Estimation of the low-latitude reflectivity of stationary waves in a GCM simulation

Abstract

Propagation of stationary waves induced by isolated orographic forcing in high latitudes was simulated in a long-term GCM run. The purpose of the study was to establish to what degree these waves are reflected from low latitudes. In this simulation, a broad unbroken belt of tropical easterlies prevents wave propagation into the southern hemisphere, and the waves are efficiently absorbed in low latitudes. The ratio of the northward to southward stationary wave activity flux can be estimated by studying the meridional distribution of the correlation between the stationary eddy horizontal velocity components. In middle latitudes of the northern hemisphere the ratio was 0.2 ± 0.04. A large proportion of the rather weak northward flux component is induced by low-latitude transient eddy forcing associated with the baroclinic instability of the flow, while reflection from the nonlinear critical layer (CL) seems to be small. This paper presents a method of partitioning the horizontal stationary wave activity flux into contributions due to various forcing functions. In the longitude belt where the mountain-induced wave train encounters the time-mean zero wind line, one can find indications of nonlinear wave breaking. In contrast to several previous simulations, wave breaking does not involve significant reflection. This may be explained by the existence of the Hadley circulation, which allows the zonal momentum balance to be maintained even under wave absorption. In the present GCM runs the reflectivity of the CL was found to be low; however, further GCM experiments are needed to assess whether this conclusion holds universally in the atmosphere.