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Australia: The Land Where Time Began |
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Oceanic Convection Chimneys
A chimney is a deep, rotating, vertical cylinder of
water that transports cold water from the surface of the ocean to great
depths, as much as 2,500 m. This contradicts all that is believed about
the stability of the ocean, which is thought of as different masses of
water in horizontal layers, which are separated by vertical differences
in salinity and temperature from which they derive their different
densities. As a whole the ocean is stable: low-density water types above
water types of greater density, all the way to the bottom. In a very few
vital places surface water can be made to sink to 1.5 miles below the
surface. One of these places is in the Greenland Sea. It is thought that
such features should be unstable, but they can in fact be stable enough
to last for years. It is not known why. The
Greenland Sea convection site The Greenland Sea is an area of open ocean that is
of unique importance, which is situated close to Europe and is linked
intimately with the climate of Europe. Western Europe is 5-10oC
warmer than the average temperature for this latitude as a result of
ocean currents transporting heat into the Greenland Sea. If this heat
transport failed the climate of Western Europe and Britain would be more
like that of Labrador. In the Greenland Sea the region of sinking covers
an area of less than 1/1000 of the world ocean, yet is vital to the
circulation of the ocean, as it is only by this sinking (aka
ventilation) that a complete circulation, vertical and horizontal, can
be achieved, allowing the gases and nutrients that are dissolved in the
surface waters to be cycled back into the depths. By way of this
downwelling dissolved CO2 is also carried down to the depths,
and thereby has a big impact on the ability of the ocean to absorb a
significant fraction of the CO2 that is emitted to the
atmosphere every year. It has been suggested that some of the rapid
climate fluctuations that have occurred in the past, evidence of which
has been detected in sediment and ice cores, have been triggered by
convection changes in the past. Some models predict a decline in the
convection in the Greenland Sea, and also predict a consequent cooling
of the climate of Western Europe.
The
convection in the Greenland Sea is located at the centre of a cyclonic
gyre, i.e. a rotating circulation that is anticlockwise in the Northern
Hemisphere, that is bounded by a cold current to the west, the East
Greenland Current (EGC) which carries polar ice and water into the
system from the Arctic Basin; to the east by a warm current that flows
in a northwards direction, the West Spitzbergen Current, which is an
extension of the Gulf Stream; and by the Jan Mayen Current to the south,
a cold current offshoot of the East Greenland Current, which diverts to
the eastward from the main East Greenland Current at about 72o-73oN
by the presence of the Jan Mayen Ridge, a subsea mountain chain. The
main highway of water and heat exchange, between the Arctic Ocean and
rest of the world, is represented by the Greenland Sea, as the only deep
water entrance to the Arctic Ocean is Fram Strait which connects the
Greenland Sea to the rest of the world. Ice that is transported down
from the Arctic Ocean into the Greenland Sea melts on its way to the
south, therefore, when averaged over a year, the Greenland Sea is an ice
sink, and so a source of fresh water. The melting of the ice contributes
around 3,000 km3 per year to the Greenland Sea.
Local ice can form in the Greenland Sea itself in
winter, within the region of cold water which has been moved to the east
in the Jan Mayen Current. As the water comes from the East Greenland
Current it is already cold. It leaves behind its polar ice cover, which
continues on southwards down the Greenland coast. The cold, though ice
free water remains behind where it is exposed to further intense cooling
from a cold atmosphere in winter, in particular during climatic phases
when the prevailing winter winds come from the west, blowing off the ice
cap of Greenland. New sea ice is caused to form on this cold open water
by the intense cooling. As a consequence of the huge amount of wave
energy in the Greenland Sea in winter new ice cannot for a continuous
sheet. It follows instead the classic ‘frazil-pancake cycle’ in which it
initially forms as what Wadhams describes as a milk-of-magnesia
suspension of frazil ice crystals in the water column, then as small
cakes 1-5 m in diameter on which the edges are raised by their frequent
collisions. Waves cause the crystals in the frazil in suspension to be
squeezed together into clumps to form the cakes. The area of the sea
carrying the cold polar water in the Jan Mayen Current is filled by
these pancakes and the frazil they float in. A tongue-shaped protrusion,
the Odden Ice Tongue, which can cover an area of up to 250,000 km2
is formed by the new ice, which can be seen in satellite images. Sealers
were the first to discover and name this in the 19th century,
as the small cakes were used by harp seals to give birth to their pups
in spring. Norwegian sealers who followed the outer edge of the ice
tongue to collect the fur from the seal pups called the area Odden
(Norwegian for headland). Early whalers also knew the area, as slow
swimming right whales (Bowhead whale) were often found in the bay of
open water to the west of the Odden, Nordbukta (‘Northern Bight’).
As pancake ice forms most of the salt that in the
sea water as it freezes is expelled back into the ocean. Wadhams and his
research group cut up pancakes on the deck of their ship. They found
that the salinity of the thinner pancakes was about 10 ppt, compared to
the 35 ppt for ocean water, and the thicker pancakes can have salinity
levels as low as 4 ppt, so they had excluded almost 90 % of their salt.
The density of the surface water is increased by the addition of the
brine from the freezing pancakes, adding to the cooling effect that
destabilises the surface layer and causes this surface water to sink
(Wilkinson & Wadhams, 2003). In the Greenland Sea this effect is much
more powerful than in the Labrador Sea because of the impact of the
extra salt. A factor that is crucial to the large amount of convection
that occurs in the Greenland Sea, and therefore the maintenance of the
thermohaline circulation of the Atlantic is the rapid growth rate of the
pancake ice, and therefore the rapid increase of the brine content of
the surface water. Wadhams says the salt rejection by the pancake ice is
exciting; since pancakes grow rapidly making it a rapid process, and
this happens in just right spot to have a big impact on the stability of
the ocean. The extent of the Odden has been recorded nearly
every year since 1855 because it was so important to the Scandinavian
sealers, and this was earlier than the founding and reporting of the
Danish Meteorological Institute. In the past it formed in November
almost every winter and lasted until April or May, so that it can be
assumed that convection occurred throughout this period. Something has
happened to disturb this since the 1990s. The Odden failed to develop in
1994-5, and since 1998 to the present. This is a major change in the
nature of the Greenland Sea. This change is mainly the result of the
climate switching to a new phase in which the prevailing winds over the
area of the Odden came from the east and were warmer (this switch
between 2 systems of atmospheric circulation is known as the North
Atlantic Oscillation (NAO)). Though what is more serious is that when
the NAO switched back into its former phase, the Odden didn’t develop,
as the air temperatures over the sea had increased enough to prevent its
formation as a result of global warming. Chimneys in
the Ocean
In order to determine the effect these changes have
on the sinking of the surface water to great depths it is necessary to
discover how convection occurs; and another process has been found, not
all parts of which are understood,
Chimney formation. In 1970
the first chimneys were discovered in a warm part of the ocean, the Gulf
of Lion in the northwestern Mediterranean, during the Mediterranean
Ocean Circulation Experiment (MEDOC), a large oceanographic experiment
(MEDOC Group, 1970). It was found that during winter, at times when the
mistral, a northwesterly wind that is intensely cold, blows out to sea
from the Alpes Maritimes, the surface water was chilled by the cold air
to such a degree that the water sank, not in a random manner, but in the
form of small coherent rotating cylinders, called chimneys. Because the
winds would change direction, a chimney would last only a few days. In
the 1990s it began to be suspected that this was the mechanism by which
convection beneath the Odden worked. It has been found that the surface
water over a distance of 20 km in diameter forms a tight cylinder that
rotates like a solid body in a clockwise direction, which is opposite to
the direction of the Greenland Sea gyre as a whole, which moved the
water downwards and extended its influence to a depth of 2,500 m, in an
ocean that has a maximum depth of 3,500 m (Wadhams et al., 2002). The
combination of ice formation and cooling makes the surface water
enormously denser so that it sinks to a depth at which it reaches water
of similar density, at which point it stops sinking. As the cylinder
sinks it cuts through any layers of water whatever its temperature,
including a deep layer of warmer water which the sinking water punches
through. The chimney can be traced whether it is plotted by temperature,
salinity or density. In the case of a smaller chimney that is present
near a large chimney, the smaller chimney has not sunk as deep as the
larger one. It has been found by using an acoustic device (an
ADCP, or acoustic döppler current sampler) that the water in the
cylinder has a high degree of coherence (Budéus et
al., 2004). The water within
the cylinder is rotating at a speed that is proportional to the distance
from the centre – i.e., like a solid rotating mass. The rotation of this
cylinder of water is clockwise (anticyclonic in oceanography), which is
directly opposite to the currents in the Greenland Sea which have a
generally anticlockwise rotation, another reason the scientists were
amazed that the cylinder can form and persist. According to Wadhams the problem with using a small
research ship when searching for chimneys is that when they found one it
took a long time to map it adequately. Because of the bad weather in the
Greenland Sea in winter the researchers often had to stop work and sit
out the worst of the weather. The largest number of chimneys that were
found in a single survey was 2 (though in most winters only 1 was found,
which was located at 75oN 0oW), and that survey
was carried out in a quiet period in winter when it was believed the
stations were close enough to detect any chimney that was present
(Wadhams et
al., 2004).
Therefore, they suspect there were only 2 chimneys in the central
Greenland Sea that year. It was found when previous studies were
re-analysed that in the past there were many more chimneys: A series of
neutrally buoyant floats had been deployed by Jean-Claude Gascard in
1997, which are weighted to float at pre-arranged depths, and it was
found that at any one time 4 of them would be turning in tight circles
at depths of 240-530 m, which Wadhams et
al. realised later must have
meant that they were trapped in chimneys. Therefore in the 1990s there
were many more chimneys than in the 2000s. Wadhams suggests it is no
accident that there was also more ice. Wadhams and colleagues visited the centre of the
gyre during the Convection project for 3 winters (2001-3), while others
in the Alfred Wegener Institute visited the gyre in the intervening
summers. It was found that a chimney is very long lived. In the first
winter an open chimney was found to be in the same position in the
subsequent summer, though in the summer it was capped by 50 m of fresher
water that was less dense which covers the surface of the Greenland Sea
in summer as a result of the melt from sea ice and glaciers. The chimney
continued to exist as a submerged rotating cell beneath this cap of
freshwater. In the subsequent summer and winter the process was repeated
until the end of the project so that it could not be followed any
further. This is the longest-lived ocean chimney that has ever been
studied (Wadhams, 2004). Such longevity in such as small, tight feature
is not known of elsewhere in the ocean, where features the size of
eddies lose energy and momentum by friction, ‘running down’ after a few
days or weeks. It is not known what maintains a chimney in such a
compressed state that is rotating so rapidly. It is not known why it
doesn’t run down. It is also not known why a chimney stays in exactly
one place, the longest-lived chimney moved 10 km during 3 years, in
spite of no feature being present on the seabed to anchor it to one
location, such as often occurs in the case of ocean eddies. In many ways
chimneys have remained a mystery. According to Wadhams in spite of
having made these key discoveries with huge climatic implications, their
repeated bids to the Natural Environment Research Council (NERC) in the
UK for further support to study chimneys in the field were all
unsuccessful.
What is now known is that there are now fewer of
these structures, which coincides with loss of ice from the Odden, and
this convection decrease in the Greenland Sea will have a serious impact
on the world ocean. It has been suggested by models that to account for
the amount of deep water formation that is occurring between 6 and 12
chimneys need to form and dissipate each year. Wadhams says it is not
known where these chimneys are now, whether they still exist in spite of
the difficulty of forming without ice, whether the formation of deep
water is slowing or stopping, or is it just happening in a different
way, or in a different place.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |