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What causes ocean currents to move?

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World map showing major ocean current systems, with arrows indicating the direction and speed of warm and cold currents in the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. Warm currents are marked in red and cold currents in blue, with currents labeled by name and symbols denoting those that reverse direction seasonally. A legend explains current speed categories and color codes. — ocean current Major ocean current systems of the world.

Ocean currents are primarily driven by two major mechanisms: wind-driven circulation and thermohaline circulation.1

Wind-Driven Circulation

The surface layer of the ocean is dominated by wind-driven currents. Wind exerts stress on the sea surface, setting the water in motion. This movement is influenced by the Coriolis effect, an apparent force caused by Earth’s rotation that deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.123

Within the top 100 meters, known as the Ekman layer, wind stress, friction, and the Coriolis force interact to create complex flow patterns, including upwelling and downwelling. These processes, combined with horizontal pressure gradients caused by variations in sea surface height, drive large-scale gyres that circulate water across ocean basins.245

Thermohaline Circulation

Deeper ocean currents are driven by thermohaline circulation, which is controlled by horizontal differences in water density. Density is determined by temperature and salinity. In polar regions, cold, salty water becomes dense enough to sink, creating a slow, global-scale movement that replaces deep water with surface water and vice versa.16