8.4: Ocean Currents

Ocean water is constantly in motion (Figure 14.7). From north to south, east to west, and up and down the shore, ocean water moves all over the place. These movements can be explained as the result of many separate forces, including local conditions of wind, water, the position of the moon and Sun, the rotation of the Earth, and the position of land formations.

Figure 14.7: Ocean waves transfer energy through the water over great distances.

Lesson Objectives

• Describe how surface currents form and how they affect the world’s climate.
• Describe the causes of deep currents.
• Relate upwelling areas to their impact on the food chain.

Surface Currents

Wind that blows over the ocean water creates waves. It also creates surface currents, which are horizontal streams of water that can flow for thousands of kilometers and can reach depths of hundreds of meters. Surface currents are an important factor in the ocean because they are a major factor in determining climate around the globe.

Causes of Surface Currents

Figure 14.14: The Coriolis Effect causes winds and currents to form circular patterns. The direction that they spin depend on the hemisphere that they are in.

Currents on the surface are determined by three major factors: the major overall global wind patterns, the rotation of the Earth, and the shape of ocean basins.

When you blow across a cup of hot chocolate, you create tiny ripples on its surface that continue to move after you’ve stopped blowing. The ripples in the cup are tiny waves, just like the waves that wind forms on the ocean surface. The movement of hot chocolate throughout the cup forms a stream or current, just as oceanic water moves when wind blows across it.

But what makes the wind start to blow? When sunshine heats up air, the air expands, which means the density of the air decreases and it becomes lighter. Like a balloon, the light warm air floats upward, leaving a slight vacuum below, which pulls in cooler, denser air from the sides. The cooler air coming into the space left by the warm air is wind.

Because the Earth’s equator is warmed by the most direct rays of the Sun, air at the equator is hotter than air further north or south. This hotter air rises up at the equator and as colder air moves in to take its place, winds begin to blow and push the ocean into waves and currents.

Wind is not the only factor that affects ocean currents. The ‘Coriolis Effect’ describes how Earth’s rotation steers winds and surface currents (Figure 14.14). The Earth is a sphere that spins on its axis in a counterclockwise direction when seen from the North Pole. The further towards one of the poles you move from the equator, the shorter the distance around the Earth. This means that objects on the equator move faster than objects further from the equator. While wind or an ocean current moves, the Earth is spinning underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line. Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west. The Coriolis Effect bends the direction of surface currents.

The third major factor that determines the direction of surface currents is the shape of ocean basins (Figure 14.15). When a surface current collides with land, it changes the direction of the currents. Imagine pushing the water in a bathtub towards the end of the tub. When the water reaches the edge, it has to change direction.

Figure 14.15: This map shows the major surface currents at sea. Currents are created by wind, and their directions are determined by the Coriolis effect and the shape of ocean basins.

Effect on Global Climate

Surface currents play a large role in determining climate. These currents bring warm water from the equator to cooler parts of the ocean; they transfer heat energy. Let’s take the Gulf Stream as an example; you can find the Gulf Stream in the North Atlantic Ocean in Figure 14.15. The Gulf Stream is an ocean current that transports warm water from the equator past the east coast of North America and across the Atlantic to Europe. The volume of water it transports is more than 25 times that of all of the rivers in the world combined, and the energy it transfers is more than 100 times the world’s energy demand. It is about 160 kilometers wide and about a kilometer deep. The Gulf Stream’s warm waters give Europe a much warmer climate than other places at the same latitude. If the Gulf Stream were severely disrupted, temperatures would plunge in Europe.

Deep Currents

Surface currents occur close to the surface of the ocean and mostly affect the photic zone. Deep within the ocean, equally important currents exist that are called deep currents. These currents are not created by wind, but instead by differences in density of masses of water. Density is the amount of mass in a given volume. For example, if you take two full one liter bottles of liquid, one might weigh more, that is it would have greater mass than the other. Because the bottles are both of equal volume, the liquid in the heavier bottle is denser. If you put the two liquids together, the one with greater density would sink and the one with lower density would rise.

Two major factors determine the density of ocean water: salinity (the amount of salt dissolved in the water) and temperature (Figure 14.16). The more salt that is dissolved in the water, the greater its density will be. Temperature also affects density: the colder the temperature, the greater the density. This is because temperature affects volume but not mass. Colder water takes up less space than warmer water (except when it freezes). So, cold water has greater density than warm water.

Figure 14.16: Thermohaline currents are created by differences in density due to temperature (thermo) and salinity (haline). The blue arrows are deep currents and the red ones are surface currents.

Figure 14.17: Surface and deep currents together form convection currents that circulate water from one place to another and back again. A water particle in the convection cycle can take 1600 years to complete the cycle.

More dense water masses will sink towards the ocean floor. Just like convection in air, when denser water sinks, its space is filled by less dense water moving in. This creates convection currents that move enormous amounts of water in the depths of the ocean. Why is the water temperature cooler in some places? Water cools as it moves from the equator to the poles via surface currents. Cooler water is more dense so it begins to sink. As a result, the surface currents and the deep currents are linked. Wind causes surface currents to transport water around the oceans, while density differences cause deep currents to return that water back around the globe (Figure 14.17).

Upwelling

As you have seen, water that has greater density usually sinks to the bottom. However, in the right conditions, this process can be reversed. Denser water from the deep ocean can come up to the surface in an upwelling (Figure 14.18). Generally, an upwelling occurs along the coast when wind blows water strongly away from the shore. As the surface water is blown away from the shore, colder water from below comes up to take its place. This is an important process in places like California, South America, South Africa, and the Arabian Sea because the nutrients brought up from the deep ocean water support the growth of plankton which, in turn, supports other members in the ecosystem. Upwelling also takes place along the equator between the North and South Equatorial Currents.

Figure 14.18: An upwelling forces denser water from below to take the place of less dense water at the surface that is pushed away by the wind.

Lesson Summary

• Ocean waves are energy traveling through the water.
• The highest portion of a wave is the crest and the lowest is the trough.
• The horizontal distance between two wave crests is the wave’s length.
• Most waves in the ocean are wind generated waves.
• Ocean surface currents are produced by major overall patterns of atmospheric circulation, the Coriolis Effect and the shape of each ocean basin.
• Ocean surface circulation brings warm equatorial waters towards the poles and cooler polar water towards the equator.
• Deep ocean circulation is density driven circulation produced by differences in salinity and temperature of water masses.
• Upwelling areas are biologically important areas that form as ocean surface waters are blown away from a shore, causing cold, nutrient rich waters to rise to the surface.

Review Questions

1. What factors of wind determine the size of a wave?
2. Define the crest and trough of a wave.
3. What is the most significant cause of the surface currents in the ocean?
4. How do ocean surface currents affect climate?
5. What is the Coriolis Effect?
6. Some scientists have hypothesized that if enough ice in Greenland melts, the Gulf Stream might be shut down. Without the Gulf Stream to bring warm water northward, Europe would become much colder. Explain why melting ice in Greenland might affect the Gulf Stream.
7. What process can make denser water rise to the top?
8. Why are upwelling areas important to marine life?

Vocabulary

amplitude

The vertical height of a wave, measured from trough to crest.

Coriolis effect

The apparent deflection of a moving object like water or air caused by Earth’s rotation.

crest

The highest point in a wave.

deep current

A current deep within the ocean, which moves because of density differences (caused by differences in water temperature and salinity).

rip current

A strong surface current of water that is returning to the ocean from the shore.

surface current

A horizontal movement of ocean water, caused by surface winds.

trough

The lowest point in a wave.

tsunami

A seismic sea wave generated by vertical movement of the ocean floor underwater earthquake, underwater volcanic eruption or landslide or meteorite impact.

upwelling

Cold, nutrient-rich water that rises from oceanic depths usually near the continents, when wind blows the overlying surface away or along the equator.

wave

A change in the shape of water caused by energy moving through the water.

wavelength

The horizontal distance between two troughs, or two crests in a wave.

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