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Progress in the Studies of Three-Dimensional Structure of Interdecadal Climate Change over Eastern China

YU Rucong1,2,ZHOU Tianjun1,LI Jian1,2,and XIN Xiaoge11 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029 2 State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,Beijing 100081  
East Asian climate has experienced a decadal scale shift by the end of the 1970s.By combining pieces of interdecadal climate change found in different regions and different seasons together,the authors have attempted to clarify the interdecadal climate change over East Asia within the context of three-dimensional(3-D) coherent structures.The progress achieved in recent five years is summarized in this paper.The results indicate that the decadal scale shift of East Asian climate has a distinct 3-D structure.The surface climate change is coherently connected with the temperature change over the middle and upper troposphere,with clear seasonal features.A prominent cooling trend is found over East Asia in the upper troposphere around 300 hPa.Accompanying this summer cooling the upper-level westerly jet stream over East Asia shifts southward and the East Asian summer monsoon weakens,which results in the tendency toward increased droughts in northern China and more flood along the Yangtze River valley.The change of the westerly jet modifies the divergence over the middle and upper troposphere,which then modulates the cloud-radiation feedback processes and contributes to the observed springtime surface cooling trend in the eastern periphery of the Tibetan Plateau.The upper troposphere cooling over East Asia and the surface cooling downstream of the Tibetan Plateau began from March.The decadal scale cooling is related to the positive trend of the North Atlantic Oscillation in recent decades.In the upper troposphere,the strongest cooling trend occurs in April and August,but in the lower stratosphere,the strongest cooling occurs in June.This sub-seasonal variation of the upper tropospheric and lower stratospheric cooling trends induces corresponding fluctuations in the tropospheric geopotential thickness,resulting in sub-seasonal variations in the subtropical westerly jet,lower-tropospheric pressure and the location of the rain band associated with the East Asian monsoon front.The rain band experiences a southward shift in April and May,causing a drought-like trend in the late spring precipitation in South China(26°N-31°N,110°E-122°E).In June,when the rain band "jumps" to the middle-lower reaches of the Yangtze River,the subsequent intensification of the westerly south of the subtropical jet during July and August resists its further northward migration,resulting in the so-called Yangtze-River-flooding-and-North-China-drought pattern and the weakening of the southerly monsoon.The observational evidence presented here provides a metric for validating climate model experiments that aim at explaining the causes of the Yangtze-River flooding-and-North-China-drought pattern.The authors further examined the outputs of nineteen IPCC AR4 coupled climate models driven by historical natural and anthropogenic forcing agents.The results show that the prescribed natural and anthropogenic forcing agents in the coupled climate models mainly reproduce the warming trends and the decadal to interdecadal scale SAT(surface air temperature) variations.Nearly all of the models failed in capturing the regional features of the interdecadal scale climate shift over East Asia.Limitations of the current state-of-the-art coupled climate models posed new questions to climate change attribution studies and improvements of model performance over the East Asian domain.
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