Linear and Nonlinear Diagnostic Models of Stationary Eddies in the Upper Troposphere during Northern Summer

Abstract

The upper tropospheric circulation during northern summer produced by a general circulation model (GCM) is studied using linear and nonlinear barotropic models and by analysing a streamfunction budget. The model experiments and the budget calculations both show a simple Sverdrup balance to he a useful first approximation for the largest scales during this season. In this Sverdrup balance, the advection of planetary vorticity by the divergent component of the flow is found to be significant, particularly in the Southern Hemisphere tropics. Nonlinear barotropic models improve the simulation of regional structures. The correct position of the Tibetan high is explained by Sverdrup balance, but its amplitude and structure are reasonably well simulated only with the nonlinear models. With climatological forcing, the time-averaged solutions of the nonlinear model are insensitive to the strength of the damping included in the model. The difference between the GCM's climatology and the GCM's flow in a particular summer is more difficult to model because of the large contribution of anomalous transients to the maintenance of the flow. However, strongly damped models produce simulations that bear some resemblance to the anomalous flow, at least in the tropics. To estimate the potential importance of vertical transport of momentum during moist convection, a damping proportional to the precipitation rate in the GCM is added in the nonlinear model. The estimated damping time scale for the eddy streamfunction is ?5 days in the northern tropics, but the changes in the predicted stationary eddy streamfunction are modest.