Geostrophic wind

The geostrophic wind is the theoretical wind that results from an exact balance between the horizontal pressure gradient force and the Coriolis force in a rotating fluid. In the geostrophic approximation, the wind flows parallel to isobars — lines of constant pressure — with low pressure to the left in the Northern Hemisphere and to the right in the Southern Hemisphere. This balance is the foundational dynamical state of large-scale atmospheric and oceanic flow, and it is the reference frame within which Rossby waves propagate.

The geostrophic approximation is not merely a simplification. It is a structural property of rotating fluids at large scales, where the Coriolis force dominates over inertial and frictional effects. At smaller scales — in the atmospheric boundary layer, in tornadoes, in oceanic eddies — the balance breaks down, and the wind crosses the isobars toward low pressure. The transition from geostrophic to ageostrophic flow marks the boundary between large-scale dynamics and small-scale turbulence, and it is one of the fundamental scale transitions in geophysical fluid dynamics.

The geostrophic wind is directly related to the concept of potential vorticity, which is conserved along geostrophic trajectories. The violation of geostrophic balance — through friction, heating, or topographic forcing — is the primary mechanism by which potential vorticity is modified and by which the large-scale circulation adjusts to external perturbations.