Mechanisms of Low-Frequency Variability in a Simple Model with Orography

Abstract

Mechanisms of low-frequency variability are examined in an extended run of a two-level nonlinear global model with a single isolated mountain. The presence of the mountain modifies the low-frequency behavior, compared with that in a zonally homogeneous model, in two ways. First, slow variations in the zonally averaged circulation, the zonal index, are manifest as variability in the train of stationary waves generated by the orography. Second, trains of low-frequency Rossby waves are modified by their propagation through the model's zonally asymmetric time-mean flow. These eddies become meridionally elongated at the jet entrance and zonally elongated at its exit, allowing them to extract energy barotropically from the mean flow in the jet exit.