Currents are steady, smooth movements of water following either a straight or circular path. All of the water in the oceans on Earth are in constant circulation. The two main types of ocean currents are surface currents and deep water or bottom currents.
Surface currents are the most obvious type of current. They are created mainly by prevailing winds, such as trade winds around the equator or westerly winds over the middle latitudes. When wind blows across the water surface, it sets the water in motion. Surface currents can extend to depths of about 65 feet (20 meters).
Landmasses—continental coasts and islands—also affect surface currents. Landmasses act as barriers to the natural path of currents. Without landmasses, there would be a uniform ocean movement from west to east at middle latitudes and from east to west near the equator and at the poles. Instead, landmasses deflect currents or split them up into branches. This deflection, combined with the rotation of Earth on it axis, forces surface currents to flow in circular patterns. These patterns, called gyres (pronounced JEYE-ers), flow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Surface currents help to moderate Earth's temperatures. As surface currents move, they absorb heat in the tropical regions and release it in colder environments near the poles. The Gulf Stream, a major surface current that originates in the Gulf of Mexico, illustrates this. Traveling at an average of 4 miles (6.4 kilometers) per hour, it carries warm water up the east coast of North America and flows across the Atlantic Ocean, where it warms the climate of England and Northern Europe.
Deep water currents move very slowly, usually around 1 inch (2.5 centimeters) per second. However, they are responsible for circulating 90 percent of Earth's ocean water. This circulation influences not only weather patterns but the overall health of the oceans.
Deep water currents are set in motion by variations in water density, which is directly related to temperature and salinity, or salt level. Colder, saltier water is heavier than warmer, fresher water. Water near the poles is colder and saltier than water near the equator. This cold water sinks and flows beneath the ocean surface toward the equator, where it is warmed. It then rises to replace the water that surface currents constantly carry toward the poles.