The Role of Eddies in Maintaining the Westerlies in the Southern Hemisphere Winter

Abstract

The distribution of the mean westerly wind over the globe is described for June, July and August, the southern winter, using analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) from 1979 to 1982. The tropospheric momentum budget is analyzed from both the traditional Eulerian and the transformed Eulerian perspectives. Thus an assessment is made of the Eliassen-Palm flux divergence and the diabatically driven residual mean circulation. The vertical mean budget is also analyzed to allow deductions about the mean surface torque by the atmosphere on the earth and the grow surface stress. In the southern winter, transient eddies dominate the poleward momentum transports in both hemispheres, and total transports are somewhat larger than found in previous studies. Values of the deduced mean westerly surface stress are therefore larger than most previous estimates, but seem very reasonable and are probably more reliable in midlatitudes. A strange vertical structure analyzed to be present in the ECMWF zonal mean meridional wind is found to be inconsistent with the momentum budget, and the analyzed Hadley circulation is shown to be much too weak. The latter was expected since diabatic effects were not included in the initialization at ECMWF for this period. The total momentum budget is determined without including vertical eddy fluxes of momentum, but the residual is fairly small outside of the tropics and can probably be accounted for by fairly small errors in the analyzed divergent wind component. The traditional Eulerian view reveals that the midlatitude westerlies are maintained mainly by convergence of westerly momentum by the transient eddies, while the induced Ferrel cell decelerates the westerlies aloft and transports momentum down to the surface to balance losses by surface friction. The transformed Eulerian view shows that the net effect of the eddies in the upper troposphere, above 300 mb, is small, but there is a marked net deceleration by the transient eddies between 700 and 300 mb, and the westerlies there are maintained by the Coriolis torque acting on the diabatically driven residual mean circulation. The observed Ferrel cell is thus revealed to be a fairly small residual of the direct diabatically driven cell and the eddy-induced indirect cell. However, the vertical mean budget clearly shows that it is the meridional transport of westerly momentum by the eddies that is primarily acting to maintain the midlatitude westerlies against losses by surface friction.