The three modes of the Atlantic overturning circulation

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.

(Rahmstorf, 2002)

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

Ganopolski & Rahmstorf (2001)

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

Rahmstorf (1995)

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?

Next time… 

Downwelling and deep water formation – what drives the THC

As we have seen in previous posts, ocean water reaches higher densities the colder and saltier it gets. Thus, to find significant areas of downwelling, we have to look for places on the globe, where ocean water is made particularly cold and salty. This leads us to the poles.

Both the Arctic and Antarctic have the potential to cool water to minimal temperatures. In the Antarctic, waters under ice sheets may lose so much heat, that the process is referred to as super-cooling. Through ocean gyres [info box], water is brought to the surface and cooled via convection by releasing heat to the atmosphere. Due to its lower temperature, the water mass will increase its density and sink to the bottom, where it is pushed away from the creation center by following water masses. The conveyor belt is moving.

Simply cooling water down will not lead to particularly dense waters. Parallel to the temperature loss, salinity needs to be increased. This is mainly possible by taking away water, but leaving the salts behind. Hence, the left behind water mass will become more saline.

There are two main processes that will accomplish the above: evaporation and ice formation.

Through evaporation, water will be removed from the oceans and enters the atmosphere as vapor. Since most salt particles are too heavy, they will be left behind. Ice formation leads to a similar process. By freezing ocean water, fresh water is taken out of the water mass, while the salts stay behind. This process is referred to as brine rejection.

Deep water formation in the Antarctic: Antarctic Bottom Water (AABW)

There are several places around the Antarctic continent where deep water formation takes place. The most famous one is the Weddell Sea, where the Atlantic Ocean hits Antarctica. Deep water formation in Antarctica is mainly connected with heat loss and brine rejection. Large year round ice sheets cool the surrounding ocean water to up to minimum temperatures of -2.2°C and increase their salinity by constantly freezing more water. This leads to the AABW being the coldest and densest water mass on earth.

Deep water formation in the Arctic: North Atlantic Deep Water (NADW)

In the Arctic, most downwelling is happening in the Barents Sea, Greenland Sea and Labrador Sea. Here, convection and mixing are the two most important processes. As explained above, gyres transport water to the surface and cool it there. In difference to the Antarctic, the Arctic ice is purely sea ice with no underlying continental mass. Thus, many areas experience a great fluctuation in ice amount with no ice during summer and little during winter. The cold open oceans lead to extreme heat loss (no sea ice that protects the upper water layer) that rapidly cools down water masses. In a complicated mixing process, many different water masses with different densities form the NADW which leaves the Arctic to flow southwards as the Atlantics deep water flow. When it reaches Antarctica, it mixes with the Antarctic Circumpolar Current, which flows all around the South Pole and the Antarctic’s AABW. From there, the new water mass intrudes other ocean basins and connects the Atlantic with other world’s oceans.

You might have noticed that there is no particular process to enhance salinity in the Arctic. Under certain circumstances the mixing of all those different water masses may lead to higher salinity, but the most important process is actually happening long before the water reaches the Arctic: Evaporation of large amounts of water at the equator and the subtropics.

Due to the Hadley Cell and the Coriolis force [info box], these large amounts of water vapor are transported east across the Atlantic and across Middle America. The flat topography of Middle America allows the water masses to be exported straight into the Pacific, which means that the Atlantic loses large amounts of water which are not coming back (Richter & Xie, 2010). The only way to counterattack this water export is by importing fresh water through river outflows. However, looking at the Atlantic, only few large rivers (e.g. the Amazon) enter the Atlantic with significant freshwater inputs. When calculating the difference of input and output, we see that the Atlantic is losing more than it gains. Thus, the water masses flowing northwards become saltier. 

By the time they reach the Arctic deep water formation places, the salt content is high enough to form deep water merely by lowering temperature.

[All of the above named processes are well described in van Aken (2007) The OceanicThermohaline Circulation, An Introduction. Atmospheric and Oceanic SciencesLibrary, Vol. 39 or Talley et al. (2011) Descriptive Physical Oceanography: An Introduction, 6.Ed.]