The ocean circulation is driven by the transfer of heat, water, and
momentum across the air-sea interface. Heat transfer is accomplished through
(1) latent heat flux, (2) sensible heat flux, and (3) radiation. Water transfer
is accomplished through evaporation and precipitation. Momentum transfer
takes place through the frictional effects of winds on the ocean surface.
If we imagine that ocean circulation is only responding simply to winds (ignoring that the ocean is in rotation), we would expect simple gyres to form in each ocean basins in response to the tropical easterly trades and midlatitude westerlies with the north-south branches along the coastlines to conserve mass. The gyre centers would be at about 30 N and 30 S in the center of the basin, and water would pile up where the eastward and westward moving branches approached land (see Fig 5-1 and the first row in the figure below)
Vorticity and western boundary currents
Vorticity is the tendency of something to rotate. It is defined so that the tendency to rotate clockwise is positive and counterclockwise is negative. If you stood at the north pole you would spin clockwise at the rate of once per day, and so the Earth imparts positive vorticity on you. If you stood at the south pole you would rotate counterclockwise and so the Earth imparts negative vorticity on you. This rotation brought about due to Earth's rotation is called planetary vorticity. I don't like this explanation of planetary vorticity because it is misleading in several ways. For one thing it does nothing to explain how Earth imparts "rotation" elsewhere without resorting to mathematics. Another way to think about it is to consider the path a fluid parcel takes when it is given a shove in the horizontal plane. Recall that the coriolis effect deflects parcels to the right in the NH. The path is actually curved, which can be considered a rotation. Ben pointed out that this argument gives the rotation in the wrong direction because planetary vorticity is positive in the NH where it is counterclockwise. Ben is absolutely right. I'm at a loss to give a physical explanation for this thing called rotation. Interestingly, planetary vorticity and the coriolis effect have magnitudes that scale the same way with latitude, even though a parcel need not be in motion to have a tendency to rotate, while the coriolis effect only applies to a moving parcel.
Relative vorticity is easier to understand. It is the tendency of
a fluid flow to cause rotation. The following figure shows how:
The sum of planetary and relative vorticity is conserved (or close
to it). This conservation helps us understand why western boundary currents
are focused near the far western side of a basin. The subpolar gyre has negative
vorticity (like red arrows on the upper right). Along the western boundary,
the flow is moving northward and so it gains positive planetary vorticity.
In order to conserve the vorticity, the relative vorticity must become even
more negative so the sum (relative plus planetary) does not change. This causes
the flow to speed up, especially near the center of the gyre (think of a
skater bringing his arm towards his body to rotate faster). The current becomes
narrow to conserve flow. The center of the gyre and this narrow fast moving
current moves closer to the western boundary to increase the friction and
slow down a little. The opposite occurs in the eastern boundary.
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Last Updated: 9/29/2004