Quasi-Geostrophic Single-Layer Models

The crudest representation of currents and winds is that given by a single, constant-density fluid layer in relative motion with respect to the rotating Earth. In the case of the ocean, the layer is bounded from below by the sea floor and from above by the free surface of the sea. In the case of the atmosphere, the layer is bounded from below by the ground and from above by a hypothetical surface, above which the density of the atmosphere goes abruptly to zero. In both cases, the horizontal pressure gradient arises from the modulation of such surfaces with respect to the geoid, while the Coriolis force tends to arrange the flow in geostrophic balance with the pressure gradient.