Surface Ocean Circulation
A bottle dropped into the Atlantic off Florida does not just drift out to sea. Within days it can be carried north along the East Coast at walking speed or faster, then across the ocean toward Europe. What steers it, and why does it follow the same route year after year? By the end of this page you will be able to explain that path.
Winds Push, the Coriolis Effect Turns
Surface currents move the top few hundred meters of the ocean. Their main engine is wind. Wind drags on the water, and the water starts to move.
Near the equator, the trade winds blow from east to west. They push surface water westward, creating the North and South Equatorial Currents. Farther north and south (about 30° to 60° latitude), the westerlies blow from west to east and push water eastward.
But moving water does not travel in a straight line on a spinning planet. Earth rotates, so moving air and water are turned sideways.
Why? Earth's surface moves fastest at the equator and slower toward the poles. A current traveling poleward carries its fast eastward speed with it and gets ahead of the ground beneath it. Seen from the ground, it veers.
Define the Coriolis effect. Then state the direction in which moving air or water is deflected in the Northern Hemisphere and in the Southern Hemisphere.
Define the Coriolis effect. Then state the direction in which moving air or water is deflected in the Northern Hemisphere and in the Southern Hemisphere.
Near the equator, the trade winds blow from east to west and push the surface water of the Pacific and Atlantic Oceans with them. In which direction does the resulting…
Near the equator, the trade winds blow from east to west and push the surface water of the Pacific and Atlantic Oceans with them. In which direction does the resulting equatorial current flow? Explain what happens to this water when it reaches the western edge of an ocean basin, such as the Caribbean or the coast of Asia.
Gyres and Western Boundary Currents
Combine the winds with the Coriolis effect and the continents. In each ocean basin, water pushed west by the trade winds and east by the westerlies runs into land and is turned. The result is a huge loop called a gyre.
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Northern Hemisphere gyres rotate clockwise.
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Southern Hemisphere gyres rotate counterclockwise.
The North Atlantic Subtropical Gyre includes the Gulf Stream (west side, along the U.S. East Coast), the North Atlantic Current (north), the Canary Current (east), and the North Equatorial Current (south).
Notice that the loop is not symmetrical. Currents on the western side of a basin are fast and narrow. Currents on the eastern side are slow and wide. These are western boundary currents. Because the Coriolis effect increases with latitude, water piles up on the western side of the basin, and the return flow is squeezed into a narrow, fast current.
The North Atlantic Subtropical Gyre includes the Gulf Stream along the east coast of the United States. State whether this gyre rotates clockwise or counterclockwise,…
The North Atlantic Subtropical Gyre includes the Gulf Stream along the east coast of the United States. State whether this gyre rotates clockwise or counterclockwise, and name the hemisphere effect (Coriolis effect) that is mainly responsible for this direction.
The Gulf Stream, on the western side of the North Atlantic, flows at about $1.5\ \text{m/s}$ and is relatively narrow. The Canary Current, on the eastern side, flows at…
The Gulf Stream, on the western side of the North Atlantic, flows at about $1.5\ \text{m/s}$ and is relatively narrow. The Canary Current, on the eastern side, flows at about $0.1\ \text{m/s}$ and is wide and diffuse.
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Calculate how many days water takes to travel $1{,}000\ \text{km}$ in each current.
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Explain how the shape of the ocean basin, together with the Coriolis effect and the winds, helps produce strong western boundary currents.
Ekman Transport and Upwelling
The wind drags the top layer of water, and the Coriolis effect turns it. The layer below is dragged by the layer above and turned again. The result is a spiral, and the net movement of the whole surface layer is about 90° from the wind direction: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is Ekman transport.
Now put a coast in the picture. Off California the wind often blows from north to south along the shore. Ekman transport moves surface water 90° to the right of the wind, which is to the west, away from the coast.
Upwelling brings cold water and nutrients to the sunlit surface. Plankton bloom, and fish, seabirds, and sea lions follow. That is why the California Current region is rich in marine life, and why the water at the beach there is often chilly even in summer.
Along the coast of California, winds often blow from the north toward the south, parallel to the shore (the coast runs roughly north-south, with the ocean to the west).…
Along the coast of California, winds often blow from the north toward the south, parallel to the shore (the coast runs roughly north-south, with the ocean to the west). Ekman transport moves the surface layer of water at about $90^\circ$ to the right of the wind direction in the Northern Hemisphere.
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Determine the direction of the net surface water movement.
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Explain what must replace the water that moves away from the coast.
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Describe one effect of this process on water temperature and on marine life.
Circumpolar Flow, Tides, and Ocean Basins
Gyres exist because continents block the flow. Around Antarctica there is no continent in the way, apart from the Drake Passage, a gap about $800\ \text{km}$ wide between South America and Antarctica. Strong westerlies drive water continuously eastward, and nothing turns it into a closed loop. The result is the Antarctic Circumpolar Current, the largest current on Earth. Water density plays a role too. Cold, dense polar water sinks and is part of the deep circulation covered under thermohaline circulation. Density differences between cold polar and warmer water to the north help steer this flow.
| North Atlantic | Southern Ocean | |
|---|---|---|
| Land in the way | Continents on both sides | Almost none |
| Flow pattern | Closed gyre | Continuous ring |
| Main wind | Trades and westerlies | Strong westerlies |
Tides are the other driver of surface motion. The Moon's (and Sun's) gravity raises water; Earth's rotation moves coasts through the bulges. The Moon's gravity pulls water toward it, and the Sun's pull adds to or subtracts from this, which raises tidal bulges that Earth's rotation carries past the coasts. Tides are not the main driver of surface currents: their water mostly sloshes back and forth every 12.4 hours with no steady net push, so they do not create gyres. They matter along coasts, where basin shape matters most. In the Bay of Fundy (Canada) a long, narrowing bay funnels the tide, and the range reaches about $16\ \text{m}$. In the Gulf of Mexico, a wide, nearly enclosed basin, the range is often about $0.5\ \text{m}$. The ratio is $\dfrac{16}{0.5} = 32$ times larger.
In the Bay of Fundy, on the Atlantic coast of Canada, the tidal range can reach about $16\ \text{m}$. In the Gulf of Mexico, the tidal range is often only about…
In the Bay of Fundy, on the Atlantic coast of Canada, the tidal range can reach about $16\ \text{m}$. In the Gulf of Mexico, the tidal range is often only about $0.5\ \text{m}$.
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Calculate how many times larger the Bay of Fundy range is.
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Explain how the shape of a basin (a long, narrowing bay versus a wide, nearly enclosed gulf) changes tidal currents.
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Explain why tides drive coastal currents rather than the large gyres.
The Antarctic Circumpolar Current flows eastward around Antarctica. No continents block it, except where it squeezes through the Drake Passage, which is about…
The Antarctic Circumpolar Current flows eastward around Antarctica. No continents block it, except where it squeezes through the Drake Passage, which is about $800\ \text{km}$ wide. In contrast, the North Atlantic has closed gyres bounded by continents.
Analyze why the Southern Ocean has a continuous circumpolar current while the North Atlantic has a gyre. In your answer, discuss the roles of (a) prevailing winds, (b) the Coriolis effect, (c) the shape of the ocean basin, and (d) water density differences between cold polar water and warmer water to the north.
A shipping container with 10,000 plastic toys falls overboard in the North Pacific at $45^\circ\text{N}$, $170^\circ\text{E}$ (a hypothetical event). The toys float at…
A shipping container with 10,000 plastic toys falls overboard in the North Pacific at $45^\circ\text{N}$, $170^\circ\text{E}$ (a hypothetical event). The toys float at the surface and travel with the current.
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Using the North Pacific gyre, the westerlies, and the Coriolis effect, describe the path the toys would most likely follow and where some might wash ashore.
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If the toys travel an average speed of $0.1\ \text{m/s}$ along a total path of $20{,}000\ \text{km}$, calculate how many years the trip takes (use $1\ \text{year} \approx 3.15 \times 10^{7}\ \text{s}$).
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Explain one reason the actual travel time might be longer or shorter than your estimate.
Back to the start: the bottle off Florida is caught by the Gulf Stream, a fast western boundary current of the clockwise North Atlantic gyre. It leaves the coast, crosses the ocean in the westerlies, and slows down in the broad Canary Current.