The Climatology and Annual Cycle of Meridional Transport of Westerly Momentum by Stationary Waves in the Northern Hemisphere

The climatology and annual cycle of zonally averaged meridional transport of westerly momentum by stationary waves(also called the eddy momentum flux) in the Northern Hemisphere are presented and analyzed using European Centre for Medium-Range Weather Forecasts(ECMWF) reanalysis(ERA40) data.Here waves are defined as the departure from zonal means.The Fourier expansion method is adopted to isolate different zonal wave numbers.The climatology of eddy momentum flux by total stationary waves,planetary-scale stationary waves,synoptic-scale stationary waves and short waves is analyzed for four seasons,respectively.The annual cycle of eddy momentum flux is also analyzed in two selected pressure levels(250 hPa and 5 hPa). The result shows that in the troposphere,the most significant flux center,whose transport direction is always towards the North Pole,lies in the upper troposphere at mid-latitudes all the year round,and its intensity and position vary with seasons.Besides this flux center,there are other two flux centers in the troposphere,with one in the mid-troposphere at high latitudes and the other in the tropical tropopause level.In the former center,which extends from the lower troposphere to the lower stratosphere,the transport direction is always towards the south.The position of this center is relatively fixed although its intensity varies obviously with seasons.The position of the latter center also changes little with time;however,the direction of eddy momentum flux in this area varies with time: it is towards the south in winter and towards the north in summer.In the stratosphere there is only one significant flux center in the upper stratosphere at middle and high latitudes.In this center the momentum transport by waves is always towards the north.The position of this center changes little with time;the transport intensity in this area varies with seasons: it is the strongest in winter,weaker in autumn,much weaker in spring and nearly zero in summer.The planetary-scale stationary waves(zonal wave numbers 13) play a crucial role in the process of eddy momentum transport and dominate the intensity variation of momentum transport.They lead to the intensity variation in the upper troposphere at mid-latitudes and dominate the intensity and position of the rest three flux centers.The synoptic-scale stationary waves(zonal wave numbers 49) mainly transport westerly momentum in the upper troposphere at mid-latitudes;they dominate the north-south movement of the flux center in this area.Moreover,they,together with the planetary-scale stationary waves,determine the transport intensity in the troposphere at mid-latitudes.Short waves,whose wave number is larger than ten,mainly transport westerly momentum in the upper troposphere at mid-latitudes.The eddy momentum flux produced by short waves is about half of that produced by stationary waves in summer.Hence,the transport effect by short waves should be considered in summer and is negligible in other seasons.The climatology and annual cycle of zonally averaged meridional transport of westerly momentum by stationary waves presented in this paper is not only a basis for further research on their interannual variability,but also an important reference for the GCM simulations.