Why does surface blow follow winds?

There are two basic circulation systems in our oceans. One is the wind-driven surface circulation, and the other is the deepwater density-driven circulation. It is controlled primarily by differences in temperature and salt content (thermohaline circulation - "thermo" for temperature and "haline" for salinity). This section focuses on surface ocean currents.

Water flow at the surface of the ocean involves about 10% of the water in the global ocean! The surface water flow is organized into many surface currents. The constant flow of wind in the latitudinal atmospheric circulation cells pulls the surface water along into these currents.

[ Waves!]

Waves! Photograph courtesy of John B. Anderson, Rice University.

As the wind blows across the water there is friction between the wind and the water surface. Some of the energy of the wind is transferred to the water to make waves and some is transferred to create surface currents. These currents travel across the surface of the ocean. The ocean surface flow would mimic wind patterns, but, of course, a few other factors complicate things!

Surface currents are influenced by other forces such as the Coriolis effect. In addition, surface currents encounter continent interference and are deflected from their original path.

[ Six major circulation cells in the global ocean ]

Six major circulation cells in the global ocean (modified from Garrison,1995).

The net sum of these influences is that a series of quasi-circular water circulation cells occur in the ocean. These current systems are called gyres. Much of the global ocean surface water is involved in gyre circulation. There are six major circulation systems: the North Atlantic gyre, South Atlantic gyre, North Pacific gyre, South Pacific gyre, the Indian Ocean gyre, and the West Wind Drift. The first five are gyres because the water flows around the edge of an ocean basin. The West Wind Drift, also called the Antarctic Circumpolar Drift, is an oceanic current that flows around the Antarctic continent.

Gyres can be subdivided into distinct currents occurring along their north-south and east-west sides: transverse currents, eastern boundary currents, and western boundary currents. Boundary currents have important consequences for the distribution of life in the surface ocean, as we will see in a little bit.

Transverse Currents
Transverse currents flow east-west. Let's use the subtropical gyres as an example. In the equatorial ocean, much of the surface water is caught up in the North and South Equatorial Currents. These transverse currents flow from east to west along each side of the equator and are driven primarily by the easterly trade winds.

[ Some transverse currents of the global ocean]

Some transverse currents of the global ocean. Transverse currents are connected by eastern and western boundary currents (modified from Garrison, 1995).

There is another east-west transverse current closing the top (or bottom) of the gyre. For the subtropical gyres, this current is called the West Wind Drift. In the Northern Hemisphere Pacific and Atlantic Oceans, the West Wind Drift is also called the North Pacific Current or the North Atlantic Current (which is a continuation of the Gulf Stream). In the Southern Hemisphere, the West Wind Drift (also called the Antarctic Circumpolar Current) is the strongest surface current. It carries the largest volume of water per second of all the surface currents - almost twice the volume of water per second of the Gulf Stream! Intense westerly winds over the Southern Ocean drive the West Wind Drift, and there are no continental land masses in its way to slow or deflect it.

[ Equatorial counter currents of the global ocean ]

Equatorial counter currents of the global ocean (modified from Garrison, 1995).

Countercurrents are another type of transverse current. The equatorial current in each hemisphere flows in the same direction, causing the water to pile up near the equator and creating convergence zones. Some of the water returns as a "down-hill" flow as a countercurrent. The subtropical gyres in each hemisphere are separated by narrow eastward-flowing equatorial countercurrents flowing opposite to the equatorial currents. Countercurrents may occur slightly offset from the original current, or may flow slightly below it. Countercurrents that flow below the surface are called undercurrents. One of these, the Pacific Equatorial Current, flows to the east along the equator at a depth of about 100 m (330 ft)! The origins of undercurrents are not fully understood.

[Adorable Penguin Cartoon]

Penguin Note: Meteorologists and oceanographers use terminology that can be quite confusing to those not used to it! When oceanographers talk about to a eastern-flowing current, they mean it is moving to the east. When meteorologists say a wind is easterly, they mean it is blowing out of the east and moving to the west!

We've described the top and bottom east-west currents of the subtropical gyres. What are the currents that link them and complete the nearly closed circulation of these cells?

Continent Interference
When currents run into something, like a continent, they must change direction. Transverse currents move large quantities of water east-west. When they meet a continent the water essentially "piles up" against the land mass. Gravity will not allow it to stay as a hill of water; it must flow to the north or south as a boundary current. (It also can flow at depth under and in the opposite direction of the surface current, as a countercurrent.)

Western Boundary Currents
[ Western boundary currents]

Western boundary currents (modified from Garrison, 1995).

Western boundary currents are among the largest and strongest ocean currents. They occur at the western side of an ocean basin (or the eastern side of a continent!). These deep, fast moving currents carry water from the equator towards the poles.

[Adorable Penguin Cartoon]

Penguin Note: Western boundary currents are very important to us land-dwellers - they help moderate our climates! Transverse currents generally stay in the same climatic zone, giving them time to adjust to the temperature of the region. When they are forced poleward by a continent, they carry heat with them, helping to partially even out Earth's surface temperatures.

Eastern Boundary Currents
[ Eastern boundary currents]

Eastern boundary currents (modified from Garrison, 1995).

Eastern boundary currents transport water from the pole to the equator. They are the return flow from the western boundary currents as they are driven back across the ocean by westerly winds. They occur on the eastern side of the basin, and are shallower, more broad, and slower than western boundary currents.

Biology and Boundary Currents
The different characteristics of boundary currents on either side of ocean basins play a large role in controlling their areas' biological productivity. The longer water has been at the surface of the ocean, the longer the tiny marine plants called phytoplankton have had to use up the available nutrients. Phytoplankton are the base of the marine food chain, and where they are abundant, other organisms tend to be abundant, too. "Old" surface water piles up along the western boundaries of the ocean basins and is usually not well mixed with the deeper, nutrient-rich water below it. This nutrient-poor water causes the area to be relatively unproductive. Regions dominated by eastern boundary currents are a different story! These areas have prevailing winds that blow offshore, pushing away surface water and allowing deeper, nutrient-laden water to come to the surface to replace it. This process is called upwelling. Upwelling causes parts of the coastal ocean along eastern basin margins to be extremely productive. Find out more about this important process in GLACIER's upwelling section!

The differences between eastern boundary currents and the western boundary currents are caused by two main factors: wind patterns and Earth's rotation.

1. The west-flowing trade winds of the Northern and Southern hemispheres converge at the equator. The trade winds push water to the west within a narrow zone of the ocean surface. This water piles against the western side of the basin, turns to flow toward the pole, and then returns with the east-flowing westerlies. The easterly winds of the hemispheres are not convergent, so there is not such an organized flow of water on the return trip of the gyre, and the eastern boundary currents are not as strong.

2. The eastward rotation of Earth comes into play; the waters are "pushed" toward the western sides of the basins, so the western boundary currents are stronger. This is due to the Coriolis effect These two factors increase the intensity of the western boundary currents, a process termed "westward intensification."

We've used subtropical gyres to illustrate what gyres are, but they are not the only ones we have. Subpolar gyres are present in each hemisphere, too. They are smaller than their lower-latitude cousins, and they circulate in the opposite direction from them (counterclockwise in the north and clockwise in the south). The position of the continents allows well developed subpolar gyres in the Northern Hemisphere but not in the Southern Hemisphere, although a few small ones do exist near Antarctica. The Antarctic Circumpolar Current flows around the Antarctic continent, essentially unimpeded by deflecting land masses to break it up.

[ Global ocean surface currents diverted by land masses today]

Global ocean surface currents diverted by land masses today (modified from Garrison, 1995).

The surface currents would be fairly simple if it were not for the arrangement of the continents! The interaction of the currents with the positions of the land results in a series of basins with roughly circular water-flow patterns. Remember that the land masses are attached to plates that are shifting on Earth's surface (plate tectonics).

[Adorable Penguin Cartoon]

Do you think that the land masses have been in the same positions forever? And, if they have moved, what do you think this means about past global ocean currents? Were currents the same in the past as they are today?

[ Reconstruction of the positions of land masses 170 million years ago]

Reconstruction of the positions of land masses 170 million years ago - dinosaur time (Jurassic)! Was the circulation pattern very different from today? Compare this map to the one showing present currents on the ocean surface. Remember that the position of land deflects the paths of currents (modified from Open University Course Team, 1992).

Coriolis Effect
The deflection of a free-moving object (air, water, an airplane, a baseball) relative to the rotation of Earth is called the Coriolis effect.

[ Path of a plane flying from the North Pole and one flying from the South
Pole toward the equator]

Path of a plane flying from the North Pole and one flying from the South Pole toward the equator. Earth rotates under the planes, so the paths look curved from the ground. The Coriolis effect, caused by the rotation of Earth on its axis, causes the east/west flow of the atmospheric circulation (modified from Lutgens and Tarbuck, 1992).

An airplane takes off from the North Pole, and flies in a straight line toward the equator. During the flight time, Earth constantly, but slowly, rotates, so the path of the airplane from the ground would look like it had curved. The plane looks like it flew to the west, or right as Earth rotated. If you were watching Earth's surface from a fixed spot in outer space, you would see the plane move in a straight path, and Earth rotate underneath.

The Coriolis effect deflects moving masses to the right of their original path in the Northern Hemisphere, and to the left in the Southern Hemisphere. In reality, the moving objects do not change their path at all; the rotation of the Earth, and us as passengers on it, gives the appearance of the change in direction.

The slower an object moves, the more influence the Coriolis effect will have. In addition, Earth spins faster at the equator than at the poles because there is more distance to cover in a single rotation at the equator. This means that the Coriolis effect will be greater at the poles than at low latitudes.

So, next time you swing the bat and miss the pitch, blame the Coriolis effect (although the time the ball takes from the mound to the plate is a little short for much rotational deflection to take place, and most coaches might want a better explanation!).