A geostrophic current results from the balance between gravitational forces and the Coriolis effect. The gravitational effect is controlled by the tilt of the sea surface and water density is controlled by horizontal changes in temperature and salinity. Geostrophic currents are zero-frequency inertial waves in the oceans. They move water horizontally, and because both heat and salinity are involved, geostrophic circulation is a type of thermohaline circulation. Thermohaline circulation is a slightly more general term that encompasses both horizontal and vertical movement of water masses. Thermohaline circulation controls the vertical distribution of temperature and salinity in oceans, and is responsible for vertical water movements that ventilate deep ocean water masses.