March 2007
Currents: Water on the Move

Can you name Earth's most important rivers? Would you guess the Nile, the Amazon, the Mississippi, or maybe the Yangtze? Did you know there are rivers that flow through the world's oceans? And that they are more important in shaping our environment than the major rivers on land? These mighty bodies of moving water are called currents.

Ocean currents move water continuously along specific pathways, often over very great distances. This happens both on the surface and in the deep ocean. Currents driven by the wind travel thousands of miles across the ocean's surface. In the process, they move heat from warmer to cooler areas. Ocean waters also move vertically, mixing waters of different temperatures and salinities (amounts of salt).

Currents influence temperature, climate, plants, and animals in the ocean and on land. They carry organisms, nutrients, and pollutants across long distances and up and down through the ocean's depths. Both short-term weather patterns and long-term changes in climate, such as global warming, are affected by currents.

Ocean in Motion

Most of the currents in the upper half-mile of the ocean are caused by winds that blow year-round in one direction across Earth. Wind pushes directly on the ocean surface, causing the ocean's top layer to move. Surface winds also set in motion other currents that extend to the ocean bottom, far below the direct influence of the wind.

However, most ocean currents do not occur at the surface and are not directly caused by wind. Deep ocean currents may move across great horizontal distances in the same way as surface currents. They also may run through the ocean vertically, although vertical currents are weak, and a body of water may take centuries to go from the surface to the bottom, Generally, the deeper a current, the more slowly it moves.

When wind-driven surface currents run into continents, the water must turn and flow toward either the poles or the equator (Earth's mid-section). The effect of Earth's rotation causes moving objects on Earth to follow curved paths (a scientific principle known as the Coriolis effect), which in turn causes the currents to turn and move in huge, oceanwide, looping circles called gyres.