1970s The 20th Century Changes In The Mid-north Pacific Sst Decadal Climate

Based on analyzing the characteristic of time and spatial distribution of North Pacific SSTA, the influence of North SSTA and its decadal variation was discussed on the rainfall in the east China during 1950 -1999 by using the methods of EOF, SVD, wavelet analysis composite analysis and correction analysis. And the relationship between SSTA, rainfall and 500hpa geopotential field are also studied. Data used in this work are North Pacific SSTA , 160 stations precipitation of China, and NCEP reanalysis data. Main results are as follow:(1) It is found that a apparent transition of north Pacific SSTA in later 1970's: eastern and middle-equatorial Pacific SSTA turns from cold to warm with area extending, and mid-latitude Pacific (west wind drift zone) turns from warm to cold. During this transition of SSTA, different characters also appear in El Nino and La Nina: before 1976, La Nina happens more frequently, and its duration is longer, El Nino zone develops from negative SSTA in the early stage; after 1976, El Nino happens a little bit frequent and longer with more intensity than before, El Nino zone develops from positive SSTA in the early stage; the course of SSTA variation has an ENSO cycle of 2-6 years, annual oscillation of 8-9 years, and decadal variation of about 22 years.(2) Around mid-1970s', precipitation of east China has also a great change: precipitation of north and south China decreases, rainfall of basin of Yangtze river and north-east China increases, the distinct areas are drainage of Yangtze river, Huanghuai and west south China. Before or after 1976, the year and the next year while El Nino or La Nina happens, the distribution of rainfall anomaly shows opposite characters in most area of east China. It shows that although the SST is not the only factor, it can influence greatly the rainfall of east China when SSTA has a great changes, meanwhile SVD analysis and correlation analysis also approve this influence and relationship between North Pacific SSTA and precipitation of China.(3)As a direct influencing factor to rainfall and a bridge between SST and rainfall, the 500hpa geopotential of Asia and pacific also has a great change before and after 1976: after 1976, geopotential of Baikal increases, west-pacific subtropical Height increases. These make west-pacific subtropical Height strengthen and extend westward and blocking form and develop in mid-high latitude, while its above west wind drift zone decreases. And around 1976, 500hpa anomaly field of winter and summer also changes. More analysis indicates that SST and 500hpa field, 500hpa field and precipitation of China have distinct correlation, and it can explain the relation between SST and China rainfall: winter SSTA can be transferred by circulation and influence precipitation of east China, in particular, the transition of SST in 1976, make the circulation and rainfall change.