Interaction of a monopolar vortex with a topographic ridge

The interaction of a monopolar vortex with a cosine-shaped topographic ridge at the equator is investigated with a two-dimensional numerical model, where the (cyclonic) monopole has a self-induced northwest-ward motion due to the beta-effect. The Fate of the monopole depends on the width and height of the ridge, but, more importantly, on the orientation of the ridge. Whereas monopoles are always seen to cross an east-west or northeast-southwest ridge, a north-south ridge can cause such deformations in the monopole's shape that it either splits into two parts (where the associated secondary vortex may or may not also cross the ridge) or it is destroyed. The computations show that the monopole can only cross the top of the ridge once it has gathered sufficient positive potential vorticity at its (north)west side. The vortex achieves this by moving along the ascending side of the ridge, westward for the east-west ridge and northward for the north-south ridge, before crossing the summit.