Thermohaline

Thermohaline circulation refers to the part of the ocean’s large-scale circulation that is driven by density differences caused by variations in temperature (thermo) and salinity (haline). It connects the surface ocean with the deep ocean and operates on long timescales, complementing wind-driven currents that shape coastal and mid‑latitude regions.

The key mechanism is density: colder water is more dense, and water with higher salinity is also more dense. In polar regions, the formation of cold, dense water leads to sinking that powers deep and bottom currents. This sinking draws water from the surface to depth and helps drive a global loop of interbasin transport. The result is a global overturning or “conveyor belt” of water that moves heat, nutrients, and dissolved gases between the surface and deep ocean.

Major components include deep waters formed in the North Atlantic and around Antarctica, as well as intermediate and bottom waters in other basins. Surface currents, winds, and evaporation/precipitation modify the surface density field, but the vertical exchange is governed by thermohaline contrasts.

Significance and variability: Thermohaline processes contribute to regional and global climate by redistributing heat and carbon. They are sensitive to freshwater input from ice melt and precipitation, which can alter salinity and potentially slow or reorganize the circulation. Ongoing observations and climate models study their behavior, responses to warming, and feedbacks with sea-ice, biogeochemistry, and atmospheric change.