Precipitation Anormaly In The Middle And Low Reaches Of Yangtze River During January-May And Its Causes

Based on the rainfall data from China Meteorological Administration,NCEP/NCAR monthly reanalysis data,monthly extended reconstructed sea surface temperature(ERSST) data supplied by NOAA and CPC (NOAA) indices, JFMAM precipitation anormaly and its causes in the middle and low reaches of the Yangtze River(MLRYR) have been analyzed by employing the EOF(empirical orthogonal function) and composite analysis in the present paper.The results as follows:(1) The average JFMAM precipitation in MLRYR area is 562.1mm, accounted for the annual precipitation of 42.5%, the climate characteristics of precipitation spatial distribution is in the south more, north much less. The wavelet energy spectrum analysis shows that MLRYR precipitation with 2-4 years variation periods. MLRYR rainfall explain 81.7% variance contribution of the annual variability (1-9 a), and the decadal variation (above 10a) variance contributionis only 18.3%, precipitation characteristics has significant interannual variations.SST key area located in the eastern and central Pacific, east of Philippines sea-Atmospheric circulation anomalies on east and central Indian ocean,WestPacific, Middle and EastPacific, WestEurope, Northeast Asia, eastern North American regional are the key regions of the MLRYR rainfall.(2) The years while precipitation more than normal, the lower(upper) tropospheric geopotential height anomaly is negative(positive) in MLRYR. SST Variations of the equatorial eastern Pacific,the South China Sea,northwest Atlantic are significantly related to the MLRYR precipitation, the eastern equatorial Pacific, South China Sea-Northwest Pacific region SSTA take on positive anomalies. WestPacific Subtropical High(WPSH) stronger, South China Sea abundant water vapor is transported to MLRYR, and MLRYR is water vapor convergence belt.(3) The years while rainfall less than normal, the lower(upper) tropospheric geopotential height anomaly is positive (negative) in MLRYR. WestPacific Subtropical High weaker, southerly wind was very weak in the West of WPSH, not conducive to the South China Sea water vapor transported to the MLRYR, at the same time MLRYR as the water vapor flux divergence zone. Significantly associated with SST of the NorthWest Pacific, North Indian Ocean, South Indian Ocean, the tropical Atlantic.The tropical Atlantic, North Pacific have positive SST anomalies.(4) The JFMAM precipitation of MLRYR vary according to the three principal modes:the first mode with in-phase change,the second with north-south out-of-phase,and the third with rainfall anormaly distributionalong the Yangtze River.Contribution of modes variance were 47%,19.6% and 6.6%, respectively. These 3 kinds of precipitation distribution type has 2～4 years variation periods.The aforementioned three modes are related to the different circulaion anomalies. WP, EU, AO respectively have better relations with the in-phase rainfall pattern, north-south out-of-phase pattern,along the Yangtze River rainfall pattern. The maintenance of the anomalous circulations related to the three patterns are found to be related to respectively the perturbations origginated from Europe and propagating easterward along the westerly belt, the wave train from Atlantic propagating across the Arctic areas to the middle and low reaches of the Yangtze River,and the Actic Oscillation.The in-phase rainfall pattern is well correlated with Nino3.4 index. The north-south out-of-phaseand along the Yangtze River rainfall pattern are linked to the eastern Pacific SSTA.