Relationship Between Dry/Wet Variation and the Pacific Decade Oscillation(PDO) in Northern China During the Last 100 Years

Abstract

The 19012002 global 0.5°×0.5° grid data of monthly precipitation and monthly mean air temperature are used to establish a humid index to detect the dry/wet variation at the surface.The relationship between dry/wet evolution of four subregions in northern China and the Pacific Decade Oscillation(PDO) is studied using the index.The precipitation analysis shows that the precipitation of the western part of Northwest China in the last 100 years is markedly increasing during the two periods of 19251955 and 19621992 including the 23 year periodic change,and a decreasing trend occurs after 1992.In the eastern part of Northwest China,the precipitation in last 100 years presents 23 year periodic change without obvious variation trend and the precipitation range decreases after 1970.The decreasing trend has shown in North China since 1950.In Northeast China,the 25-year periodic change is very obvious,and a dropping trend is superimposed upon the periodic oscillation after 1950.The analysis result from the humid index is different from that of precipitation variation,especially in the two subregions of Northwest China.In the western part of Northwest China,the humid index does not show the increasing trend despite the enlarging trend of precipitation,but the dropping trend(not in precipitation) of humid index occurs in the eastern part of Northwest China.The decreasing trend of humid index is more obvious than that of precipitation in North China and Northeast China.This accounts for the temperature variation as one of the important reasons inducing and enhancing aridification in addition to the precipitation variation,especially in arid and semi-arid regions.In North China and Northeast China,the dry/wet variation at interannual and interdecadal scales is obvious.PDO is closely related to the dry/wet variation of northern China.The correlation is significant in North China,the eastern part of Northwest China and the western part of the Tibetan Plateau south of the Tianshan Mountain.The correlation between dry/wet variation and PDO in Southwest China and South China is also significant.The negative correlation is shown in North China and the eastern part of Northwest China.The positive correlation exists between the surface humid index and PDO in the western part of the Tibetan Plateau.The reverse sign of correlation coefficients between PDO and humid index occurs in eastern and western China with the boundary of 100°E.The anomaly analysis shows that the interdecadal anomaly phase of PDO is contrary to the phase of humid index in North China and the eastern part of Northwest China,and the same phase duration is about 2030 years.According to the interdecadal periodic calculation,the pattern shifts will occur possibly in future in North China and the eastern part of Northwest China,which means the wetting shifts for the two subregions.But the temperature increase enlarges the uncertainty for the calculation,and warming may delay or prevent this kind of shift.