The Nonlinear Effects of Transient and Stationary Eddies an the Winter Mean Circulation. Part I: Diagnostic Analysis

Abstract

The nonlinear effects due to transient eddies and stationary eddies are analysed for the large-scale winter mean circulation, based on the potential vorticity equation for the zonally asymmetric time-averaged quasi-geostrophic motions. Using the geopotential height for January 1963, the individual terms of the potential vorticity equation are evaluated. Throughout the troposphere the linear advections dominate the nonlinear which, however, are by no means negligible. At the mid-tropospheric level the transient eddy term is large north of 40°N. The distribution appears to be correlated negatively with the time-mean asymmetric potential vorticity, suggesting its dissipative role. On the other hand, the non-linear advection by stationary eddies has no clear correlation with the time-mean potential vorticity. In order to estimate the role of these nonlinear terms in maintaining the stationary geopotential field, the potential vorticity equation is solved by regarding these nonlinear terms as empirical forcing functions. An additional source was assumed as estimated from the residual of the potential vorticity equation to represent diabatic forcing. Mountain forcing was included in the lower boundary condition. A series of perturbation versus control experiments was run to determine the effect of the various apparent sources. The transient eddy source term appears to decrease the stationary geopotential as much as a few hundred meters at 500 mb. The nonlinear stationary eddy source generates geopotential perturbations which are comparable to the effect of transient eddies, but the correlation with the observed geopotential is highly irregular indicating no specific characteristics of the effects.