Analysis on Seasonally Evolutive Main Modes of North China Precipitation and Their Influence Factors

The seasonally evolutive main modes of North China precipitation and their influence factors are analyzed based on precipitation data from 120 stations in North China,NCEP/NCAR reanalysis data,and Kaplan extended SST data (Kaplan et al., 1998). The results show that there are two main seasonally evolutive modes in North China precipitation,with summer rainfall of the first mode above normal in the central,northern areas and below normal in the southern areas,with that of the second mode less in the northern areas and more in the central,southern ones,both are significantly different. The first mode is mainly controlled by the evolution of ENSO and the Indian Ocean Dipole event,and the second mode mainly by the evolution of the Indian Ocean Dipole. Although the sea surface temperature anomalous amplitude in the Indian Ocean is significantly smaller than that in the equatorial eastern Pacific,but it has a greater influence on the North China precipitation. The evolution of monsoon circulation influencing the two modes was different. To the first mode,in the spring,abnormal circulation similar to the summer wind appeares in the north of East Asia,in the summer,the spring anomalous monsoon circulation further strengthens,with westerly winds circulation of the India summer monsoon being absorbed into the East Asian summer monsoon in South China. To the second mode,in the spring,southerly wind anomalies also come into being in the East Asia region,in the summer,the East Asian monsoon further strengthens,but quickly turns to westerly wind after crossing the Yangtze River,no obvious Indian monsoon circulation combined into the East Asia summer monsoon in South China. In addition,the water vapor channel and the vapor source of the two modes are significantly different. In the summer of the first mode,there are three water vapor channels to affect North China precipitation,namely,southwest,southeast,and west wind water vapor channels,with the southwest water vapor channel being most important and its main source of water vapor being in the South China Sea and the East China Sea. In the summer of the second mode,the water vapor channel affecting North China precipitation is mainly southwesterly water vapor channel,and the main source of water vapor is in the Bay of Bengal and the South China Sea.