Abstract
The nature and extent of the interactions between the zonally averaged current (ū) and synoptic-scale eddies, including the contribution of the eddy-induced men meridional circulation, are assessed. Eddy fluxes of heat and momentum determined by Oort and Rasmussen are used as forcing functions in the diagnostic equation for the streamfunction corresponding to the mean meridional circulation. This equation is solved by relaxation and the solution is used to evaluate the contribution of the eddy-induced meridional circulation to the acceleration of the zonal current (∂ū/∂t). The net acceleration (i.e., the sum of the accelerations due to this meridional circulation and to the convergence of the eddy flux of momentum) is evaluated and is presented as a function of latitude and pressure height for each of the four seasons. Also presented is the meridional distribution of the net kinetic energy generation (ū∂ū/∂t) by the eddies, and the hemispheric integral of this quantity. The net annual generation of zonal kinetic energy by the eddies, including the contribution of the eddy-induced meridional circulation, is found to be ∼70% of the net annual generation neglecting the contribution of this meridional circulation. The distribution of ∂ū/∂t and of ū∂ū/∂t due to the eddies reveals, however, that the maxima of these quantities are centered poleward of the zonally-averaged jet core.
In an effort to assess the extent to which diabatic beating is responsible for the location and seasonal changes in the position of the jet core, a similar calculation is made using only the diabatic heating distribution given by Newell et al. as a forcing function for the mean meridional circulation. The acceleration (∂ū/∂t) and the net kinetic energy generation (ū∂ū/∂t) due to diabatic heating are found to be centered in the tropics, equatorward of the zonally averaged jet core. Hence, diabatic heating alone does not maintain the zonally averaged jet in the position we find it. Apparently, the combined contributions of the eddies and of diabatic heating, where neither has its maximum, create the positive acceleration and kinetic energy generation required to maintain the jet core where we find it.
In middle latitudes, poleward of the jet core, internal dissipation and the meridional Ekman circulation induced by surface friction must combine to decelerate the air being accelerated by the eddies. In the tropics, equatorward of the jet core, the acceleration of the westerlies by diabatic heating is largely counterbalanced by the deceleration by the eddies, leaving a small residual acceleration to be taken care of by dissipative processes.
