Currents

Surface Currents

Ocean water moves in predictable ways along the ocean surface. Surface currents can flow for thousands of kilometers and can reach depths of hundreds of meters. These surface currents do not depend on weather; they remain unchanged even in large storms because they depend on factors that do not change.

Surface currents are created by three things:

  • global wind patterns
  • the rotation of the Earth
  • the shape of the ocean basins

Surface currents are extremely important because they distribute heat around the planet and are a major factor influencing climate around the globe.

Global Wind Patterns

Winds on Earth are either global or local. Global winds blow in the same directions all the time and are related to the unequal heating of Earth by the Sun — that is, more solar radiation strikes the equator than the polar regions –- and the rotation of the Earth — that is, the Coriolis effect. The causes of the global wind patterns will be described in detail in the Earth’s Atmosphere chapter.

Water in the surface currents is pushed in the direction of the major wind belts:

  • trade winds: east to west between the equator and 30oN and 30oS
  • westerlies: west to east in the middle latitudes
  • polar easterlies: east to west between 50o and 60o north and south of the equator and the north and south pole

Earth’s Rotation

Wind is not the only factor that affects ocean currents. The Coriolis effect describes how Earth’s rotation steers winds and surface ocean currents (Figure  below). Coriolis causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve.

An example might make the Coriolis effect easier to visualize. If an airplane flies 500 miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly 500 miles, that city moved, along with the Earth it sits on. The airplane will therefore arrive at a city to the west of the original city (in the Northern Hemisphere), unless the pilot has compensated for the change. So to reach his intended destination, the pilot must also veer right while flying north.

As wind or an ocean current moves, the Earth spins 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 to the right in the Northern Hemisphere and left in the Southern Hemisphere.

The Coriolis effect causes winds and currents to form circular patterns. The direction that they spin depends on the hemisphere that they are in.

Coriolis effect is demonstrated using a metal ball and a rotating plate in this video. The ball moves in a circular path just like a freely moving particle of gas or liquid moves on the rotating Earth (5b): http://www.youtube.com/watch?v=Wda7azMvabE (2:04).

Shape of the Ocean Basins

When a surface current collides with land, the current must change direction. In the Figure  below, the Atlantic South Equatorial Current travels westward along the equator until it reaches South America. At Brazil, some of it goes north and some goes south. Because of Coriolis effect, the water goes right in the Northern Hemisphere and left in the Southern Hemisphere.

The major surface ocean currents.

You can see on the map of the major surface ocean currents that the surface ocean currents create loops called gyres (Figure  below). The Antarctic Circumpolar Current is unique because it travels uninhibited around the globe. Why is it the only current to go all the way around?

The ocean gyres. Why do the Northern Hemisphere gyres rotate clockwise and the Southern Hemisphere gyres rotate counterclockwise?

This video shows the surface ocean currents set by global wind belts (5a): http://www.youtube.com/watch?v=Hu_Ga0JYFNg (1:20).

Effect on Global Climate

Surface currents play an enormous role in Earth’s climate. Even though the equator and poles have very different climates, these regions would have more extremely different climates if ocean currents did not transfer heat from the equatorial regions to the higher latitudes.

The Gulf Stream is a river of warm water in the Atlantic Ocean, about 160 kilometers wide and about a kilometer deep. Water that enters the Gulf Stream is heated as it travels along the equator. The warm water then flows up the east coast of North America and across the Atlantic Ocean to Europe (Figure  below). The energy the Gulf Stream transfers is enormous: more than 100 times the world’s energy demand.

The Gulf Stream’s warm waters raise temperatures in the North Sea, which raises the air temperatures over land between 3 to 6oC (5 to 11oF). London, U.K., for example, is at the same latitude as Quebec, Canada. However, London’s average January temperature is 3.8oC (38oF), while Quebec’s is only -12oC (10oF). Because air traveling over the warm water in the Gulf Stream picks up a lot of water, London gets a lot of rain. In contrast, Quebec is much drier and receives its precipitation as snow.

In a satellite image of water temperature in the western Atlantic it is easy to pick out the Gulf Stream, which brings warmer waters from the equator up eastern North America.

Deep CurrentsThermohaline circulation drives deep ocean circulation. Thermo means heat and haline refers to salinity. Differences in temperature and in salinity change the density of seawater. So thermohaline circulation is the result of density differences in water masses because of their different temperature and salinity.

What is the temperature and salinity of very dense water? Lower temperature and higher salinity yield the densest water. When a volume of water is cooled, the molecules move less vigorously so same number of molecules takes up less space and the water is denser. If salt is added to a volume of water, there are more molecules in the same volume so the water is denser.

Changes in temperature and salinity of seawater take place at the surface. Water becomes dense near the poles. Cold polar air cools the water and lowers its temperature, increasing its salinity. Fresh water freezes out of seawater to become sea ice, which also increases the salinity of the remaining water. This very cold, very saline water is very dense and sinks. This sinking is called downwelling.

This video lecture discusses the vertical distribution of life in the oceans. Seawater density creates currents, which provide different habitats for different creatures (5d): http://www.youtube.com/watch?v=LA1jxeXDsdA (6:12).

Two things then happen. The dense water pushes deeper water out of its way and that water moves along the bottom of the ocean. This deep water mixes with less dense water as it flows. Surface currents move water into the space vacated at the surface where the dense water sank (Figure  below). Water also sinks into the deep ocean off of Antarctica.

Cold water (blue lines) sinks in the North Atlantic, flows along the bottom of the ocean and upwells in the Pacific or Indian. The water then travels in surface currents (red lines) back to the North Atlantic. Deep water also forms off of Antarctica.

Since unlimited amounts of water cannot sink to the bottom of the ocean, water must rise from the deep ocean to the surface somewhere. This process is called upwelling (Figure  below).

Upwelling forces denser water from below to take the place of less dense water at the surface that is pushed away by the wind.

Generally, upwelling occurs along the coast when wind blows water strongly away from the shore. This leaves a void that is filled by deep water that rises to the surface.

Upwelling is extremely important where it occurs. During its time on the bottom, the cold deep water has collected nutrients that have fallen down through the water column. Upwelling brings those nutrients to the surface. Those nutrient support the growth of plankton and form the base of a rich ecosystem. California, South America, South Africa, and the Arabian Sea all benefit from offshore upwelling.

An animation of upwelling is seen here: http://oceanservice.noaa.gov/education/kits/currents/03coastal4.html.

Upwelling also takes place along the equator between the North and South Equatorial Currents. Winds blow the surface water north and south of the equator so deep water undergoes upwelling. The nutrients rise to the surface and support a great deal of life in the equatorial oceans.

Lesson Summary

  • Ocean surface currents are produced by global winds, the Coriolis effect and the shape of each ocean basin.
  • The Pacific and Atlantic Oceans have a circular pattern of surface currents called gyres that circle clockwise in the Northern Hemisphere and counterclockwise in the Southern. The Indian Ocean only has a counterclockwise gyre.
  • Surface ocean circulation brings warm equatorial waters towards the poles and cooler polar water towards the equator.
  • Thermohaline circulation drives deep ocean currents.
  • Upwelling of cold, nutrient-rich waters creates biologically rich areas where surface waters are blown away from a shore, or where equatorial waters are blow outward.

Review Questions

  1. What causes the patterns of surface currents in the ocean?
  2. How do ocean surface currents affect climate?
  3. What is the Coriolis effect?
  4. What process can make deep, dense water rise to the surface?
  5. Why are upwelling areas important to marine life?

Further Reading / Supplemental Links

Points to Consider

  • Some scientists have hypothesized that if enough ice in Greenland melts, the Gulf Stream might be shut down. Why might this happen?
  • If the Gulf Stream shuts down, what would be the result on climate in Europe?
  • How do the movements of ocean water contribute to the ocean’s life?