Ocean currents have been steering the course of sailing vessels for centuries. These bands of water, which may extend several hundred meters below the surface and move at up to 10 kilometers (6 miles) per hour, have aided the discovery of new lands. For example, Indonesian natives settled Madagascar more than 1,500 years ago at the end of a 6,400-kilometer (3977-mile) canoe journey along the South Equatorial Current. In other instances, currents have doomed sea voyages. In the year 990, only 14 out of 25 Viking ships bound for Greenland landed safely. For weeks, the aspiring settlers aboard these ships battled storms and the eastward-moving currents of the North Atlantic Drift and the Gulf Stream.

The Gulf Stream was important to the founding and settling of North America. This current, which carries warm water from the southern tip of Florida, along the east coast of the United States, and across the North Atlantic Ocean, enabled ships to travel from colonial America to Britain in less than a month. However, for those who did not know to steer clear of the Gulf Stream on their return trip, their journey was much more arduous, taking two months on average. Those who tried to avoid the current by hugging the coastline instead of taking the more southerly route sometimes ended up shipwrecked.

The movement of ocean currents is affected by global wind patterns around Earth. Global winds form three separate bands in the northern and southern hemispheres. These wind patterns result from a combination of greater solar radiation at the equator than at the poles, Earth's rotation, and the Coriolis effect. The uneven distribution of heat from solar radiation creates pressure differences, which cause the movement of air, or wind. Earth's rotation and the Coriolis effect cause airflow to be deflected in an eastward or westward direction in addition to its northward and southward flow. Ocean currents mirror these wind patterns to an extent. However, because continents impede the flow of water, the currents are not aligned in three distinct bands. Instead, they form cyclical patterns, called gyres, around the perimeter of Earth's oceans and seas. In the northern hemisphere, gyres move in a clockwise direction and in the southern hemisphere they move in a counterclockwise direction.

Over the years, scientists have used many techniques to map and model the movement of ocean currents. For example, Benjamin Franklin — who named the Gulf Stream — relied on the observations of whalers and his own temperature measurements to map the warm waters of this current. Others have used records of the paths, or trajectories, of abandoned ships, called derelicts, as well as reports of recovered cargo lost from ships, to plot the direction and speed of ocean currents that propelled the objects. This information, combined with what has been obtained from more sophisticated instruments such as satellite-tracked buoys, has helped scientists better understand ocean currents and has made ocean travel safer and more efficient.

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