The last
100,000 years of Earth’s climate history show that the THC has flipped between
active and inactive states depending on the freshwater input in the Northern
Atlantic and the temperature of the Earth. In fact, it seems like the Atlantic
circulation switches between three distinctive modes of operation.
- warm mode: strong and active overturning circulation
- cold mode: weakened and slow overturning circulation, with deep water convection sites (the location where surface water is turned into deep water) moved south of the Arctic, somewhere north of Portugal. This leads to less cold and less dense deep water. Thus, it does not sink all the way to the bottom, but rather flows in the intermediate space. The deep current from Antarctica (AABW) can now flow all the way to the north
- Heinrich or “off” mode: the THC is fully shutdown. All deep water is coming from Antarctica and ocean mixing slows down. The result is an ocean with many stratified layers.
The first
to notice this phenomenon was Stommel in 1961. Since then, many other
scientists have accepted and expanded the hypothesis (Broecker et al., 1985;
Rahmstorf, 2002).
One
discovery was the hysteresis behavior of the Atlantic circulation. This means
that changes are not always gradual. Instead, there are moments when only a
very small forcing can lead to a big change in THC flow strength. This also
means that, after the flow strength has fallen to a minimum, an extremely large
backwards-forcing is needed to push the flow strength back to its normal flow
rate (Rahmstorf, 1995; Ganopolski & Rahmstorf, 2001).
Figure produced by the blog author
Using a
coupled climate model, Ganopolski & Rahmstorf (2001) showed that indeed the
flow of North Atlantic Deep Water (NADW) seems to follow the hysteresis loop
(see following figure). However, interestingly there seems to be a great
difference in the shape of the loop depending on glacial or warm period. During
a warm period (which we have today), the “fall” and “rise” of the hysteresis
loop are much steeper than during the ice ages. This implies that changes in
flow strength today may be much larger than during an ice age.
(a) Hysteresis reaction in the warm period; (b) Hysteresis reaction during the glacial period
Black lines: response for the high latitudes; red lines: response for the low latitudes
However, as
Hu et al. (2012) show, this might not be true. According to their study, the
hysteresis effect becomes much greater when the Bering Strait (the Pacific
inlet to the Arctic Ocean) is closed. This will only happen during ice ages
(unless the continents crash into each other), since then frozen ice sheets
will stop the flow. So maybe the hysteresis effect is not as pronounced in our
warm world today, after all. To be certain about the hysteresis effect today,
more research is needed in the future.
Some new
interesting discoveries have been made. As we know the flow strength of the THC
is strongly dependent upon the production of deep water in the Arctic. This
happens in two main spots: west of Greenland in the Norwegian Sea + surrounding
Seas, and east of Greenland in the Labrador Sea.
http://www.climate.unibe.ch/main/jobs/Master/naoc/schematic_SPG.jpg |
You might remember the North Atlantic
subpolar gyre from one of the earliest posts. The North Atlantic subpolar gyre
is a big counter-clockwise circulating mass of water. It is partially the
reason why the Gulf Stream is pushed from the eastern North American coastline
to western Europe and further past Iceland into the Greenland Sea.
Schulz et al. (2007); Jongma et al. (2007) |
Schulz etal. (2007) as well as Jongma et al. (2007) found out that deep water production
in the Labrador Sea is part of a big feedback-loop. If ice is melting in the
Arctic, a strong spinning subpolar gyre (SPG) will send a lot of freshwater to
the Labrador Sea, due to its, counterclockwise rotation. This leads to
over-freshening of the Labrador Sea and turns off local deep water production. This
in turn weakens the whole conveyor belt and also the subpolar gyre. The spin
becomes slow and weak and no more freshwater is imported into the Labrador Sea.
In turn the subtropical gyre (STG) is now much stronger and pushes high
salinity equator water preferably into the Labrador Sea, due to its clockwise
rotation. So slowly salinity is restored and deep water convection/production
resumes.
In contrast
to the upper findings, Thornalley et al. (2009) present evidence from deep sea
cores that the subpolar gyre may also buffer possible weakening in circulation
strength by transporting salts between the Labrador Sea and the Nordic Seas. So
if one spot reduces deep water production, the other may keep it up. However,
this is an ongoing field of research and more answers are expected in the
future.
To come
back to the hysteresis theory, there is one particular question scientists have
pondered about: how far is our current anthropogenic climate change pushing the
thermohaline circulation?
Will it never be strong enough to push the THC over
the edge (a)?
What if it can weaken the circulation? Will it be easy to bring
it back to present day strength(b)?
Or will it never be possible again to
regain today’s flow speed (c)?
How likely is the last/worst option?
Hysteresis response example from Stocker & Marchal (2000)
The
bifurcation problem (option c) is not as unlikely as you might think. Already
Stommel (1961) had spoken of a bifurication point, meaning a passing point of
difficult return. Later, Rahmstorf (1995) modeled the NADW (North Atlantic Deep
Water) flow response to freshwater input and found a possible “double loop
hysteresis” in the high latitudes.
Upper graph: hysteresis loop for high latitudes; lower graph: hysteresis loop for low latitudes
Again the question where are we now and where are we heading?
Then again, Stocker & Marchal (2000) remind us, that model results are just as good as the model that computed them. Many responses of the climate system can be well reconstructed with the current models. However, some problems, such as ice sheet dynamics and cloud cover evolution, remain unresolved (IPCC, 2013).
I want to end
this post with this very famous graph from the IPCC (2007):
Models can
be overestimating. But we have to always remember, that they can as much
underestimate reality. With arctic sea ice declining at such a rapid state…
what will happen to our ocean conveyor belt?
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