Linear Diagnosis of Stationary Waves in a General Circulation Model

Abstract

A linear stationary wave model is used to diagnose the causes of stationary waves in integrations of a general circulation model (GCM) and to indicate the sources of differences between the stationary waves of separate integrations. The GCM generates solutions to the equations of motion. The linear model, constructed to be as similar as possible in structure to the GCM, is employed in various attempts to approximate and understand the time averages of the GCM solutions. When GCM values for internal dissipation time constants are used in the linear model, significant differences between the linear model and GCM solutions are found. These differences can be interpreted as errors due to the linear approximation. The linear simulation is improved somewhat by enhancing the scale selective horizontal diffusion. The linear model with enhanced dissipation is used to simulate the differences between the stationary waves of two consecutive months of a GCM integration. Transient forcing turns out to be the major cause for these differences, according to the linear model. The phase structure of the errors of the linear solutions indicates that the error source is located primarily in the tropics and subtropics. Possible explanations for the errors are inaccurate representation of the topographic forcing and reflections from critical latitudes. The latter possibility is subjected to a crude test and cannot be rejected.