Orographically Induced Stationary Waves: Dependence on Latitude

Abstract

A general circulation model (GCM) with idealized boundary conditions is used to study the effects of a mountain's latitudinal position on the stationary wave response. In each of a series of experiments the only asymmetry in the boundary conditions is a Gaussian-shaped mountain with an e-folding width of 15° latitude placed at 0°, 15°, 30°, 45°, and 60° latitude in separate integrations. The stationary wave response in the GCM is analyzed using a linearized primitive equation model, a 3D wave-flux vector, and a barotropic ray-tracing technique. Stationary waves in the GCM are generated by modifications to the diabatic heating field, termed thermal forcing, and by obstructing the surface winds, termed mechanical forcing. With a mountain at 0° latitude, latent heating anomalies provide the forcing mechanism. In the 15° mountain experiment, forcing by anomalous latent heating is also found, but mechanical forcing (which occurs within the easterlies) seems to dominate. In the 30°, 45°, and 60° mountain experiment, the stationary wave response results from mechanical forcing in westerly flow. As the mountain's latitudinal position is moved poleward, two distinct regions of stationary wave forcing appear, as indicated by a 3D wave-flux vector. This results in two wave trains emanating from the mountain placed at 60° latitude. One region of forcing occurs in the region where the westerlies are perturbed, while the other region occurs on the east and poleward flank of the mountain. In this idealized setting, the propagation patterns of the stationary waves can, for the most part, be understood through quasigeostrophic theory. The dispersion of stationary wave energy throughout the atmosphere is largely dependent upon the upper-level flow.