Momentum Transfer Across an Asymmetric Jet

Abstract

The northward momentum transfer across an asymmetric jet in a three-dimensional atmosphere is examined by means of an initial-value problem. The flow is contained between two latitude circles in the horizontal and between the ground and the tropopause in the vertical. The motion is governed by the potential vorticity equation. The initial horizontal flow consists of a disturbance superimposed upon a slightly asymmetric west-cast zonal current. The initial momentum transfer vanishes identically. A solution is obtained for the tendency of the momentum transfer. It is found that the small asymmetry in the zonal current is responsible for the strong northward momentum transport in the region of the strongest zonal flow. The variation of the momentum transfer both in the horizontal and in the vertical agrees satisfactorily with the observed transfer. It is found that an initial maximum meridional velocity of 10 m sec?1 is required in order that after one day the predicted transfer becomes equal to the observed transfer at 200 mb and 3ON. The transfer of energy between the zonal flow and the disturbance is discussed. The transformation from perturbation kinetic energy to basic-flow kinetic energy has a small positive value and the transformation from basic-flow potential energy to perturbation potential energy has a larger positive value. The disturbance will increase its energy by about 10 per cent after one day.