Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy and mass around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

The Atlantic meridional overturning circulation (AMOC) is the "main current system in the South and North Atlantic Oceans". As such, it is a component of Earth's oceanic circulation system and plays an important role in the climate system. The AMOC includes currents at the surface as well as at great depths in the Atlantic Ocean. These currents are driven by changes in the atmospheric weather as well as by changes in temperature and salinity. They collectively make up one half of the global thermohaline circulation that encompasses the flow of major ocean currents. The other half is the Southern Ocean overturning circulation.