Thermohaline circulation

The thermohaline circulation is a term (see also global conveyor belt) for the global density-driven circulation of the oceans. Derivation is from thermo- for heat and -haline for salt, which together determine the density of sea water. Surface currents (such as the Gulf stream) head polewards from the equatorial Atlantic Ocean, cooling all the while and eventually sinking at high latitudes. This dense water then flows downhill into the deep water basins, only resurfacing in the NE Pacific 1200 years later. 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 piggyback both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the ciculation has a large impact on the climate of our planet.

Table of contents

1 The System 1.1 Poleward surface currents in the Atlantic 1.2 Formation of the Deep Water Masses 1.3 Movement of Deep Water Masses 1.4 Upwelling 2 Impacts on Climate 3 References

The System

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. In this case, however, the predominant driving force is differences in density.

The density of ocean water is not globally homogeneous, but varies significantly and discretely. Sharply defined boundaries exist between water masses which form at the surface, and subsequently maintain their own identity within the ocean. They position themselves one above or below each other according to their density, which depends on the surface conditions under which they formed: lighter water masses float over denser ones (just as a piece of wood or ice will float on water). In order to take up their most stable positions they must flow, providing a driving force for deep currents.

Poleward surface currents in the Atlantic

Formation of the Deep Water Masses

The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean. Here the water is intensively cooled by the wind, then becomes salty as sea ice forms and excludes the salt fraction of the water. The increasing salinity pushes the freezing temperature of the brine down, so cold liquid brine is formed in inclusions within a honeycomb of ice. Being extremely dense, it slowly drips out of the ice matrix and sinks to the sea bottom. These deep water masses are so dense they flow downhill, like a stream within the surrounding less dense fluid, and fill up the basins of the polar seas. Just as river valleys direct streams and rivers on the continents, the bottom topography steers the bottom water masses.

In the Norwegian Sea wind cooling is predominant, and the sinking water mass (the North Atlantic Deep Water, or NADW) fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Britain. Flow into the Pacific, however is blocked. It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction.

By contrast in the Weddell Sea north of Antarctica near the edge of the ice pack, the effect of wind cooling is intensified by brine exclusion. The resulting Antarctic Bottom Water (AABW) sinks and flows north into the Atlantic Basin, but is so dense it actually underflows the NADW. Again, flow into the Pacific is blocked, this time by the Drake Passage between the Antarctic Peninsula and the southernmost tip of South America.

Movement of Deep Water Masses

-route of dw from atlantic to indian to pacific oceans -aging of the water masses and their chemical signatures

Upwelling

All these dense water masses sinking into the ocean basins displace the water above them, so that elsewhere water must be rising in order to maintain a balance. However, because this thermohaline upwelling is so widespread and diffuse, its speeds are very slow even compared to the movement of the bottom water masses. It is therefore fiendishly difficult to measure where upwelling occurs using current speeds, given all the other wind-driven processes going on in the surface ocean. Deep waters do however have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth. This signature can be found in surface waters in the North Pacific, indicating that this is where most upwelling happens.

Impacts on Climate

-redistribution of heat from equator to poles -meltwater disruption of dw formation (Younger Dryas)

References

Apel, JR. Principles of Ocean Physics

Knauss, JA. Introduction to Physical Oceanography