The ocean's surface is always moving. Steady global winds drag the top hundred meters or so of water along, and Earth's spin and the shapes of the continents turn these flows into huge loops called gyres.
- Gyres turn clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
- On the western side of each ocean, currents carry warm water from the tropics toward the poles, like the Gulf Stream along the U.S. East Coast.
- On the eastern side, currents carry cool water toward the equator, like the California Current along the U.S. West Coast.
Key idea
Surface currents are driven mainly by steady winds and turned by Earth's spin into gyres. They move heat: warm currents carry it toward the poles, and cool currents bring cold water toward the equator.
Why it works
In the North Atlantic, the trade winds push water west near the equator and the westerlies push it east farther north. Between them the water circles around, and the Coriolis effect bends it to the right, giving a clockwise loop. Water flowing north from the tropics along the western side brings tropical warmth with it; water flowing south along the eastern side has been cooled at high latitudes.
Worked example: Warm or cool?
The Gulf Stream flows north along the east coast of the United States. Is it warm or cool, and why?
- It flows from the tropics toward the pole.
- So it carries warm tropical water: a warm current.
Answer: Warm
Worked example: A coastal climate
San Francisco, next to the cool California Current, often has foggy, cool summers. Why?
- The current brings cool water from the north.
- Air over the cool water is chilled, and its water vapor condenses into fog that drifts ashore.
Answer: The cool current chills the air above it
Common mistake
Thinking tides or the Moon drive ocean currents. Tides only move water back and forth twice a day. Surface currents are pushed mainly by steady winds and turned by Earth's spin. Check: the gyres line up with the global wind belts, not with the Moon.