The Influence Of Midlatitude Transient Eddy Activities On The Summer Mean Circulation And Precipitation In East Asia

The mid-to-high latitude circulation and its accompanying transient eddy activity have an important influence on the summer weather and climate in East Asia.Based on the CFSR reanalysis data,the structure of summertime mean circulation and the characteristics of transient eddy activities in East Asia are first analyzed.Then by using the WRF model,in north boundary experiment,a control run and a sensitivity run are designed to simulate the East Asian mean circulation and precipitation in summer that with/without the influence of the midlatitude transient eddy activities in northern East Asia,respectively.Likewise,in west boundary experiment,a control run and a sensitivity run are designed to simulate the East Asian mean circulation and precipitation in summer that with/without the influence of the midlatitude transient eddy activities in upstream region,respectively.By contrast,the contribution of midlatitude transient eddy activities in the northern East Asia and the upstream region to the East Asia summertime mean circulation and precipitation is revealed.The main conclusions obtained are as follows:The East Asian monsoon region presents a baroclinic structure south of 35°N with tropical circulation characteristics,driven by heat,and a barotropic structure north of35°N,with mid-latitude circulation characteristics,mainly driven by transient eddies.The mid-latitudes of East Asia,especially the northern Mongolia-Northeastern China and the upstream area centered on Lake Balkhash,are the main sources of transient eddies in summer.The midlatitude transient eddy activities can change the background field by systematically transporting momentum,heat,moisture and vorticity,thereby affecting the mean circulation and precipitation in summer.Numerical simulation studies further show that when the midlatitude transient eddy activities in northern East Asia are greatly weakened,their induced poleward momentum,heat and moisture flux transport are also significantly weakened.On the one hand,the decrease of poleward heat and moisture flux transportation increases the mean temperature and water vapor in the lower layers in the eastern continent of East Asia,which provides unstable conditions and moisture conditions for precipitation.On the other hand,the change of transient eddy dynamic transportation causes an equivalent-barotropic structure response in the circulation field.The entire eastern part of China is controlled by stronger southwesterly,thus the East Asian summer monsoon strengthens and extends to further north.The combined effect of these two factors result in moisture convergence and enhanced convection in most parts of eastern China,leading to increased precipitation in this area.When the transient eddy activity in the upstream area is greatly weakened,the polar momentum,heat and water vapor transport produced by it are also significantly weakened.On the one hand,the weakening of the transient meridional moisture transport is most obvious in southern China.The moisture transport converges in the southeast coast of China and the South China Sea,causing the increase of moisture in this area and providing moisture conditions for precipitation.On the other hand,the dynamic transport changes of the transient eddies form a high response with a equivalent-barotropic structure over the entire East Asian continent,and a low response with a equivalent-barotropic structure from eastern East Asia to the southeast coastal area,which weakens the southwest wind along the southeastern coast of China.As a result of dynamic effect,the upward movement in China's southeast coast and the South China Sea increased,and the East Asian summer monsoon weakened and retreated southward.Under the combined effect of dynamic and heat,moisture convergence and enhanced convection occurred along the southeastern coast of China and Taiwan,resulting in increased precipitation in the southern area.