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An ocean current is organized water motion that transports mass, heat, salt and other properties through the ocean. Surface winds, pressure gradients, buoyancy changes, Earth's rotation and friction all contribute, with their relative importance depending on scale and depth.

A steady large-scale interior flow can approximately balance a horizontal pressure gradient against the Coriolis force, while boundary currents require additional dynamics. Henry Stommel showed that the latitude dependence of rotation, combined with friction in a wind-driven basin, can concentrate the return flow on the western side. The circulation is therefore not simply water moving in the direction of the local wind.^[ref]

A physical oceanographer studies the ocean's motion and physical state, including ocean currents, waves, temperature, salinity and mixing. Observations and mathematical models connect local measurements with transport over much larger distances.

For example, temperature and salinity influence mass density, which affects stratification and buoyancy-driven motion. Interpreting a current therefore requires both fluid dynamics and the physical properties of seawater.