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How do temperature and density differences drive ocean circulation?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

Ocean circulation is driven by horizontal and vertical differences in water density, which are primarily controlled by temperature and salinity—a process known as thermohaline circulation.

In high-latitude regions, intense cooling and the rejection of brine during ice formation create cold, salty water that is significantly denser than surrounding waters. This dense water sinks toward the seafloor and spreads throughout the global ocean basins, eventually upwelling to replace surface waters. Conversely, in areas where surface water becomes less dense due to warming or freshwater input, the sinking motion is inhibited, which can stabilize the water column.123

While wind-driven currents dominate the surface layer, thermohaline circulation operates much more slowly, extending to the seafloor to form a global system that transports heat and nutrients. Because seawater density increases as temperature decreases and salinity increases, these physical properties act as the primary engines for this deep-reaching, vertical movement of water.32