Abstract:

China is located in the largest land on the world the Eurasia,and close to the largest sea the Pacific Ocean. This strong contrast distribution of sea and land produces great thermal differences,strongly destroys the lower troposphere planetary wind zone distribution,and forms a strong monsoon circulation. Therefore,in China,the precipitation in the most area is controlled by monsoon somewhat. Predecessors research shows that the strength of monsoon gets the strong influence of the El Nino/Southern Oscillation ( ENSO ) events.ENSO phenomenon is the anomaly change of ocean temperature,and this kind of change can strengthen or weaken the thermal differences between sea and land,cause monsoon circulation strengthen or weaken. With the strength of the monsoon changing,the strength and the move for the Western Pacific Subtropical High have also been affected and further lead to rainband position and precipitation changes. Therefore,to most parts of China,ENSO incident has significant influence to the precipitation,and it reveals regional differences in the aspect of the influence way,the strength and sustained time. This pape analyzed the relationship between precipitation change and floods with ENSO events by X2test and compare analysis from 1951 to 2010 in the upper reaches of the Hanjiang River. The results show that the annual precipitation manifested a decreasing trend in the El Nino events years; in the La Nina years,compared with the years that no La Nina events occurred,precipitation slightly increased,but did not reach significant change level. Flood had close relation with ENSO events in the upper reaches of the Hanjiang River. At the end of last year or in the beginning of this year,or the years that strong El Nino and strong La Nina mutual conversed,large flood occurred easily,the correlation was more than significant level. At the end of last year or in the beginning of this year,as well as El Nino converse to La Nina in this year,the probability that occurred flood was the biggest. The correlation was more than a significant level,flood peak was big,and disaster was also heavy. It might cause the huge floods occurred, in which the maximum of peak discharge exceeded 20 000 m3/s or even more than 30 000 m3/s（at Ankang station）.In the years that no El Nino also no La Nina events happened,the probability of occurred large flood was lower relatively；in the years that had occurred the events of El Nino or La Nina,the frequency of big flood was also not high. Relative to El Nino event years,La Nina event years,especially in the years that La Nina Event occurred successively,the possibility of flood was larger. These results are of great importance in agricultural production，water resources development and flood mitigation over the upper reaches of the Hanjiang River