Seawater is in constant motion. Large-scale, continuous flows are called ocean currents. They are driven by wind, by differences in temperature and salinity that create density gradients, by tides, and by the shape of the basins and the rotation of the Earth.

Surface currents are largely wind-driven and organize into basin-scale gyres. Deep currents form the slow overturning circulation that ventilates the interior ocean. Boundary currents along the western sides of basins are swift and narrow; eastern boundary currents are slower and broader. Currents transport heat toward the poles, redistribute nutrients and larvae, and influence the routes of ships and the tracks of storms.

Measuring currents involves current meters, drifts of floats and surface buoys, satellite altimetry, and the motion of ships. Models simulate current systems for forecasting and climate projection. Changes in current strength or path can alter regional climates and marine ecosystems on interannual to centennial timescales.

• Continuous directed movement of seawater
• Driven by wind, density, tides, and rotation
• Transports heat, nutrients, and organisms
• Organized into gyres and overturning cells