Definition
Ignoring friction or depth changes; if a parcel of water with no initial relative vorticity (i.e. no rotation) is moved northward, as f (the Coriolis parameter) increases the parcel will gain negative relative vorticity and will circulate clockwise. The Coriolis force will be greater on the poleward side of the parcel than on the equatorward side and hence the parcel will be subjected to a net southward restoring force. This force will push the parcel south of the latitude of zero relative vorticity overshoots and the circulation becomes counterclockwise. Due to the Coriolis variation the parcel will now experience a northward restoring force. Thus the variation of f provides a restoring force (in the horizontal plane) allowing oscillation to occur just as the effect of gravity does (vertically) for surface or internal waves. In reality Rossby waves are complicated by depth variations and frictional effects.