冬季中东急流中心位置的年际变化及其与大气环流的联系

利用NCEP/DOE再分析资料,通过EOF分解、合成分析和线性回归等多种统计学方法,对年际时间尺度上冬季中东副热带西风急流(Middle East subtropical westerly Jet stream,MEJ)中心位置的变化进行研究,分析了MEJ中心位置的年际变化与大气环流的联系,找到了与MEJ中心位置相联系的关键大气环流因子。MEJ中心东西向位置变化显著,最西位于17.5°E,最东位于67.5°E,中间跨度达50个经度,表现出显著的年际变化特征。MEJ中心东西向位置变化与中高纬大气环流系统东大西洋遥相关(East Atlantic teleconnection,EA)和地中海辐合(Mediterranean Convergence,MC)异常存在紧密联系。当EA偏强时,北大西洋副热带地区、非洲北部和欧洲南部与北大西洋中高纬度地区南正北负的位势高度场分布,EA通过动力的直接作用使得MEJ西部轴线上纬向风减弱。另外,在Ekman抽吸作用下形成二级环流,偏强的EA环流会使对流层上层地中海地区出现辐合风异常,MC增强,地中海地区出现正异常的Rossby涡源,涡源所激发沿亚非急流东传的Rossby波活动出现异常,在波流相互作用下MEJ东部轴线上纬向风增强,该过程体现了EA对MEJ影响的间接作用。在EA和MC的共同作用下,200 hPa纬向风在MEJ轴线上西侧减弱东侧增强,急流中心出现向东移动。反之亦然。此外,除了EA的作用外,MC异常可能还受到其他中高纬环流大气因子的影响。

Interannual variation of wintertime Middle East subtropical westerly jet stream core location and its relation with atmospheric circulation

By using of National Centers for Environmental Prediction/Department of Energy reanalysis data,through some statistical methods,such as empirical orthogonal function analysis,composite analysis,regression analysis,we research the variation of wintertime Middle East subtropical westerly Jet stream (MEJ) core location based on the interannual time scale.The link between the interannual variations of MEJ core location and the atmospheric circulation is studied,and the critical circulation factors associated with the interannual variations of MEJ core location are found out.The east-west movement of the MEJ core is very pronounced.MEJ core extends from 17.5°E to 67.5°E,which spans 50 longitudes in the east-west direction.MEJ core''s location shows apparent interannual variation features.There is closed relation between the variation of MEJ core location and East Atlantic teleconnection (EA) and Mediterranean Convergence (MC) anomaly.When EA is robust,negative 200 hPa geopotential height anomaly occurs in the middle and high latitudes of the North Atlantic,and positive anomaly exists in the subtropical North Atlantic,northern Africa,and southern Europe.The geopotential height anomaly distribution with positive in the south and negative in the north makes the 200 hPa zonal wind at the western MEJ axis week under direct dynamical effect.Furthermore,through Ekman pumping to bring secondary circulation,the anomalous geopotential height with positive EA phase produces convergence wind anomaly in the Mediterranean Sea at the high level of troposphere,which strengthens MC with a positive anomalous vorticity source in the Mediterranean.Eastward propagating Rossby wave activities along Asian-African subtropical westerly jet motivated by vorticity source are anomalous,making the zonal wind at the eastern MEJ axis strong under the interaction of current flow and wave.The above process shows the indirect effect of EA on MEJ.Under the joint influence of EA and MC anomaly,200 hPa zonal wind weakens at the west part of the MEJ axis and strengthens at the east part,which makes MEJ core eastward movement.Vice versa.Moreover,besides the EA''s effect,other atmospheric circulation factors in middle and high latitudes maybe impact the MC anomaly.