thermohaline circulation - Impacts on global climate

Marine circulation caused by differences in the temperature and salinity of sea water. These differences are caused by heating or cooling, evaporation or precipitation, and freezing and thawing, and result in density differences in surface sea water. An increase in salinity or a decrease in temperature produces an increase in density, and conversely a decrease in salinity or an increase in temperature produces a decrease in density. Though the density differences appear small (1·02–1·07 g/cm³), they produce a vertical circulation. When the density of surface water is greater than that of the water below it, it sinks, pushing less dense water aside, until it reaches a level where the water below is denser. Here it spreads laterally. The result is an ocean made up of layers of water with the most dense on the bottom and those of progressively lower densities lying above. The oceans for the most part are vertically stratified in this manner. The water layers having characteristic temperatures and salinities are known as water masses or water types.

The thermohaline circulation differs from the wind-driven surface water circulation pattern, which is arranged in latitudinal belts following climatic zones. The well-mixed surface layer and the thermocline zone are the zones which experience the greatest changes in physical properties. Below the base of the main ocean thermocline (about 1000 m/3250 ft), variations in physical properties tend to be much smaller. This deep thermohaline circulation is almost completely unconnected with the surface circulation except around Antarctica, where the Antarctic Circumpolar Current extends from the surface to the bottom, and forms the main link between the three major ocean basins. In fact, most deep water masses are formed at high latitudes, the bottom waters of all oceans coming from the Antarctic.

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. 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. 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. 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. 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). 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. 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 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. The resulting Antarctic Bottom Water (AABW) sinks and flows north into the Atlantic Basin, but is so dense it actually underflows the NADW.

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.

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

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.

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. 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. 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. 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 .

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.