The Role Of Transient Eddy Forcing In The Variability Of The Northern Hemisphere Annular Mode During Boreal Winter

Using ERA-40reanalysis data for the period1958-2001during boreal winter,diagnostic analyses are firstly carried out for the role of transient eddy forcing in the time-mean flow in this paper. And then the interannual variability of the Northern Hemisphere Annular Mode(NAM) during boreal winter and its influence to the general circulation are investigated. On these base,the different roles of synoptic and low-frequency transient eddy forcing in the variability of the NAM is finally discussed. The conclusions are as follows:(1) As the climatic background field for the variation of general circulation,time-mean flow during boreal winter is affected by lots of factors,among which the effect of transient eddy forcing is indispensable.The transient thermal forcing with baroclinic structure can offset parts of the influence of diabatic heating in middle-high latitudes.lts low-frequency component,which mainly acts on the high latitudes,is stronger than the synoptic one.Acting on the mid-latitudes,the thermal forcing due to synoptic transient eddies is characterized by dipole-like structures over the oceanic storm tracks,and weakens the vertical shear of the eastward wind in these areas.The transient momentum forcing with equivalent barotropic structure can enhance the eastward wind in the midlatitude.Its low-frequency component is conspicuous,and also has dipole-like structures over the eastern part of North Pacific and Atlantic Oceans.The synoptic part of transient momentum forcing shows a banded distribution in the northern hemisphere,which strengthens the eastward wind in the north of jet with dipole-like structures too. On the whole,the transient eddy forcing can weaken the stationary wave in the upper troposphere during boreal winter.(2) The NAM during boreal winter,which refers to a anti phase oscillation between midlatitude and high-latitude, has a equivalent barotropic vertical structure, but its horizontal distribution is nonuniform.The NAM during boreal winter for the period1958-2001is persistently enhanced.After removing this linear tendency,the NAM is dominated by a quasi-8yr period. (3) The influence of the NAM to the general circulation and climate is very important during boreal winter.When it’s in its negative phase,the trough-ridge system and atmospheric active centers in East Asia are all strengthened.This would be benifit to the southward moving of the cold air. With the cold air,the surface air temperature of the northern part of Eurasia and China would be decreased.But the one in South Asia and most areas of South China would be increased because of the weakened South Asian winter monsoon and East Asian winter monsoon in South China.The precipitation of most areas in South China would be decreased due to the weakened southern westerlies trough. Vice versa.(4) The variability of NAM is described as a zonally symmetric latitudinal shift of the midlatitude jet during boreal winter.The role of synoptic transient eddies(2.25-9days) in the variability of the NAM is different from that of the low-frequency transient eddies(10-90days) because of the difference between where they generate.(5)The synoptic transient eddies(2.25-9days) always act on the center of the midlatitude jet where they generate during the latitudinal shift.Its thermal forcing cools the southern part of the jet,but warms the northern one,and thus weakens the lower-level baroclinicity of the jet. However,the eastward wind at the center of the jet is always enhanced by the momentum forcing of the synoptic transient eddies,which is beneficial to the persistence of the NAM.(6) The low-frequency transient eddies(10-90days), which generate at the midlatitude jet flanks, still shift with the jet. Its momentum forcing always acts on the jet flanks and enhances the eastward wind there.But it has a opposite effect at the center of the jet. The thermal forcing due to low-frequency transient eddies strays away from the midlatitude jet during the shift.When the jet moves southward(northward),the thermal forcing will move further more.It strengthens the lower-level baroclinicity and leads to eastward wind in the the upper troposphere in the south(north) of the jet,but the opposite effect in the north(south),which pushes the jet to move further south(north). During this process,the low-frequency transient eddy forcing can destroy the anti phase oscillation between midlatitude and high-latitude in the NAM,and goes against the persistence of the NAM.Compared to the synoptic transient eddy forcing,the role of the low-frequency transient eddy forcing in the variability of the NAM during boreal winter is more significant.