Physiochemical properties of nanomaterials and its nanotoxicity.pptx
THERMOHALINE CIRCULATION
1.
2. The consequences of a warming Earth on its oceans are
not, of course, confined to their peripheries but chemistry,
circulation, and biology of the wide-open ocean will
change in numerous small and large ways and over short
and long periods of time.
Insights about what likely will happen to the open seas
are derived largely from computer simulation models,
bolstered by field research and satellite observations.
As explained before, it’s best to consider qualitative rather
than quantitative results, because model representations,
although complicated, are predicated on numerous
assumptions and simplifications of what is a deeply
complex reality.
Apart from thermohaline circulation , the Arctic’s sea-ice
cover, and the biological productivity and acidification of
the oceans,I will the discuss likely effects of a warming
Earth on thermohaline circulation.
3. Thermohaline circulation (THC) is a part of the
largescale 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.
The thermohaline circulation is sometimes called the
ocean conveyor belt, the great ocean conveyor, or the
global conveyor belt.
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.
4. Consider the voluminous inflow of freshwater to the
North Atlantic Ocean supplied by the rapid melting of
the Greenland ice sheet.
The area east and south of Greenland is a major site
of downwelling, which drives thermohaline circulation
as far away as the Indian and Pacific Oceans.
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 (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.
5. The exchange of warm surface water and cold deep water is accomplished by an
enormous conveyour belt system,driven largely by the sinking of water near Greenland.
6. As the Greenland ice sheetmelts and the Arctic Sea ice cover disappears, the
inflow of freshwater will inhibit, possibly even shut down, this global exchange of
surface and deep water.
7. As per the belief of early oceanographorers the wind
easily produces ripples on the surface of a
pond.Thus the deep ocean—devoid of wind—was
assumed to be perfectly static .
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 (increasing salinity and
lowering the temperature of a fluid both increase its
density).
Note that ocean currents due to tides are also
significant in many places.
8. 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 are known as
thermocline.
9. 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.
10. 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 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.
11. All these dense water masses sinking into the
ocean basins displace the older deep water
masses 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.
12. 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.
According to biological oceanographers, marine ecosystems
worldwide are undergoing “regime shifts,” meaning that
biogeographical boundaries that define the spatial extent of
particular organisms are changing in significant ways as
warmwater species displace cold-water species because of global
climate change.
In the Northern Hemisphere,for example, subtropical squid,
finfish, and jellyfish are now invading the less frigid water of the
temperate latitudes. Plankton abundances in the North Pacific,
the North Atlantic, and the North Sea are changing as well
