Why the ocean current is elliptical with strong western boundary current?

1. The tropical easterly wind causes poleward Ekman transport and the midlatitude westerly wind causes equatorward Ekman transport.

2. The Ekman transport converges at the subtropics causes downwelling. The diverging region of Ekman transport cause upwelling.
(Reason: Continuity and conservation of mass)

3. The downwelling push the thermocline down and squeeze the underling geostrophic layer. The upwelling pull the thermocline up and stretches the geostrophic layer.
(Reason: Continuity and conservation of mass)

4. The squeezing causes equatorward Sverdrup transport and stretching lead to poleward Sverdrup transport.
(Reason: Potential vorticity conservation)

5. The equatorward Sverdrup transport diverges from the midlatitude westerly and converges to the tropical easterly forms an elliptical clockwise gyre in the interior layer of open ocean

6. This large scale equatorward mass transport is compensated by the poleward western boundary transport.
(Reason: Continuity and conservation of mass)

7. Thus the ocean current is elliptical with strong western boundary current.


Key words:

Atmospheric surface wind
Ekman layer mass transport
Potential vorticity
Geostrophic layer
Sverdrup transport