The term
thermohaline circulation (THC) refers to the hypothesis
[citation needed] of global
density-driven
circulation of the oceans. The phrase is derived from
thermo- referring to temperature and
-haline referring to salt content, factors which together determine the
density of sea water. Wind-driven surface currents (such as the
Gulf Stream) head polewards from the equatorial
Atlantic Ocean, cooling all the while 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 1600 years) upwell in the North Pacific (Primeau, 2005). Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's ocean 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.
The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyer, or the global conveyor belt. On occasion, it is mistaken to mean the meridional overturning circulation (often abbreviated as MOC), but is distinct from it in two ways. Firstly, the MOC only occurs around the meridian - hence the name - and so is confined to the Atlantic Ocean. Secondly, there is published data on the MOC's existence, whilst the thermohaline circulation remains a purely theoretical conjecture.
The movement of surface currents pushed by the wind is intuitive we have all seen wind ripples on the surface of a pond. Thus the deep ocean – devoid of wind – was assumed to be perfectly static by early oceanographers. However, modern instrumentation shows that current velocities in deep water masses can be significant (although much less than surface speeds).
In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature (the more saline the denser, and the colder the denser). There is often confusion over the components of the circulation that are wind and density driven[1]. Note that ocean currents due to tides are also significant in many places; most prominent in relatively shallow coastal areas, tidal currents can also be significant in the deep ocean.
