Mean meridional circulations in the Northern Hemisphere lower stratosphere during 1964 and 1965

Abstract

AbstractZonally‐averaged mean monthly circulations in the meridional plane, which have been forced from momentum and heat sources, are presented for the 100–50 mb, 50–30 mb and 30–10 mb pressure layers, between 25°N latitude and 80°N latitude, during 1964 and 1965. The zonal momentum and continuity equations, written in spherical coordinates, are combined, using an iteration process, to compute mean meridional and vertical motions.Fixed boundary values are required for the vertical component at 10 mb where the downward vertical integration starts and for both components at 22·5°N and 82·5°N latitudes. These values are computed from the initial guess fields, which are based on approximate thermodynamic balance for the vertical component and a momentum balance for the meridional component. The effects of these boundary conditions on the interior motions are discussed.The results of the computed velocity components show a reasonable and consistent pattern, both in time and space. In winter months, motions are much more intense than in summer months and the flow patterns are characterized by cell‐like structures, having upward motions over the pole and downward motions over middle latitudes. It appears as though the lower stratospheric motions over mid‐latitudes are an extension of the tropospheric indirect mean cell. There is a poleward shift in the motions as winter progresses into spring and an equatorward shift as autumn progresses into winter. In summer, the motions are small and not too well organized.In the January and July 1964 momentum budgets, it is shown that a near balance exists between the Coriolis torque and the convergence of eddy momentum flux, which gives an approximate expression for calculating mean meridional velocities. In the January 1964 heat budget, at middle latitudes, a near balance is shown between adiabatic subsidence and heat sources due to radiation and eddy heat‐flux convergence. This balance implies an approximate solution for mean vertical motions. However, this approximation does not appear to be valid near the latitudinal boundaries during January or in certain regions during July.