The four main surface ocean current gyres in the north and south Pacific Ocean, as well as the north and south Atlantic are quite similar. The monsoon circulation over the Indian Ocean and south Asia are responsible for some differences in the gyres in the Indian Ocean. Gyres are driven by similar wind systems although both the winds and ocean circulate in opposite directions on either side of the equator because of the reversal in Coriolis Effect across the equator. That is, the gyres are mirror images on either side of the equator.
In each of the main (subtropical) oceanic gyres, the circulation is asymmetrical in the east-west direction. The gyre circulation is shoved toward the western side of the basin. Strong, fast, narrow, deep currents occur on the west sides of the ocean close to the east coasts of continents. These are called western boundary currents and include the Gulf Stream, Kuroshio Current, Brazil Current, and the East Australian Current. The Gulf Stream and the Kuroshio Current are the two strongest. Conversely, on the eastern sides of the ocean basins, the currents are much wider, slower, shallower and not as close to the coast. These eastern boundary currents include the Canary, California, Benguela, and Peru Currents. They are characterized by coastal upwelling that make them important biologically and economically for commercial fishing.
The western boundary currents are the fastest currents in the ocean. Flows in the major western boundary currents transport 50 to 100 times the total water discharged by all the world's rivers. To explain these strong western boundary currents, why the currents on the eastern sides of the ocean basins are weak, why gyre circulation is not centered in the ocean basin but is shifted toward the west, we must invoke the conservation of angular momentum. This is the same principle that explains why figure skaters performing a spin slow down then they extend their arms and spin faster when they pull their arms closer to their bodies.
The conservation of angular momentum (or rotation) in an ocean gyre involves three factors that must balance for the current to flow in a curved motion. These factors are wind, the effect of Earth's rotation (the Coriolis Effect), and frictional drag of the coast, that is the continental slope, on the edge of the ocean.
The large-scale surface winds in the North Atlantic subtropical high-pressure system rotate the gyre in a clockwise direction over almost the entire ocean basin. Near the equator, little horizontal spinning or rotation of the ocean water occurs because the Coriolis Effect is almost zero. Along the western side of the ocean basin (the U.S. Eastern Seaboard), the Gulf Stream carries this non-rotating ocean water northward. In traveling northward, the Coriolis Effect increases (shifting motion to the right in the Northern Hemisphere). Therefore, the water, which had no spin at the equator, picks up clockwise spin or rotation as it travels northward.
Once this ocean water reaches the northwest quadrant of the North Atlantic, it then flows eastward across the North Atlantic (the North Atlantic Current) in balance with its acquired Coriolis Effect (clockwise spin). Upon reaching the northeast quadrant of the North Atlantic, the ocean water then turns again, but this time toward the south on the east side of the ocean. Now, the Canary Current is carrying water with clockwise spin from high latitude toward the equator where the water started, but where the Coriolis Effect is weak. That is, in the transit from north to south, the water now must lose the clockwise spin it gained at northern latitudes (equivalently, gain counterclockwise spin) to stay in balance and come to nearly zero Coriolis spin again near the equator.
On the eastern side of ocean basins, in the eastern boundary regions, wind adds clockwise spin while movement southward subtracts clockwise spin to create an approximate balance. However, on the west side of the gyres, both wind and northward movement (increasing Coriolis Effect) add clockwise spin so there is too much clockwise spin, and thus an imbalance. The question then is, how does the west side of the ocean gain counterclockwise spin or rotation to balance the excessive clockwise spin it gained from the wind and increasing Coriolis and not break into rotational eddies all over the North Atlantic?
The answer is that the western boundary current gains counterclockwise spin by accelerating, deepening, and pushing up against the coast to increase frictional drag. Shoving its shoulder against the coast contributes a counterclockwise turning. The coastal edge of the gyre (e.g., the Gulf Stream) slows compared with the currents farther offshore, and in this way generates a counterclockwise turning or rotation. The Gulf Stream, as well as the other western boundary currents, accelerates and produces enough friction against the coast to generate sufficient counterclockwise turning or spinning to balance the clockwise spin. But in doing so, to generate enough spin, the current becomes stronger, deeper, faster, and flows closer to the eastern seaboard.
Conversely, in the eastern boundary region of the ocean basin, the gyre does not need to generate much clockwise spin to balance angular momentum and so does not push against the coast, but rather remains relatively wide, slow, shallow and away from the coast.
We have used the Gulf Stream in the Northern Hemisphere as an example, but the same thing happens in the Southern Hemisphere except the spins or circulations are in the opposite direction because the Coriolis Effect is in the opposite direction. In the Southern Hemisphere, the circulation of gyres and winds are in the opposite direction but then so is the Coriolis Effect so that the strong boundary currents also occur on the west side of the gyre and flow poleward.
Return to DataStreme Ocean Website
Prepared by H.J. Niebauer, Ph.D. and Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2006, The American Meteorological Society.