On the Role of Large-Scale Transient Eddies in the Maintenance of the Vorticity and Enstrophy of the Time-Mean Atmospheric Flow

Abstract

The global distribution of the forcing of the time-mean flow due to large-scale, horizontal Reynolds stresses (u? u?, v? v?, u? v?) is determined from upper wind statistics for the period 1968?73. The role of this forcing in the maintenance of the vorticity and enstrophy of the time-mean flow is discussed. The most striking effect of transient eddy stresses is the tendency to shift the subtropical maxima in the time-mean flow and the associated vorticity patterns poleward. However, significant longitudinal Variations in forcing occur, also. Calculations of the dominant terms in vorticity budgets of the North Pacific Low, the North Atlantic Low, and the Siberian High, which may he called the centers of action of winter-time circulation at sea level in the Northern Hemisphere, are presented. In all three cases, transient eddies are found to be important in maintaining the centers against the dissipative action of surface friction. In terms of the enstrophy budget, the hemispheric and global-mean effects of transient eddies on the mean flow are small. both in December?February and June?August. In the Northern Hemisphere, where the results are most reliable, the eddies are weakly dissipative with a time scale on the order of several months. When separating the time-mean flow into the contributions from the axisymmetric component and from the stationary disturbances, it is found that the transient eddy stresses tend to maintain the axisymmetric mean flow, but to weaken the stationary disturbances. There are significant latitudinal variations in the enstrophy forcing of the stationary disturbances. Thus eddy forcing is an important factor in maintaining the enstrophy of stationary disturbances in the extratropies, while it tends to destroy their enstrophy in the tropics.