Gradient currents

From Glossary of Meteorology
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gradient current

In oceanography, a current determined by the condition that the horizontal pressure gradient due to the (hydrostatic) distribution of mass balances the Coriolis force due to the earth's rotation.

The gradient current corresponds to the geostrophic wind in meteorology. In practice, the distribution of density is determined by measurements of salinity and temperature at a series of depths in a number of positions. From this the geopotential topography of any isobaric surface relative to any other isobaric surface may be computed and the horizontal pressure gradient may be expressed by the geopotential slope of the isobaric surface. In this way relative isobaric surface currents are obtained, corresponding to thermal wind in meteorology. If one isobaric surface is known to be level, the absolute geopotential topography of any other surface may be computed by reference to this, and hence absolute gradient currents are obtained. Where no isobaric surface is known to be level, the total gradient current will consist of the relative gradient current, due to the distribution of density, and the slope current, due to that portion of the inclination of the isobaric surfaces that is not the result of the distribution of density.
See also geostrophic current.