How does gravity affect the movement of ocean water?

Gravity plays a fundamental role in the movement of ocean water by driving both vertical and horizontal pressure gradients, as well as influencing tidal phenomena.

Pressure Gradients and Currents

Gravity is a primary component of the hydrostatic equation, which describes the vertical pressure distribution in the ocean. Because gravity acts downward, it creates hydrostatic pressure that increases with depth. Horizontal variations in water density—caused by differences in temperature and salinity—result in horizontal pressure gradients along geopotential surfaces (surfaces perpendicular to the direction of gravitational acceleration). These horizontal pressure differences drive ocean currents, as water tends to move from areas of high pressure to low pressure.1

Density Currents

Gravity also drives density currents, often referred to as "dense overflows." When water becomes denser than its surroundings—due to cooling or brine rejection during ice formation—gravity pulls this water downslope along the continental shelf and slope. This movement is a critical mechanism for filling deep ocean basins, such as the formation of Antarctic Bottom Water.2

Tides

On a larger scale, the gravitational forces exerted by the Moon and the Sun cause cyclic deformations in the sea surface known as tides. These gravitational interactions create periodic vertical movements of the sea surface and alternating horizontal tidal currents.3