Dictionary Definition

A horizontally moving mass of water.

Anatomy

• Set: The direction of flow. It can be either constant or changing.
• Velocity: The speed of the current, which can also change a lot.

Power

Any wind blowing on the ocean's surface has an effect, and when the wind is strong and steady the water begins to move. The surface film is first moved, and gradually the water beneath also begins to move. The depth of the current depends on the strength of the wind and the length of time it blows in one direction.

The Coriolis effect also helps to power the ocean currents. This concept was discovered by the French mathematician Gaspard Coriolis in the 19th century. He calculated how the path of a body in motion on a spinning surface curves in relation to any fixed body on the same surface.

Therefore, the Earth causes a deflection of the objects on its surface because it turns on its axis. The Coriolis effect deflects currents to the right of the prevailing wind direction in the Northern Hemisphere and the left in the Southern Hemisphere. Due to this deflection, the currents in the Northern Hemisphere usually flow clockwise and the currents in the Southern Hemisphere usually flow counterclockwise.

Temperature and salinity affect the density of any mass of water. The colder and saltier the water, the heavier it is. Seawater sinks in a small area where the ocean is very cold and rises gradually across a large area in warmer parts of the ocean. These upwellings and downwellings cycle the water from the poles to the floor and across to the equator. From there, it flows to the surface and returns to the poles. Density causes currents because the heavier water falls while the less dense water rises.

The waves can also cause currents to form. When the waves break, the water is sent to shore and must somehow return. The surf water first runs along the beach, and then goes back through the waves in a rip current. These currents can be dangerous because they can sweep even a moderately strong swimmer into deep water. To escape, a swimmer should swim parallel to the shore until they are outside the pull of the rip current.

Wind

Explanation

Wind has a major effect on ocean currents because it causes the water to move when it blows across the surface. Try the experiment below to see how winds can cause currents to form.

Experiment #1: Winds on Currents

What You'll Need:

• shallow baking pan
• 1 sheet of dark-colored construction paper
• paper hole punch

Directions:

1. Fill the pan with water.
2. Punch ten circles from the construction paper and place them on the surface of the water near the left side of the pan.
3. Blow across the surface of the water where the paper is floating.
4. Watch the motion of the paper as you continue to blow.

What's Happening?

The paper moves clockwise around the outside of the pan because your breath starts a surface current. The water moves horizontally, just like in the tropics when powerful trade winds drive the ocean water before them. The water travels far away from the winds, moving clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. There are also many other contributing factors to surface currents, such as the rotation of the earth, changes in the temperature of ocean water, and differences in the height of the ocean.

Temperature

Explanation

Like explained above, temperature differences cause currents in the ocean. Warm water sinks and cold water rises, so the water moves from the poles to the ocean floor, then travels to the equator and back up to the surface, then returning to the poles to complete the cycle and start over again. Look at the experiment below to see how temperature affects currents in the ocean.

Experiment #2: Temperature on Currents

What You'll Need:

• blue food coloring
• two clear drinking glasses
• two coffee cups
• jar, 1 quart (liter)
• eyedropper
• ice

Directions:

1. Fill the jar half full with ice and add water to fill it up the rest of the way. Let it sit for five minutes.
2. Fill one of the coffee cups 1/4 full with the cold water from the jar.
3. Add enough food coloring to the water in the glass to make it dark blue.
4. Fill one of the drinking glasses with hot water from the faucet.
5. Fill the eyedropper with the cold colored water.
6. Put the eyedropper into the hot water in the glass and release several drops.
7. Watch what happens to the colored water.
8. Fill the other coffee cup 1/4 full with hot water from the faucet.
9. Add enough food coloring to the coffee cup to make the hot water dark blue.
10. Fill the remaining drinking glass with cold water from the jar.
11. Fill the eyedropper with hot water and release several drops into the cold water glass.
12. Watch what happens to the colored water.

What's Happening?

The hot colored water rose in the cold water, while the cold colored water sank in the hot water. This happens because cold water contracts and hot water expands. So the cold water is more dense than the hot water since it occupies less space. The denser cold water sinks and the hot water rises. Convection currents occur when water and air moves and changes temperature.

Density

Explanation

When denser water moves under water that is less dense, currents can form. This merging of the water causes the water to move, forming currents. Look at the experiment below to see how density can cause currents to form.

Experiment #3: Density on Currents

What You'll Need:

• glass bowl, two quart (2 liter)
• table salt
• 250 ml measuring cup
• 15 ml tablespoon measuring tool
• blue food coloring

Directions:

1. Fill the cup about 3/4 full (200 ml) with water.
2. Add six tablespoons (90 ml) of salt to the water and stir.
3. Add several drops of food coloring to make the water dark blue.
4. Fill the bowl 1/2 full with water.
5. Watch the bowl at the side as you slowly pour the blue, salty water down the side of the bowl.

What's Happening?

The colored water sinks to the bottom of the bowl, creating waves under the clear water above it. A density current, such as this, is the movement of water due to the difference in density of water. All sea water has salt in it, but when two bodies of water mix, the water with the most salt will move under the other water.

Types

• Downwelling: Water sinking from the surface downward.
• Upwelling: Water moving up from the depths of the ocean.
• Rip Currents: Currents that are set in motion by waves on the shore and allow the water breaking there to get back out to sea.
• Countercurrents: Each major oceanic current has a powerful countercurrent moving in the opposite direction beneath it. The Gulf Stream has a countercurrent flowing from north to south in the western Atlantic Ocean, as does the eastern coast of South America where the countercurrent moves north.

Major

Southern Hemisphere

• Gyres: Huge whirlpools in the South Atlantic, the South Pacific, and the southern part of the Indian Ocean that circle counterclockwise.
• West Wind Drift: Located in the Southern Hemisphere circling the Antarctic continent, this current flows east. It is also known as the Antarctic Drift or the Antarctic Circumpolar Current after its location. As this drift approaches other landmasses to the north, such as South America, Africa, and Australia, its northern portion deflects north and carries cold water up the west coast of each land area.
• Peru Current: Formerly known as the Humboldt Current, this current is found off the western coast of South America by Peru flowing northward. This is the branch of the West Wind Drift found in the Pacific Ocean.
• Benguela Current: This current was named after the Benguela district of Angola and is found along the western coast of Africa. This is the branch of the West Wind Drift found in the Atlantic Ocean.
• West Australia Current: This current is found off the western coast of Australia, and is the branch of the West Wind Drift in the Indian Ocean.
• South Equatorial Current: Where the currents merge with a broad westward flow. When it reaches the Atlantic and Indian Oceans, it divides so that one part crosses the equator and heads north and the other moves south. In the Pacific, it stays as one and turns south.
• Brazil Current: The southward-moving current of warm water in the South Atlantic.
• Agulhas Current: The southward-moving current of warm water in the Indian Ocean.
• Falkland Current: This current is a deflection of the West Wind Drift that flows between Cape Horn and the South Shetland Islands, and between and around the Falkland Islands. It then moves up to the east coast of South America as a cold current.

Northern Hemisphere

• North Equatorial Current: Flowing westward, this slow and wide current is found in the equatorial regions of the North Atlantic and North Pacific Oceans.
• Monsoon Drift: The monsoon winds in the Indian Ocean cause this current. It flows west in the winter, and east in the summer.
• Antilles Current: In the southwestern part of the North Atlantic, part of the North Equatorial Current flows into the West Indies and becomes this current.
• Florida Current: Part of the North Equatorial Current joins the South Equatorial Current, and this combination flows into the Caribbean and the Atlantic to join the Antilles Current off the coast of Florida. The part of the Gulf Stream system from the Florida Straits to Cape Hatteras, North Carolina, is known as the Florida Current.
• Gulf Stream: The part of the Gulf Stream system from Cape Hatteras to south of the Grand Banks.
• Japan Current, or Kuroshio: The North Equatorial Current divides in the Pacific off the coast of the Philippines and one part turns north to form the Japan Current.
• Equatorial Countercurrent: The other part of the North Equatorial Current that divides in the Pacific, goes south and then east to form the Equatorial Countercurrent.
• North Atlantic Current: Part of the Gulf Stream turns east to form this current.
• Canaries Current: Part of the North Atlantic Current turns south to form this current.
• Irminger Current: Some of the North Atlantic Current curves back to the west to form this current south of Iceland.
• Norwegian Current: The rest of the North Atlantic Current enters the Arctic Ocean as the Norwegian Current.
• North Pacific Current: Part of the Japan Current turns east to form this current.
• California Current: Part of the North Pacific Current turns south to form this current.
• Alaska Current: Another part of the North Pacific Current turns north and becomes this current.
• Labrador Current: The cold current in the Atlantic that flows south from the Arctic regions, and at about 45 degrees north latitude it divides with one part curving to the east to join the North Atlantic Current. The other part moves southwest to form a wedge between the coast and the warm waters from the south.
• Oyashio Current: The cold current in the Pacific that flows south from the Arctic regions, and at about 45 degrees north latitude it divides. One part goes east to join the North Pacific Current, while the other part moves southwest to form a wedge between the coast and the warm waters from the south.
• East Greenland Current: This flows south along the eastern coast of Greenland, and is joined by the Irminger Current to curve around the southern tip of Greenland and north through the Davis Strait into the Baffin Bay where it turns again to become part of the Labrador Current.

Effects

Currents can have many effects both on the ocean and on us. For example, currents affect navigation by adding speed to ships sailing with it and slowing down ships sailing against it. Also, major currents affect the climate because they transport warm water to higher latitudes and cold water to lower latitudes. The winds that flow over the currents determine the effect the current has on climate because stronger winds will make the currents carry more water with them.

Studying

In 1885, Prince Albert of Monaco conducted an experiment with the currents by throwing bottles over the side of his yacht and seeing where they landed. By the end of his experiment, he had tossed 1,675 bottles and 1,448 were lost. But the remaining 227 bottles helped scientists chart two of the Atlantic's major currents, which later helped mariners avoid drifting World War I sea mines. This method is still used today. Scientists put a card with their name and address into a bottle that floats just beneath the surface and push them into the sea. Then they may be picked up in the nets of fishing boats, or end up eventually on land. The person who finds the bottle is asked to write on the card when and where they found it and put it in the mail.

Currents were first studied by observing the drift of some flotsam to measure the direction and speed of a surface current. And this method is still used because the onshore observer has a fixed position to measure the relative speed and direction of the current. However, we have much more advanced techniques today, as well. Satellites take vivid images of ocean currents, and the National Oceanic and Atmospheric Administration makes detailed studies of important currents using heat-sensing radiometers on its satellites. Computers help by converting the differences in water temperature found by the satellite into brightly colored maps detailing the movement of the currents. Another satellite system, called TOPEX/Poseidon was launched in 1992 to confirm and update current movement and also to predict rough seas.

A current's speed can be determined by a current meter, which is a tube that is directed toward the flow with a propeller inside that turns when the water flows through the tube. The current's velocity is then calculated by the number of turns the propeller makes per second or per minute. The direction of the current can be found by setting buoys and observing the buoys direction and distance at various intervals.

Ships can also be helpful in measuring currents. Using fancy equipment to map the currents, or a bathythermograph to measure the temperature, or just sending in routine information about how far they've been pushed by the current, the ships can be very helpful in charting currents.