Ocean currents are driven by wind, water density differences, and tides.
Oceanic currents describe the movement of water from one location to another. Currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour). Oceanic currents are driven by three main factors:
1. The rise and fall of the tides. Tides create a current in the oceans, which are strongest near the shore, and in bays and estuaries along the coast. These are called "tidal currents." Tidal currents change in a very regular pattern and can be predicted for future dates. In some locations, strong tidal currents can travel at speeds of eight knots or more.
2. Wind. Winds drive currents that are at or near the ocean's surface. Near coastal areas winds tend to drive currents on a localized scale and can result in phenomena like coastal upwelling. On a more global scale, in the open ocean, winds drive currents that circulate water for thousands of miles throughout the ocean basins.
3. Thermohaline circulation. This is a process driven by density differences in water due to temperature (thermo) and salinity (haline) variations in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents.
Currents affect the Earth's climate by driving warm water from the Equator and cold water from the poles around the Earth. The warm Gulf Stream, for instance, brings milder winter weather to Bergen, Norway, than to New York, much further south.