Geostrophic current

The direction of geostrophic flow is parallel to the isobars, with the high pressure to the right of the flow in the Northern Hemisphere, and the high pressure to the left in the Southern Hemisphere.

The concept is familiar from weather maps, whose isobars show the direction of geostrophic winds.

A geostrophic current may also be thought of as a rotating shallow water wave with a frequency of zero.

The principle of geostrophy or geostrophic balance is useful to oceanographers because it allows them to infer ocean currents from measurements of the sea surface height (by combined satellite altimetry and gravimetry) or from vertical profiles of seawater density taken by ships or autonomous buoys.

Seawater naturally tends to move from a region of high pressure (or high sea level) to a region of low pressure (or low sea level).

In particular, it is assumed that there is no acceleration (steady-state), no viscosity, and that the pressure is hydrostatic.

The equations governing a linear, rotating shallow water wave are: The assumption of steady-state (no net acceleration) is: Alternatively, we can assume a wave-like, periodic, dependence in time: In this case, if we set

Thus a geostrophic current can be thought of as a rotating shallow water wave with a frequency of zero.

An example of a geostrophic flow in the Northern Hemisphere.
An example of a geostrophic flow in the Northern Hemisphere.
A northern-hemisphere gyre in geostrophic balance ; paler water is less dense than dark water, but more dense than air; the outwards pressure gradient is balanced by the 90 degrees-right-of-flow coriolis force The structure will eventually dissipate due to friction and mixing of water properties.