What drives the oceanic circulation ?

Thermohaline driven (-> exercise)

Wind driven (-> Sverdrup, Ekman)

some of the observed main global surface current systems.

The water column can be broadly divided into four segments:

• At the top lies the mixed layer that is stirred by the surface wind stress. With a depth on the order of 10 m, this layer includes Ekman dynamics and is characterized by d rho/dz 0. ≃

• Below lies a layer called the seasonal thermocline, a layer in which the vertical stratification is erased every winter by convection. Its depth is on the order of 100 m.

• Below the maximum depth of winter convection is the main thermocline, which is permanently stratified. Ist thickness is on the order of 500 to 1000 m.

• The rest of the water column, which comprises most of the ocean water, is the abyssal layer. It is very cold, and its movement is very slow. (deep sea)

Basic equations

Geostrophy

Hydrostatic balance

continuity equation (mass conservation for an incompressible fluid)

conservation of heat and salt (density)

Definitions

• u, v and w are the velocity components in the eastward, northward and upward directions,

• rho0 is the reference density (a constant), • rho is the density anomaly, the difference

between the actual density and rho0, • p is the hydrostatic pressure induced by the

density anomaly

• This set of five equations for five unknowns (u, v, w, p and rho) is sometimes referred to as

Sverdrup dynamics.

Sverdrup Relation

vertical stretching (+), or squeezing (-)

change in meridional velocity

Pressure eliminated

Using: Conservation of mass

Vert. expansion -> shrink laterally -> (zeta+f)/h requires vorticity to increase

Sverdrup: meridional velocity

Wind-driven circulation

Turbulence terms,

deviations from geostrophy

Wind-driven circulation

Interior: geostrophy

Introduce u + iv, assume geostrophic flow is constant.

A schematic of the envisioned upper ocean structure

Ekman Spiral and Mass Transport

ship-observed windstress (N m-2).

main global surface current systems

ship-observed windstress (N m-2).

main global surface current systems

the wind-driven nature of the oceanic gyres

A schematic of the envisioned upper ocean structure

To gain some insights into the workings of the gyre system, we idealize the upper ocean. The vertical flow from the surface Ekman layer into the geostrophic interior is

small latitudinal extent of the motion f=f0

Ekman

• the vertical flow from the surface Ekman layer into the geostrophic interior is

Ekman

… relates the integral meridional flow throughout the vertical extent of the treated layer to the local windstress curl.

Sverdrup Balance

we can introduce a Sverdrup streamfunction

Sverdrup Balance

Being that the curl is negative throughout the subtropics, it follows that the meridional flux must be everywhere equatorward. But such a situation, if sustained, will progressively empty the midlatitude oceans, while piling-up more and more water along the Equator; a clear physical impossibility! There must be somewhere a return poleward flow that `drains' the Equatorial region while replenishing the midlatitude missing volume.

Boundary Current

• The vorticity generation by the interactions of boundary currents: northward-flowing boundary current,

• The sense of the generated vorticity is shown for northern hemisphere flows.