Questions: Wind-Driven Ocean Circulation and Surface Currents

Question 1

In the Northern Hemisphere, the trade winds blow from the northeast toward the southwest. In which direction does Ekman transport move the surface water?

• A. Northeast to southwest — the same direction as the wind.
• B. Roughly 90° to the right of the wind, toward the northwest.
• C. Roughly 90° to the right of the wind, toward the southeast.
• D. Directly downward into the deep ocean.

Answer: (C)

The Coriolis effect deflects moving water to the right in the Northern Hemisphere. Trade winds blow toward the southwest, so the net Ekman transport is 90° to the right of that direction — roughly toward the northwest. A common error is assuming water moves in the same direction as the wind; it does not.

Question 2

Wind-driven surface currents can extend to several kilometers depth, and are the primary driver of thermohaline circulation in the deep ocean.

• T. True
• F. False

Answer: False

Wind-driven currents are largely confined to the upper few hundred meters (the Ekman layer). Deep thermohaline circulation is driven by density differences — cold, salty water sinking at high latitudes — not by direct wind stress. The two circulation systems are connected but distinct in their driving mechanisms.

Question 3

What causes the subtropical convergence zones in the ocean, and what happens to surface water that accumulates there?

Model answer: Trade winds drive Ekman transport poleward (to the right of easterly winds in the Northern Hemisphere), while westerlies drive Ekman transport equatorward (again to the right of westerly winds). These two opposing flows converge in the subtropics, piling water up in a mound. The elevated sea surface then drives geostrophic currents flowing around the subtropical gyre, and the excess water slowly sinks (downwells) into the ocean interior.

Convergence of Ekman transport creates a downwelling zone. The raised sea surface sets up a horizontal pressure gradient that — balanced by the Coriolis force — produces geostrophic flow. This is the mechanism behind the large subtropical gyres like the North Atlantic Gyre.