Diagnostic Study of the Momentum Balance in the Northern Hemisphere Winter Stratosphere

Abstract

An attempt was made to determine the contribution of small-scale eddies to the zonal mean momentum balance of the Northern Hemisphere stratosphere between 100 and 0.4 mb during four December and four Januaries (1976–80). The approach adopted was to compute the Eulerian mean meridional circulation from a zonal mean heat budget obtained using heat fluxes from the large-scale eddies resolved by NMC analyses. This meridional circulation was then employed in an examination of the zonally averaged zonal momentum budget. In general, the eddy momentum flux convergences needed to satisfy the momentum balance cannot be entirely supplied by the eddies in the NMC analyses. The residual in the momentum budget is tentatively attributed to the effects of eddies with space or time scales too short to be resolved in the NMC data. The results suggest that small-scale eddies must contribute zonal mean easterly accelerations of about 10 m s−1 day−1 at the midlatitude stratopause. This is in rough agreement with the recent theoretical predictions of the eddy momentum flux convergence due to vertically propagating gravity waves (Lindzen, 1981; Holton, 1982).