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Thermohaline circulation is density-driven overturning of the global ocean, powered by differences in temperature and salinity. In polar regions, cooling and sea ice formation make surface water dense enough to sink, forming North Atlantic Deep Water and Antarctic Bottom Water. These dense water masses spread through the deep ocean basins, eventually upwelling and returning as surface flow. This global overturning circulation redistributes heat, carbon, and nutrients on timescales of hundreds to thousands of years.
Trace the path of a water parcel through the global conveyor belt using maps showing deep water formation sites and surface return flows. Connect density-driven sinking to the T-S properties covered earlier.
Thermohaline circulation is the ocean's global overturning system — a slow, density-driven flow that moves water through the full depth of all ocean basins over timescales of hundreds to thousands of years. The name comes from its two driving variables: thermo (temperature) and haline (salinity), which together control seawater density. Unlike wind-driven surface currents that circulate water in shallow gyres, thermohaline circulation connects the surface to the deep ocean and redistributes heat, carbon, and nutrients on a planetary scale.
The circulation begins where surface water becomes dense enough to sink. This happens primarily in two regions: the North Atlantic, where warm surface water flowing north from the tropics cools dramatically as it loses heat to the atmosphere, and around Antarctica, where intense cold and sea ice formation concentrate brine, further increasing salinity. When surface water in these regions becomes denser than the water column beneath it, it sinks — forming North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). These dense water masses then slowly spread through the deep ocean basins, filling the world's deep ocean with cold, relatively oxygen-rich water.
As deep water spreads away from its formation sites, it eventually reaches regions where it gradually upwells back to the surface. This upwelling is slow and diffuse, occurring most actively around Antarctica (where strong winds drive surface divergence) and in the deep Pacific and Indian Oceans. At the surface, water warms, picks up CO₂ and nutrients from biological activity, and eventually joins a surface current that carries it back toward the next formation site — completing the loop. The entire circuit is sometimes called the global conveyor belt, though this label oversimplifies what is actually a three-dimensional, multi-branching flow.
The consequences of this circulation for climate are profound. The North Atlantic branch carries an enormous amount of heat northward, making Western Europe far warmer than its latitude would otherwise suggest. If this flow weakens — for example, due to freshwater input from melting Greenland ice diluting the salinity of North Atlantic surface water — regional cooling in Europe and parts of North America would follow. A common misconception is that this would cause a global ice age; in reality, it would produce significant regional disruption while global warming from greenhouse gases continued. The system's sensitivity to freshwater forcing makes it one of the potential tipping elements of the climate system.