Geostrophic motion

Geostrophic motion occurs when the horizontal components of the pressure and Coriolis forces are in approximate balance and frictional effects are confined to thin ‘boundary’ layers. Two special cases have proved useful in studying atmospheric and oceanic motions: type 1, characterized by a horizontal length scale ( L ) smaller than the radius of the earth, and type 2, which has L comparable with the radius of the earth. A detailed scale analysis of type 1 motion is presented, including a derivation of the so‐called ‘β plane.’ The energetic properties of type 1 motion are summarized, and several successful applications to atmospheric and oceanic problems are described (wave instability, vertical propagation of energy, numerical weather prediction, Gulf Stream meanders). Motion of type 2 has quite different properties; its principal application has been in analysis of the very slow motion and thermocline structure of the interior ocean. In the atmosphere, observations suggest that there is appreciable interaction between the motions of very large scale (type 2) and those of smaller scale (type 1). Laboratory analogs of certain features of geostrophic flow in the atmosphere and ocean have also been constructed.