Quasi-Geostrophic Theory

Quasi-geostrophic (QG) theory is the leading-order dynamics of slowly varying, rotation-dominated, stratified flow, derived by asymptotic expansion in small Rossby number Ro = U/(fL) much less than 1. The velocity is geostrophic at leading order, but the small ageostrophic part advects the flow, so motion is nearly, not exactly, geostrophic. The theory reduces to one prognostic equation: conservation of quasi-geostrophic potential vorticity following the geostrophic flow, Dq/Dt = 0, with q combining relative, planetary (beta), and stretching vorticity. QG describes Rossby waves, baroclinic instability, and midlatitude gyre and weather-scale dynamics.