Chapter 10 - Geostrophic Currents

10.10 Important Concepts

1. Pressure distribution is almost precisely the hydrostatic pressure obtained by assuming the ocean is at rest. Pressure is therefore calculated very accurately from measurements of temperature and conductivity as a function of pressure using the equation of state of seawater. Hydrographic data give the relative, internal pressure field of the ocean.



2. Flow in the ocean is in almost exact geostrophic balance except for flow in the upper and lower boundary layers. Coriolis force almost exactly balances the horizontal pressure gradient.



3. Satellite altimetric observations of the oceanic topography give the surface geostrophic current. The calculation of topography requires an accurate geoid, which is known with sufficient accuracy only over distances exceeding a few thousand kilometers. If the geoid is not known, altimeters can measure the change in topography as a function of time, which gives the change in surface geostrophic currents.



4. Topex/Poseidon is the most accurate altimeter system, and it can measure the topography or changes in topography with an accuracy of ± 4.7cm.



5. Hydrographic data are used to calculate the internal geostrophic currents in the ocean relative to known currents at some level. The level can be surface currents measured by altimetry or an assumed level of no motion at depths below 1–2m.



6. Flow in the ocean that is independent of depth is called barotropic flow, flow that depends on depth is called baroclinic flow. Hydrographic data give only the baroclinic flow.



7. Geostrophic flow cannot change with time, so the flow in the ocean is not exactly geostrophic. The geostrophic method does not apply to flows at the equator where the Coriolis force vanishes.



8. Slopes of constant density or temperature surfaces seen in a cross-section of the ocean can be used to estimate the speed of flow through the section.



9. Lagrangean techniques measure the position of a parcel of water in the ocean. The position can be determined using surface or subsurface drifters, or chemical tracers such as tritium.



10. Eulerian techniques measure the velocity of flow past a point in the ocean. The velocity of the flow can be measured using moored current meters or acoustic velocity profilers on ships, CTDs or moorings.

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