Temporal Variation Of Coherent Drought And Flood In Yunnan Over The Past 500 Years

Yunnan Province is located in the southwest of China and southeastern Qinghai-Tibet Plateau.It is influenced by the South Asian monsoon,the East Asian monsoon and the Qinghai-Tibetan Plateau monsoon.Droughts and floods are the major meteorological disasters restricting the social and economic development of Yunnan.In this thesis,time series of dryness/wetness index over recent 55,160,and 530 years were developed using the classification method of drought and flood based on the observational precipitation and historical archives in Yunnan.These data together with PDSI data reconstructed by tree-ring were analyzed using principal component analysis(PCA)to detect spatial mode of drought and flood in Yunnan.Regime shift detection(RSD)and wavelet analysis were used to analyze the time series of major spatial mode scores for revealing their temporal variations and possible cycles.Correlation analysis and slide-window correlation analysis were also used to explore effects of different climatic factors on temporal variation of major spatial modes of drought and flood in Yunnan and their possible driving mechanisms.The main conclusions are as follows:1 In the past 55 years,the two main modes of spatial patterns of drought and flood were revealed by PCA on rainy-season dryness/wetness index in Yunnan.One mode is coherent pattern,and another one is northwest-southeast dipole.In coherent pattern,climatic conditions in the central region are more severe than others although whole Yunnan experiences drought or flood;in the northwest-southeast dipole,the northwest see drought(flood)as the southeast of Yunnan experiences flood(drought).Time series of the 1st PC(coherent mode)scores derived from rainy-season dryness/wetness index shows a significant shifts in 2009.The correlation test suggests that the South Asian summer monsoon(SASM)and the South China Sea summer monsoon(SCSSM)are dominant factors influencing two modes of spatial patterns during the rainy-season respectively on the interdecadal scale.2 In the past 160 years,the 1st PC reveals coherent mode of spatial patterns during the rainy season.The time series of the 1st PC scores shows a slight downward trend,indicating that climatic conditions change gradually from wet to dry.RSD of the 1st PC scores suggests the occurrence of a shift in 1979.On the decadal scale,Sunspots,South Asian summer monsoon(SASM),Pacific decadal oscillations(PDO)and North Atlantic oscillations(NAO)all would have a significant effect on the temporal variation of this coherent mode of rainy-season dryness/wetness index in Yunnan.3 In the past 530 years,the 1st PC reveals coherent mode of spatial patterns during the rainy season again,but it explains more variance(69.5% of total variance)than PCs above.RSD of the 1st PC scores shows that regime shifts from dry to wet occur in 1505 and 1652,and from wet to dry in 1603 and 1979.Wavelet analysis of the 1st PC scores indicates the occurrence of 6a,14 a,35a and 130 a periods in temporal variation of coherent spatial mode of rainy-season dryness/wetness index in Yunnan.The comprehensive analysis also shows that sunspots,temperature,South Asian summer monsoon and Pacific decadal oscillations might play important roles in coherent drought and flood in Yunnan.4 At different time scales,coherent mode is a significant spatial pattern for rainy-season dryness/wetness in Yunnan,in which the central part of Yunnan is the most severe area of drought and flood.This study suggests that solar activity probably is an external factor affecting drought and flood during rainy-season in Yunnan.As the solar activity weakens,the surface temperature difference between sea and land reduces,the South Asia and the East Asian summer monsoon thus weakens,leading to less precipitation in Yunnan,and vice versa.