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… 

When could the Thermohaline circulation have shut down during the last 100,000 years?

Last time we found a record showing temperature differences from today over the last 100 000 years. Now let’s see whether we can find possible THC-shutdown incidences…

On the first blink, the temperature record just looks like a sequence of undefinable scribbles. However, we have to bear in mind that those scribbles show temperature dropping and rising again within less than 100 years! From roughly 70 000 to 100 000 years ago, the scribbles are rather boring and show no significantly extreme changes. But the time from 10 000 to 60 000 years ago shows a row of very extreme changes over only short periods of time. These could possibly give us insights into ocean circulation changes.

The first question: do we see those extreme temperature changes also in the Atlantic Ocean?

Yes we do! Look at this record found by Grootes et al.(1993) in the GISP2 Greenland ice core (blue line) and another one found by Sachs & Lehman (1999) in a subtropical North Atlantic deep sea sediment core (green line) for the last 60 000/30 000 years:

(Rahmstorf, 2002)

Looking at all those rapid climate change events more closely, scientists have found out that there are two distinct happenings which keep showing up in the record. They named one set Dansgaard-Oescher cycles (event 1-20) and Heinrich events (event H1-H5).

Daansgard-Oescher cycles are characterized as being high frequency climate oscillations (Maslin et al., 2002). The short warm phases appear in the ice core records as 5-10 degree warming phases within only a few decades. At first, cooling is happening gradually, then abrupt over less than 30 years (Rahmstorf, 2002). Both records show the D/O cycles, meaning that the rapid warming/cooling was not confined to the North Atlantic, but happened across the whole ocean. However in sediment records the cold phases are recorded, since substantial layers of ice rafted debris (IRD, see INFO BOX) show up in the record (Maslin et al., 2002). A study done by Voelker et al.(2002) shows that evidence actually exists throughout the globe making this a significant global climate event.

Here we have our first candidate. Could it be possible that changes in Atlantic thermohaline circulation caused these abrupt climate events?

The next question is: how?


Could you imagine how the THC can collapse? Post your ideas :)

( Info Box links: Bond et al. 1992; Bond & Lotti 1995; Alley & Macayeal 1994)

The 1 million "currency" question:

The one question everyone is currently worrying about in terms of global warming:

Could our release of greenhouse gases and the current increase in global mean temperatures lead to a shutdown of the THC and would this send the whole of Europe and North America into ice age like conditions?

 

(btw if you think this "freeze" is completely overexaturated.... check out the FUN FACT about Brinacles! ;) )

To investigate this question, we will take a paleoclimatic approach and look back in time to see whether the THC has shut down before, under which circumstances it does this, and what the consequences were for the North Atlantic region.

But then, where do we search? 1000 years ago? 100 000 years ago? 1 million? When the Earth was created??....

Let’s review what we know about the THC (see other posts):

-          It is sensitive to temperature and salt content (deep water production)

-          Heat is transported from south to north via currents

-          Currents may flow very fast

-          The mixing time for the whole world ocean is roughly 1000 years

-          We believe that changes called rapid climate changes are connected to ocean circulation

This gives us the hint that turning off the THC probably will happen on a rather short geological time scale. So luckily, we do not have to travel back to the Earth’s first birthday. However, 1000 years might be too short aswell, since one water drop needs this long to have gone through the whole conveyor belt system. So we expect to find something in the 100 000 year range!

Now we just need data! And if we look around the world, there are an uncountable number of archives that have recorded the last 100 000 years of climate! Especially important are ice cores! (for more information see SCIENCE FACT). Ice cores on Antarctica go back 800 000 years (Luthi et al.,2008)! Greenland ice does not reach back that far, but still enough to cover our period of interest (Svensson et al., 2008). The great thing about ice cores is that little bubbles of air have been trapped in the ice (Alley, 2000). With careful extraction, you can get an air sample from the 400 000 year old atmosphere! You gotta admit... that's pretty cool!

And this is what we are going to look at. Here you see "real" temperature values for the last 100 000 years:

(Ganopolski & Rahmstorf, 2001)

Let me know what you see/feel/think/notice!

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.]

What drives Ocean Circulation: The Thermohaline Circulation

We were interested in the factors that drive and control Broecker's ocean conveyor belt. The most influencial process is called...

The Thermohaline Circulation (THC):

What is that? If you've come across some latin terms you might have figured out that thermo obviously refers to temperature, while haline refers to salinity. Thus, you have just named the two most important factors that drive the ocean conveyor belt.

Here is how it works:

Density of salty water happens to be positively correlated with salt content and negatively correlated with temperature. This means that the density of cold and salty water is higher than that of warm and fresh water and the less dense water will swim ontop of the denser water.
In the oceans, this phenomenon leads to large water masses being "sorted" by their density, with coldest and saltiest waters on the bottom of the ocean and warmest, freshest waters at the surface (Broecker, 1997).

http://omp.gso.uri.edu/ompweb/doee/science/physical/cipatt1.htm

Are you the kind who doesn't like jumping into the salty ocean during your beach holidays??... well be happy you're not a diver ;)

The upper figure shows that under normal circumstances there is no mixing between the layers. However, imagine you change the density of one water parcel in the corner of the figure by making it colder, then the parcel would sink and push away the last water parcel in the bottom layer which is forced to rise. Due to the rise it heats up slightly and becomes less dense, rising even better. And just like that you end up with a cycle that gets your water layers to rotate. In principle, the ocean conveyor works just like that. At upwelling areas bottom deep water comes to the surface, while downwelling areas surface waters sink to the bottom (Broecker, 1997).

graph produced by the author


To fully understand the process, let's take a trip to the most presigious downwelling areas on Earth: ARCTICA and ANTARCTICA

wooooooo.... you'll need your mittens!

http://www.pinterest.com/tristess/aleut-inuit-pantheon-people-of-the-pacific-northwe/

What is Ocean Circulation and how does it work?

During your last read you have found out that Ocean Circulation may answer your question of how rapid climate changes can occur.In the next few sessions an important and famous scientist will help us understand how Ocean Circulation and Climate are interconnected.

Nothing in nature is static. Everything moves, forms and reforms constantly throughout the geological timescale. Constant movement is also found in the world’s oceans. Due to wind, rain and evaporation (e.g. Richter & Xie, 2010; Clark et al., 2002) water is being moved in, out and within each large mass of water. In certain location this movement becomes directed and forms large ocean currents flowing past landmasses and through continent openings. This whole system consisting of many directed flows of current is referred to as ocean circulation.

In 1982, a scientist by the name of W.S. Broecker published his first idea of what the global ocean circulation may be like. He thought of ocean circulation as a conveyor belt transporting water along a route through all world oceans and back to its original position. This means that oceans throughout the earth are interconnected. Thus, changing the climate in one place will automatically influence the whole Earth.

The Great Ocean Conveyor Belt, taken from W.S. Broecker's "The biggest chill" (1982)

However, as young critical scientists you have to ask Mr. Broecker one particular question to verify his hypothesis: what are the important factors that drive and influence the ocean conveyor belt?

If you have an idea, post your answer below.

Here is the Answer: Abrupt Climate Changes

Last posted question:

What is the difference between warming phases and cooling phases?

Here is the answer:

During cooling phases, the Earth tends to gradually cool down over a period of thousands of years. In contrast, warming phases happen over a few centuries! 

This discovery has put scientists infront of a difficult question: Slow climate changes are explainable with extraterrestrial forcing through the Milankovich cycles, but how do we explain those observed rapid climate changes?

When reconstructing climate, it is important to look not only at one component (eg the atmosphere) or one resource (eg ice cores), but many possible variables.
Thus, let’s have a look at a different climate record that highlights another component: coral reefs in the ocean:

Sea level fluctuation time series reconstructed from coral reef terraces, taken from Siddall et al. (2003)

When looking at past sea level stands we see almost identical fluctuation structures in the ocean compared to the fluctuations of temperature or greenhouse gases in the atmosphere.

Thus, mechanisms for changing climates may not only be found in the atmosphere, but also in the oceans.

And here is the big clue: Ocean Circulation is believed to be one possible mechanism that may trigger these otherwise unexplainable abrupt climate changes!

Why are we interested in ocean circulation? A short trip to the Earth's past.

Many people today are worried about the future of the earth when thinking of climate change. To most humans, climate and natural settings were considered the one stable concept in their otherwise unstable life. Hence, the thought of a 4˚C warmer Earth, as it is postulated by the IPCC and many newspapers often triggers communal fears of mass extinction that may even include the human race.

You may panic.

http://www.demonsinthedark.com/panicandalcohol.htm

However! Here is the interesting bit. During the last century, climate scientists have found evidence for long term climate changes throughout the earth’s history (e.g. Daansgard et al., 1982; Daansgard et al., 1993). Proxy data (eg by converting information found through biological and abiological indicators into climate variables) or even direct measurements (eg trapped air bubbles in ice cores) in geological records have shown that Earth has went through climates much more aggressive than the one we know.

-          Precambrian (600-3500 Ma): Snowball Earth: The whole of the earth’s surface was covered in ice (Hydeet al., 2000; Donnadieu et al., 2004)

-          Cretaceous (65-144 Ma): Greenhouse Earth: Ice did not exist. Fun fact: CO₂ pressure was above 1000 ppm vs today at 400 ppm (Barron et al., 1981; Barron,1983; Kuypers et al., 1999)

This shows that the Earth has a much greater climate span than we would have initially believed. Since our climate today includes ice on Earth (glaciers and permafrost) we are actually closer to the snowball earth climate than the Cretaceous version; meaning, we are stuck in an Icehouse Climate, while an ice free earth represents a Greenhouse Climate.

To get a more accurate understanding of our current climate situation, we should take a closer look at the recent past of our climate.

Vostok Ice core time series and Insolation, taken from Petit et al. (1999)

Petit et al. (1999) have managed to characterize climate for the last 400,000 years. We see that temperature (line b) has fluctuated strongly between cold glacial and warm interglacial periods. The Insolation curve (line e) suggests that those fluctuations are initiated by changes in solar energy on earth. Thanks to Milankovitch, we know today that climate over the last ice ages was indeed forced by three external variables (Eccentricity, Obliquity and Precession) with different oscillations (Zachos et al., 2001).

Now put yourself in the shoes of a climate change scientist and try to answer the following question using the graph above:

What is the difference between warming phases and cooling phases?

Post your answer if you like and see whether you were right next time J

Now what?

Ok now what? 

Have you ever wondered about the actual scientific consensus on the doom stories known about climate change and ocean circulation?

Well you have found the right place! Together with me and many other people, you will find out about the scientific base that lies underneath the ocean circulation theory.

Come by each week and learn about ocean circulation, the conveyor belt model, the threshold hypothesis, future projections and ways of doing climate research.