On generation of zonal flows by interacting Rossby waves

Generation of zonal flows by finite amplitude resonantly interacting discrete Rossby waves is investigated within the context of the quasi-geostrophic barotropic model on the beta plane. Under the “channel” assumption, in which the north-south extent of the domain is taken as finite, analysis shows that a triad of Rossby waves is capable of generating a zonal flow on the time scale of the resonant interaction. Zonal currents thus produced exhibit a spatial structure similar to that obtained from the baroclinic instability mechanism: a jet in the central portion and weaker reversed flow in the northern and southern portions of the channel. If the north-south extent of the domain is taken as infinite, zonal flows of sinusoidal latitudinal structure can be generated by discrete Rossby waves via the quartet resonance mechanism. The time scale required for their generation is an order of magnitude longer than that associated with the “channel” case.