On hysteresis-like effects in orographically forced models

Time integrations with an orographically forced high resolution, barotropic model have shown hysteresis-like behaviour. The orography consists of an isolated, mid-latitude hump and the flow is forced with a solid body rotation. For an increasing forcing, a marked instability is found, but it is not connected with a bifurcation, as the model only has one stable equilibrium state. It is found that the instability is coupled to orographic effects and that a slow time evolution of the flow is required for its existence. It is argued that the hysteresis-like effects found here are perhaps more relevant to atmospheric behaviour than bifurcations leading to multiple equilibria.