4.1. Geostrophic balance and Geostrophic Currents
Geostrophic balance is the basic balance in the open ocean. It is the balance between Coriolis forces and the horizontal pressure gradient that determines the first order circulation patterns (large scale patterns) at the surface of the oceans. Using satellite altimetry, it is possible to determine the level of the sea surface relative to a geoid which is used as a reference system. The large-scale flows of the ocean can be determined by balancing the Coriolis force with the observed pressure gradients.
Geostrophic currents, are the surface currents caused by the variability of sea surface elevation. Due to the difference in surface elevation there is a horizontal pressure gradient and subsequently a current (water flow) from areas with higher elevation to areas with lower elevation. Due to the large scale of geostrophic current Coriolis is important and the current direction is such that a balance between pressure gradient and Coriolis force is possible.
Example: Calculate the geostrophic current…
4.2. Satelite Altimetry
The satellite Topex/Poseidon was the first satellite dedicated to provide constant ocean surface topography. It made precise measurements of the ocean surface from 1992 to 2006. Nowadays, satellites Jason-1 and Jason-2 continue the ocean surface topography mission. More info at NASA Ocean Surface Topography site and Wikipedia.
4.3. Derivation of Geostrophic balance equations
To derive the equations of geostrophic balance we assume:
1. The flow has no accelerations
2. Horizontal velocities are much larger than the vertical
3. Friction is small and can be neglected.
With the above assumptions the Momentum Equations (Navier-Stokes) are simplified to:
1.
2.
3.
Geostrophic wind