Vorticity waves produced in a continental-shelf area where there is a sea-bed slope. In the northern hemisphere, if a water column is displaced into shallower water it develops negative relative vorticity, or anticyclonic motion; if displaced into deeper water it will develop positive relative vorticity, or cyclonic motion. The net result is for shelf waves to progress in a poleward direction along the west coasts of continents, or equatorwards along east coasts.