Geostrophic Balance

Geostrophic balance is the leading-order steady state of large-scale ocean and atmosphere flow: the horizontal pressure-gradient force balances the Coriolis force, so the current runs along isobars rather than across them. It holds where the Rossby number is small, away from the equator, friction, and rapid acceleration. Most of the open-ocean interior flow is geostrophic, which is why satellite altimetry of sea surface height yields surface geostrophic currents directly.