on the Extended Reconstructed Sea Surface Temperature(ERSST) data supplied by NOAA. The main results are as follows:1) From climatic mean perspectives, during the SPR period, the southwest wind speed center located on the southeast of Tibetan Plateau has always existed, and there is a strong upward motion over the SPR area. This configuration is conductive to water vapor transport to the SPR area and further produces precipitation. In addition,Also in the 850hPa height field, the equipotential line on the southeast of Tibet Plateau is intensive and the cyclonic curvature here is large, which is an important reason for the enhanced southwest wind.2) The meridional and zonal land-sea thermal differences have important effect on the SPR, especially the zonal land-sea thermal differences between East Asia continent and the Western Pacific, for which has a good corresponding relationship with the SPR. The land-sea thermal differences is likely to be an important cause of the SPR, and it is significant for the definition of the nature of the SPR, because it is the fundamental driving force of the monsoon.3) When the SPR shows the whole droughts or floods anomaly, in drought years, the southwester on the southeast of Tibetan Plateau strengthens, and the western Pacific subtropical high enhances and extends westward, which are conducive to water vapor transport from the southern of the plateau and the subtropical high to the SPR area, and further induce the enhancement of ascending motion and precipitation over the SPR area. But in drought years, the contrary is the case. In addition, the zonal land-sea thermal differences between East Asia continent and the Western Pacific plays an important role in the onset date of the SPR, which indicates the subtropical monsoon precipitation characteristics of the SPR.4) When the SPR shows south-north reverse type, the southwest wind in the upstream area of the SPR has weakened whether it is in "northern drought and southern flood" years or "northern flood and southern drought" years, however, the southwest wind decreases more apparently when "northern flood and southern drought" happens. Analysis of the water vapor flux during the SPR period, it shows that the water vapor flux over the northern SPR area enhances and the southern part is reduced. Moreover, the upward motion over the northern SPR area is enhanced in "northern flood and southern drought" years, while the case is on the contrary in "northern drought and southern flood" years. This circulation configuration results in the north-south anti-phase pattern of the SPR.5) The relationship between the temporal and spatial distribution of the SPR and previous SST.Whole droughts or floods of the SPR have a greater negative correlation with the previous winter SST in Western Pacific warm pool area, When whose SST is higher than normal, the sea temperature near Philippines is sacked and become warmning under the drive of east wind in the north of the equator, therefore, the convection here becomes strong and Hadley circulation enhances which inhibits the ascending motion over the SPR area. At the same time, the southwest wind in the upstream area of the SPR weakens significantly, which causes rainfall less than normal. On the other hand is abundant rainfall.The north-south anti-phase variation of the SPR is in much close relationship with the SST near the Andaman Sea in the northeast of India Ocean. When SST of Andaman Sea is higher than normal in previous winter, the SPR area is easier to be "northern flood and southern drought". When the SST is abnormally low, the SPR area tends to be "northern drought and southern flood".