Thermohaline circulation
The thermohaline circulation (THC) is a term 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. 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 our planet.
The thermohaline circulation is sometimes called the ocean conveyor belt, the global conveyor belt, or, most commonly, the meridional overturning circulation (often abbreviated as MOC).
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 (although much less than surface speeds). In the deep ocean, however, the predominant driving force is differences in density and temperature. There is often confusion over the components of the circulation that are wind and density driven; this is partially addressed. [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.
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 both temperature and salinity. Warm seawater expands and is thus less dense than cooler seawater. Saltier water is more dense than fresher water because the dissolved salts fill interstices between water molecules, resulting in more mass per unit volume. Lighter water masses float over denser ones (just as a piece of wood or ice will float on water, see buoyancy). This is known as "stable stratification". When dense water masses are first formed, they are not stably stratified. In order to take up their most stable positions, water masses of different densities must flow, providing a driving force for deep currents.
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 in the Southern Ocean. In these polar regions, seawater at the surface of the ocean is intensively cooled by the wind; wind moving over the water also produces a great deal of evaporation. Evaporation removes only molecules of pure water, resulting in an increase in the salinity (saltiness) of the seawater left behind. The combination of these two wind-driven processes is known as evaporative cooling.
The formation of sea ice can also contribute to an increase in seawater salinity; saltier brine is left behind as the sea ice forms around it. Increasing salinity depresses the freezing temperature of seawater, so cold liquid brine is formed in inclusions within a honeycomb of ice. The brine progressively melts the ice just beneath it, eventually dripping out of the ice matrix and sinking. This process is known as brine exclusion.
The dense water masses formed by these processes flow downhill at the bottom of the ocean, 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 deep and bottom water masses.
Note that, unlike fresh water, saline water does not have a density maximum at 4 °C but gets denser as it cools all the way to its freezing point of approximately -1.8 °C.
In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Britain. It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait.
By contrast in the Weddell Sea off the coast 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
The North Atlantic Deep Water, formed by dense water sinking from the surface of the North Atlantic, is not a static mass of water but rather a slowly southward flowing current. The route of the deep water flow is through the Atlantic Basin around South Africa and into the Indian Ocean and on past Australia into the Pacific Ocean Basin. The vertical exchange of dense, sinking water with lighter water below it is known as overturning. Hence, a recent and popular name for the thermohaline circulation, emphasizing the vertical nature and pole-to-pole character of this kind of ocean circulation, is the meridional overturning circulation.
The deep water masses that participate in the MOC have chemical and isotopic ratio signatures and can be traced, their flow rate calculated, and their age determined.
Upwelling
- Main article: 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- and a number of authors have tried to use these tracers to infer where the upwelling occurs.
Wallace Broecker, using box models, has asserted that the bulk of deep upwelling occurs in the North Pacific, using as evidence the high values of silicon found in these waters. However, other investigators have not found such clear evidence. Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean, associated with the strong winds in the open latitudes between South America and Antarctica. While this picture is consistent with the global observational synthesis of William Schmitz at Woods Hole and with low observed values of diffusion, not all observational syntheses agree. Recent papers by Lynne Talley at the Scripps Institution of Oceanography and Bernadette Sloyan and Steven Rintoul in Australia suggest that a significant amount of dense deep water must be transformed to light water somewhere north of the Southern Ocean.
Impacts on global climate
The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions. Changes in the thermohaline circulation are thought to have significant impacts on the earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters are exposed to the surface, it may also play an important role in determining the concentration of carbon dioxide in the atmosphere. While it is often stated that the thermohaline circulation is the primary reason that Western Europe is so temperate, this is largely incorrect as Europe is warm mostly because it lies downwind of an ocean basin [2] . However, the thermohaline circulation does have a mild impact, warming Western Europe by about 2 °C relative to the similarly located west coast of Canada.
Large influxes of low density meltwater from the Greenland ice sheet is thought to have led to a disruption of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas.
For a discussion of the possibilities of changes to the thermohaline circulation under global warming, see shutdown of thermohaline circulation.
References
- ^ http://www.realclimate.org/index.php?p=159
- ^ Seager,R., 2005,'The Source of Europe's Mild Climate',American Scientist
- Apel, J. R., 1987, Principles of Ocean Physics, Academic Press, (ISBN 0-12-058866-8)
- Gnanadesikan, A., R. D. Slater, P. S. Swathi, and G. K. Vallis, 2005: The energetics of ocean heat transport. Journal of Climate, 18, 2604-2616.
- Knauss, J. A., 1996, Introduction to Physical Oceanography, Prentice Hall (ISBN 0-13-238155-9)
- Primeau, F., 2005, Characterizing transport between the surface mixed layer and the ocean interior with
a forward and adjoint global ocean transport model, Journal of Physical Oceanography,35, 545-564.
- Rahmstorf, S., 2006, Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier, Amsterdam.
- Rahmstorf, S., 2003, The concept of the thermohaline circulation. Nature, 421, 699.