The structure of the most unstable baroclinic mode in the presence of elongated topography oriented parallel to the basic state zonal current has been investigated in a number of studies. The topographic modification leads to enhanced baroclinic activity on the southern side of the mountain. The results found in simple quasi-geostrophic models with a ridge were generalized to primitive equations and realistically high and steep isolated topography; the same type of north–south asymmetry apt to explain lee cyclogenesis was observed.
In the present study we use a quasi-geostrophic two-layer model with a bottom ridge to investigate the dynamical balance leading to this type of asymmetry. In the case of a purely barotropic Rossby wave, we find that the topographic correction always leads to intensification of the undisturbed wave south of the mountain barrier. In the case of unstable baroclinic waves in the f-plane, a simple criterion holds that determines the conditions favoring lee cyclogenesis in terms of the basic state potential vorticity. The effect of β modifies the criterion and contributes to lee deepening.
The computation of the most unstable normal mode with a primitive equation model and a basic state derived from an Alpex lee cyclogenesis case further supports our findings concerning the robustness of the normal mode topographic correction in relation to the initial value problem and the basic state properties.