Chapter 10 - Geostrophic Currents
10.10 Important Concepts
- 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.
- 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.
- 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.
- Topex/Poseidon is the most accurate altimeter system, and it can measure
the topography or changes in topography with an accuracy of ± 4.7cm.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.