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 referring to
salt content, factors which together determine the density of sea water.
Wind-driven surface currents (such as the
Gulf Stream) travel polewards from the equatorial
Atlantic Ocean, cooling en route, and eventually sinking at high
latitudes (forming
North Atlantic Deep Water). This dense water then flows into the
ocean basins. While the bulk of it
upwells in the
Southern Ocean, the oldest waters (with a transit time of around 1000 years) 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. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the
climate of the Earth.