ON MEAN MERIDIONAL CIRCULATIONS IN THE ATMOSPHERE

Abstract

The purpose of the study is to make a detailed investigation of mean meridional circulations forced by given eddy transports of heat and momentum and to describe the vertical variation of the energy conversions for the zonally averaged flow. A nonhomogeneous second-order partial differential equation for the vertical p-velocity, ?, is obtained from the quasi-geostrophic vorticity and thermodynamic equations. The method of separation of variables is used to solve the zonally averaged form of this equation such that zonally averaged vertical p-velocity, ?z, is expressed as a series of Legendre polynomials. The boundary conditions used are that ?z is zero at the top of the atmosphere and that at the surface it is equal to that value of ?z which is produced by the topography of the earth. After the solution for ?z is obtained, the mean meridional velocity is determined from the zonally averaged continuity equation. The diabatic heating in the meridional plane is estimated from the zonally averaged steady-state thermodynamic equation. Computations of the zonal available potential energy and the conversion from zonal available potential energy to zonal kinetic energy are made using the distributions of diabatic heating, the vertical p-velocity and the temperature in the meridional plane. The general conclusions which can be drawn on the basis of the calculations are: (1) Three-cell meridional circulations are produced by the eddy transport processes in the atmosphere. (2) The eddy transport of momentum is twice as effective as the eddy transport of heat in forcing the meridional circulations. (3) The influence of the planetary scale motion on the circulation is predominant during winter whereas that of the baroclinically unstable waves dominates the forcing mechanism during the other seasons. (4) The seasonal variation of the meridional circulations shows that the circulation cells move toward the pole and undergo a decrease in their intensity from winter to summer. (5) The net diabatic heating in the meridional plane is positive south of 40°N. and negative north of that latitude during winter months. In the upper troposphere, the heating decreases gradually with height in the region of net heating whereas the cooling decreases sharply in the region of net cooling. (6) The generation of zonal available potential energy is maximum in the lower troposphere, decreases sharply with height, and becomes negative in the lower stratosphere. (7) The conversion from zonal available potential energy to zonal kinetic energy is positive in the lower troposphere and negative in the upper troposphere.