Chapter 10 - Geostrophic Currents

10.1 Hydrostatic Equilibrium

Within the ocean's interior away from the top and bottom Ekman layers, for horizontal distances exceeding a few tens of kilometers, and for times exceeding a few days, horizontal pressure gradients in the ocean almost exactly balance the Coriolis force resulting from horizontal currents. This balance is known as the geostrophic balance.

The dominant forces acting in the vertical are the vertical pressure gradient and the weight of the water. The two balance within a few parts per million. Thus pressure at any point in the water column is due almost entirely to the weight of the water in the column above the point. The dominant forces in the horizontal are the pressure gradient and the Coriolis force. They balance within a few parts per thousand over large distances and times (See Box).

Scaling the Equations: The Geostrophic Approximation

We wish to simplify the equations of motion to obtain solutions that describe the deep-sea conditions well away from coasts and below the Ekman boundary layer at the surface. To begin, let's examine the typical size of each term in the equations in the expectation that some will be so small that they can be dropped without changing the dominant characteristics of the solutions. For interior, deep-sea conditions, typical values for distance L, horizontal velocity U, depth H, Coriolis parameter f, gravity g, and density r are: L » 106m f» 10-4s-1 U» 10-1m/s g» 10 m/s2 H» 103m r» 103kg/m3 From these variables we can calculate typical values for vertical velocity W, pressure P, and time T: The momentum equation for vertical velocity is therefore: and the only important balance in the vertical is hydrostatic: The momentum equation for horizontal velocity in the x direction is: Thus the Coriolis force balances that pressure gradient within one part per thousand. This is called the geostrophic balance, and the geostrophic equations are: This balance applies to oceanic flows with horizontal dimensions larger than roughly 50 km and times greater than a few days.

Both balances require that viscosity and nonlinear terms in the equations of motion be negligible. Is this reasonable? Consider viscosity. We know that a row boat weighing a hundred kilograms will coast for maybe ten meters after the rower stops. A super tanker moving at the speed of a rowboat may coast for kilometers. It seems reasonable, therefore that a cubic kilometer of water weighing 10¹⁵kg would coast for perhaps a day before slowing to a stop. And oceanic mesoscale eddies contain perhaps 1000 cubic kilometers of water. Hence, our intuition may lead us to conclude that neglect of viscosity is reasonable. Of course, intuition can be wrong, and we need to refer back to scaling arguments.

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10.1 Hydrostatic Equilibrium
Before describing the geostrophic balance, let's first consider the simplest solution of the momentum equation, the solution for an ocean at rest. It gives the hydrostatic pressure within the ocean. To obtain the solution, we assume the fluid is stationary:

the fluid remains stationary:

(10.2)

and, there is no friction:

With these assumptions the momentum equation (7.12) becomes:

(10.4)

where we have explicitly noted that gravity g is a function of latitude φ and height z. We will show later why we have kept this explicit.

Equations (10.4a) require surfaces of constant pressure to be level surface. A surface of constant pressure is an isobaric surface. The last equation can be integrated to obtain the pressure at any depth h. Recalling that r is a function of depth for an ocean at rest.

(10.5)

For many purposes, g and ρ are constant, and p = g h ρ. Later, we will show that (10.5) applies with an accuracy of about one part per million even if the ocean is not at rest.

The SI unit for pressure is the pascal (Pa). A bar is another unit of pressure. One bar is exactly 10⁵ Pa (Table 10.1). Because the depth in meters and pressure in decibars are almost the same numerically, oceanographers prefer to state pressure in decibars.

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