Climatic Effect Of Springtime Extratropical Cyclones Over Changjiang River-Huaihe River Valleys And Its Possible Mechanism

Based on the data 6-hourly 850 hPa geopotential height field from ERA-Interim 1979-2013,extratropical cyclones that occur during spring in Changjiang River-Huaihe River Valleys(CHV)were detected by using 2D cyclone area objective identification method that recognizing the outermost enclosed contours of cyclones.The spatial-temporal characteristics of springtime extratropical cyclones over CHV were analyzed.According to the horizontal scale,springtime extratropical cyclones over CHV were divided into different categories,and the intensity index of synoptic-scale cyclones and sub-synoptic scale cyclones were defined respectively and their relationship with regional precipitation anomalies were investigated.The results show that:(1)The 2D cyclone area objective identification method has a good expression ability to describe spring extratropical cyclones over CHV,which have many kinds,but most of them were formed and died in CHV.According to the classification of horizontal scale,sub-synoptic scale cyclones are the most and synoptic-scale cyclones are the next.(2)The interannual variations of synoptic-scale cyclone activity index display significant positive correlation with the precipitation anomalies and the occurrence of frequency of heavy precipitation events over CHV.A significant quasi-barotropic high-pressure center exhibits in northeast Asia when synoptic-scale cyclone activity index intensifying,which is mainly induced by the upper troposphere warming that weakens the meridional temperature gradient over the south of Japan.The circulation configuration is conducive to a decrease upper westerly jet and a positive vorticity advection in the northern part of CHV accompanied by an anomalous southwesterlies in the middle-low troposphere in the south of CHV.The horizontal trough cyclonic circulation anomalies in CHV results in an increase of the vorticity in this area.The associated low-level convergence and high-level divergence provide favorable dynamic lifting conditions for the development and the activation of synoptic scale cyclone in CHV.The abnormal circulation configuration of the lower level horizontal trough is favorable to the intensified of southwest warm-wet air flow in south China and the anomalous easterlies/southeasterlies of water flux in northern CHV,and thus leading to the increase of the regional cloud cover and precipitation anomalies.(3)The inter-annual variations of sub-synoptic scale cyclone activity index display significant positive correlation with the precipitation anomalies and the occurrence of frequency of heavy precipitation events over southeast CHV as well.When sub-synoptic scale cyclone activity index intensifying,the geopotential height field of CHV is a baroclinic structure with upper positive and lower negative,and lower layer is a cyclonic wind field.The increase of atmospheric baroclinic and warm advection in the lower layer in southern China,including southern CHV,are both conducive to the increase of vorticity over CHV,corresponding to the configuration of convergence in the lower layer and divergence in the upper layer,which provides the dynamic conditions for development and maintenance of sub-synoptic scale cyclone.Southwest warm-wet air flow in southeast CHV and cyclonic water flux lead to the increase of the regional cloud cover and precipitation anomalies.(4)The sub-synoptic scale cyclone activity index decline in the early 2000s.The standardized average monthly precipitation of key area has a same interdecadal decline as I1.The warming anomaly structure in the middle and upper troposphere near Tibetan Plateau weaken meridional temperature gradient on the south side,resulting in EASJ land branch weakened,and the corresponding vertical wind shear in south of CHV is decreased,thereby weakening the low layer atmospheric baroclinicity.The cyclonic wind field in CHV is also weakened,and activity of sub-synoptic scale cyclone is decreased,with the reduced water vapor transport and water vapor convergence,which eventually lead to reduction of precipitation anomalies and occurrence of frequency of heavy precipitation events in southeast CHV.