On the mean meridional circulation and the flux of angular momentum over the northern hemisphere

Abstract

The mean meridional circulation north of the latitude 15° N has been determined by an indirect method, which uses the angular momentum and continuity equations under the assumption that the vertical eddy flux of the zonal momentum is negligible in the free atmosphere. The method requires a knowledge of the (time-space) mean zonal motion and the mean meridional eddy flux of angular momentum. Data for these quantities have been derived for four seasons of the year (December-February, March-May, June-August and September-November from the upper winds statistics charts as published by CRUTCHER (1959). For each season an essentially three-cell circulation pattern is obtained with the strongest cell intensities occurring in winter and the weakest in summer. Except in summer, the direct cell in the lower latitudes is stronger than the indirect cell in the middle latitudes. The direct cell in the high latitudes is found to be very weak in all seasons.

The poleward transport of angular momentum and its different mechanisms are discussed. The role of the mean meridional circulation in the net flux across different latitudinal circles is found to be small except in the low latitudes in winter and spring. The transient eddies are in every season the main agency of the flux, but the standing eddies are also important, particularly in winter. The mean zonal stress of the atmosphere on the earth is derived from the convergence of the angular momentum flux.