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How do differences in temperature and salinity create deep-ocean currents?

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World map showing major ocean current systems, with arrows indicating the direction and speed of warm and cold currents in the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. Warm currents are marked in red and cold currents in blue, with currents labeled by name and symbols denoting those that reverse direction seasonally. A legend explains current speed categories and color codes. — ocean current Major ocean current systems of the world.

Deep-ocean currents are primarily driven by thermohaline circulation, a process controlled by horizontal differences in temperature and salinity that determine the density of seawater.41

Seawater density increases as temperature decreases and salinity increases. In high-latitude regions, such as the North Atlantic and near Antarctica, intense atmospheric cooling and the rejection of brine during ice formation create exceptionally cold, salty, and dense water. Because this water is denser than the surrounding layers, it sinks toward the seafloor, forming deep and bottom water masses that spread throughout the global ocean.2314

Similarly, in marginal seas like the Mediterranean and Red Sea, high rates of evaporation increase salinity, creating dense water that overflows into the open ocean. As these dense water masses descend and spread, they displace existing water, driving a slow, global-scale vertical circulation that replaces deep water with surface water and vice versa.3124