How does ocean circulation move heat around Earth?

A coastal place can receive heat that sunlight first supplied to ocean water far away. Water absorbs and stores thermal energy; when currents carry that water elsewhere, they carry energy too. Heat can then pass between the ocean and the air above it. Ocean circulation is the connected movement of this water through currents, swirling eddies and mixing, not simply a flow drawn on a map.

Wind transfers momentum to the ocean surface and helps set water moving. Earth's rotation deflects large-scale moving flows, while continents and the shapes of ocean basins guide their routes. Some surface currents carry relatively warm water toward higher latitudes. Others return cooler water or bring deeper water upward, so different regions exchange both water and heat.

Temperature and dissolved salt also change seawater's density: its mass in a given volume. Cooling generally makes seawater denser, and adding salt does too. Where surface water becomes denser than water below it, it can sink and displace other water. This connects changes at the surface with movement at depth. The effect depends on temperature and salinity together; cold water is not automatically the densest water in every situation.

Deep and shallow flows are connected by mixing, winds and buoyancy effects over different distances and times. A conveyor-belt picture introduces the idea of a connected ocean, but real circulation has many branching and changing routes rather than one fixed pipe. Changes in those routes can change where heat is delivered or stored. The water's journey matters to climate because it changes where energy is available to warm the air and surroundings.