Thermohaline circulation 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 -haline referring to salt
content, factors which together determine the density of sea water. Wind-driven surface
currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually
sinking at high latitudes. This dense water then flows into the ocean basins. While the
bulk of it upwells in the Southern Ocean, the oldest waters 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 and matter 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 conveyor, or the global conveyor belt.
On occasion, it is used to refer to the meridional overturning circulation. The term MOC, indeed,
is more accurate and well defined, as it is difficult to separate the part of the circulation
which is actually driven by temperature and salinity alone as opposed to other factors
such as the wind and tidal forces. Moreover, temperature and salinity gradients can also
lead to circulation effects that are not included in the MOC itself.
Overview
The movement of surface currents pushed by the wind is fairly intuitive. For example,
the wind easily produces 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 the deep ocean, the predominant driving force is differences in density, caused by
salinity and temperature variations. There is often confusion over the components of
the circulation that are wind and density driven. 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 denser 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. 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. The thermohaline circulation is mainly triggered
by the formation of deep water masses in the North Atlantic and the Southern Ocean caused
by differences in temperature and salinity of the water.
The great quantities of dense water sinking at polar ocean basin edges must be offset
by equal quantities of water rising elsewhere. Note that cold water in polar zones sink relatively
rapidly over a small area, while warm water in temperate and tropical zones rise more
gradually across a much larger area. It then slowly returns poleward near the surface to
repeat the cycle. The continual diffuse upwelling of deep water maintains the existence of the
permanent thermocline found everywhere at low and mid-latitudes. This slow upward movement
is estimated to be about 1 centimeter per day over most of the ocean. If this rise were
to stop, downward movement of heat would cause the thermocline to descend and would reduce
its steepness. Formation of 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. In the North Atlantic, seawater at the surface
of the ocean is intensely cooled by the wind. Wind moving over the water also produces a
great deal of evaporation, leading to a decrease in temperature, called evaporative cooling.
Evaporation removes only water molecules, resulting in an increase in the salinity of
the seawater left behind, and thus an increase in the density of the water mass. In the Norwegian
Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic
Deep Water, fills the basin and spills southwards through crevasses in the submarine sills that
connect Greenland, Iceland and Great 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.
In the Southern Ocean, strong katabatic winds blowing from the Antarctic continent onto
the ice shelves will blow the newly formed sea ice away, opening polynyas along the coast.
The ocean, no longer protected by sea ice, suffers a brutal and strong cooling. Meanwhile,
sea ice starts reforming, so the surface waters also get saltier, hence very dense. In fact,
the formation of sea ice contributes to an increase in surface seawater salinity; saltier
brine is left behind as the sea ice forms around it. Increasing salinity lowers the
freezing point 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 rejection.
The resulting Antarctic Bottom Water sinks and flows north and east, but is so dense
it actually underflows the NADW. AABW formed in the Weddell Sea will mainly fill the Atlantic
and Indian Basins, whereas the AABW formed in the Ross Sea will flow towards the Pacific
Ocean. 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, seawater 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.
Movement of deep water masses Formation and movement of the deep water masses
at the North Atlantic Ocean, creates sinking water masses that fill the basin and flows
very slowly into the deep abyssal plains of the Atlantic. This high latitude cooling and
the low latitude heating drives the movement of the deep water in a polar southward flow.
The deep water flows through the Antarctic Ocean Basin around South Africa where it is
split into two routes: one into the Indian Ocean and one past Australia into the Pacific.
At the Indian Ocean, some of the cold and salty water from the Atlantic — drawn by
the flow of warmer and fresher upper ocean water from the tropical Pacific — causes
a vertical exchange of dense, sinking water with lighter water above. It is known as overturning.
In the Pacific Ocean, the rest of the cold and salty water from the Atlantic undergoes
haline forcing, and becomes warmer and fresher more quickly.
The out-flowing undersea of cold and salty water makes the sea level of the Atlantic
slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher
than the Pacific. This generates a large but slow flow of warmer and fresher upper ocean
water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago to
replace the cold and salty Antarctic Bottom Water. This is also known as 'haline forcing'.
This warmer, fresher water from the Pacific flows up through the South Atlantic to Greenland,
where it cools off and undergoes evaporative cooling and sinks to the ocean floor, providing
a continuous thermohaline circulation. 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.
Quantitative estimation The deep water masses that participate in
the MOC have chemical, temperature and isotopic ratio signatures and can be traced, their
flow rate calculated, and their age determined. These include 231Pa / 230Th ratios.
Gulf Stream
The Gulf Stream, together with its northern extension towards Europe, the North Atlantic
Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of
Florida, and follows the eastern coastlines of the United States and Newfoundland before
crossing the Atlantic Ocean. The process of western intensification causes the Gulf Stream
to be a northward accelerating current off the east coast of North America. At about
40°0′N 30°0′W, it splits in two, with the northern stream crossing to northern Europe
and the southern stream recirculating off West Africa. The Gulf Stream influences the
climate of the east coast of North America from Florida to Newfoundland, and the west
coast of Europe. Although there has been recent debate, there is consensus that the climate
of Western Europe and Northern Europe is warmer than it would otherwise be due to the North
Atlantic drift, one of the branches from the tail of the Gulf Stream. It is part of the
North Atlantic Gyre. Its presence has led to the development of strong cyclones of all
types, both within the atmosphere and within the ocean. The Gulf Stream is also a significant
potential source of renewable power generation. Upwelling
All these dense water masses sinking into the ocean basins displace the older deep water
masses which were made less dense by ocean mixing. To maintain a balance water must be
rising elsewhere. 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 difficult to measure where upwelling occurs using current speeds, given all the
other wind-driven processes going on in the surface ocean. Deep waters have their own
chemical signature, formed from the breakdown of particulate matter falling into them over
the course of their long journey at depth. A number of scientists 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. 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 Stephen Rintoul in Australia suggest that a significant amount
of dense deep water must be transformed to light water somewhere north of the Southern
Ocean. Effects 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, although poleward heat transport
outside the tropics is considerably larger in the atmosphere than in the ocean. 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,
it has been suggested that this is largely incorrect, and that Europe is warm mostly
because it lies downwind of an ocean basin, and because of the effect of atmospheric waves
bringing warm air north from the subtropics. However, the underlying assumptions of this
particular analysis have likewise been challenged. Large influxes of low-density meltwater from
Lake Agassiz and deglaciation in North America are thought to have led to a shifting of deep
water formation and subsidence in the extreme North Atlantic and caused the climate period
in Europe known as the Younger Dryas. In 2005, British researchers noticed that
the net flow of the northern Gulf Stream had decreased by about 30% since 1957. Coincidentally,
scientists at Woods Hole had been measuring the freshening of the North Atlantic as Earth
becomes warmer. Their findings suggested that precipitation increases in the high northern
latitudes, and polar ice melts as a consequence. By flooding the northern seas with lots of
extra fresh water, global warming could, in theory, divert the Gulf Stream waters that
usually flow northward past England and Norway, and cause them to instead circulate toward
the equator. If this were to happen, Europe's climate would be seriously impacted.
See also Atlantic Multidecadal Oscillation
Climate change Contourite
Downwelling Halothermal circulation
Hydrothermal circulation Shutdown of thermohaline circulation
Upwelling References
Other Sources Apel, JR. Principles of Ocean Physics. Academic
Press. ISBN 0-12-058866-8. Gnanadesikan, A., R. D. Slater, P. S. Swathi,
and G. K. Vallis. "The energetics of ocean heat transport". Journal of Climate 18: 2604–16.
Bibcode:2005JCli...18.2604G. doi:10.1175/JCLI3436.1. Knauss, JA. Introduction to Physical Oceanography.
Prentice Hall. ISBN 0-13-238155-9. United Nations Environment Programme / GRID-Arendal,
2006, [2]. Potential Impact of Climate Change Rahmstorf, S. "Thermohaline Ocean Circulation".
In Elias, S. A. Encyclopedia of Quaternary Sciences. Elsevier Science. ISBN 0-444-52747-8.
External links Ocean Conveyor Belt
THOR FP7 projects http:www.eu-thor.eu investigates on the topic "Thermohaline overturning- at
risk?" and the predictability of changes of the THC. THOR is financed by the 7th Framework
Programme of the European Commission. Google Map of the Geography of the Thermohaline
Circulation of our Oceans