A monopolar vortex encounters a north-south ridge or trough

Results are described from a two-dimensional numerical model in which a cyc lonic monopole that moves due to the beta-effect encounters a north-south o riented ridge or trough, the height and depth of which is varied. The fate of the monopole depends on its initial north-south position y(0) (or, equiv alently, the value of the constant f(0) in the Coriolis parameter): the mon opole can be rebounded, destroyed or trapped by the topography. Only for a narrow range of y(0)-positions, depending on the height (depth) of the ridg e (trough), can the monopole actually cross the topography, sometimes after being heavily deformed. it is the gradient of the topography-induced vorti city, which deforms the background potential vorticity field due to the bet a-effect, that determines the fate of the monopole. This topography-induced vorticity is larger for initial positions more to the north or south, and for higher ridges and deeper troughs. (C) 2000 The Japan Society of Fluid M echanics and Elsevier Science B.V. All rights reserved.