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Multiple equilibria in the Atlantic meridional overturning circulation

Multiple equilibria in the Atlantic meridional overturning circulation is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Multiple equilibria in the Atlantic meridional overturning circulation rather than just read about it. In short: The Atlantic meridional overturning circulation (AMOC) is a large system of ocean currents, like a conveyor belt. It is driven by differences in temperature and salt content and it is an important component of the climate system.

Multiple equilibria in the Atlantic meridional overturning circulation — main illustration
Multiple equilibria in the Atlantic meridional overturning circulation — illustration

Key takeaways

  • Multiple equilibria in the Atlantic meridional overturning circulation belongs to earth science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Multiple equilibria in the Atlantic meridional overturning circulation to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Multiple equilibria in the Atlantic meridional overturning circulation from memory before moving on to harder problems.

Reference excerpt

The Atlantic meridional overturning circulation (AMOC) is a large system of ocean currents, like a conveyor belt. It is driven by differences in temperature and salt content and it is an important component of the climate system. However, the AMOC is not a static feature of global circulation. It is sensitive to changes in temperature, salinity and atmospheric forcings. Climate reconstructions from δ18O proxies from Greenland reveal an abrupt transition in global temperature about every 1470 years. These changes may be due to changes in ocean circulation, which suggests that there are two equilibria possible in the AMOC. Stommel made a two-box model in 1961 which showed two different states of the AMOC are possible on a single hemisphere. Stommel’s result with an ocean box model has initiated studies using three dimensional ocean circulation models, confirming the existence of multiple equilibria in the AMOC.

The AMOC

The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic. It is driven by differences in temperature and salt content. The present-day AMOC is mainly temperature-driven, which means that there is a strong AMOC characterized by sinking in the North. It is, in principle, also possible that upwelling can take place at low latitudes. This was studied by Stommel in 1961. The climate of the Northern hemisphere is influenced by the oceanic transport of heat and salt from the tropics to the sub-polar regions. The ocean releases heat to the atmosphere in the sub-polar Atlantic region. This Northward heat transport is responsible for the relatively warm climate in Northwest Europe. Changes in the strength of the AMOC are thought to have been responsible for significant changes in past climate. A collapse of the AMOC would have large consequences on the temperatures in the North-Atlantic region. It could lead to a reduction of air temperatures up to 10 °C.

Geological record of abrupt changes in the climate

The Cenozoic Era The Cenozoic Era covers the period from 65.5 Ma to present. It is the most recent of the three classic geological eras (Paleozoic, Mesozoic, Cenozoic). The earth is mostly characterized as a "Greenhouse world" during the early Cenozoic times, with no ice and high temperatures. The widespread occurrence of large glaciations started on Antarctica ~34 Ma in the Eocene-Oligocene transition (EOT). During this time, the world became an "Icehouse world" like we know it today, with ice sheets present in both poles simultaneously.

Dansgaard-Oeschger events There are also abrupt changes in the climate in the last glacial period. Willi Dansgaard analyzed the isotopic composition of ice cores from Camp Century in Greenland in 1972. He reported that the last glacial period showed more than 20 abrupt interstadials marked by very intense warming. Hans Oeschger reported 12 years later that the abrupt changes were accompanied by sudden increases in CO2 in the Greenland ice cores. These abrupt and dramatic changes in climate were from then on known as Dansgaard-Oeschger events (DO-events) and they occur approximately every 1470 years. Paleo-proxy records from δ18O proxies have been linked to the evidence of temperature fluctuations of this magnitude. The cause for these fluctuations is still uncertain, but recent research suggests that they are due to changes in ocean circulation. These changes could be induced by North Atlantic freshwater perturbations.

Stommel box model Several simple box models were used to study the changes in AMOC caused by for example changes in freshwater fluxes or salinity fluxes. Stommel was the first one to do so and devised a single-hemispheric box model in 1961 (Stommel box model). He made this model to explore the existence of stable responses to a constant forcing with either a temperate-driven or a salinity-driven AMOC. Stommel made use of a fundamental assumption that the strength of AMOC is linearly proportional to the equator–pole density difference. This assumption implies that AMOC is driven by surface thermohaline forcing.The model consists of two boxes. One box is at a high latitude (polar box) and the other one is at a low latitude (equatorial box). The high-latitude box has uniform temperature and salinity (T1,S1), this holds as well for the equatorial box (T2,S2). A linear equation of state is assumed:

ρ = ρ 0 − α ( T − T 0 ) + β ( S − S 0 ) {\displaystyle \rho =\rho _{0}-\alpha (T-T_{0})+\beta (S-S_{0})} , where ρ0, T0 and S0 are the reference density, temperature and salinity, respectively. The thermal and haline coefficients are indicated by α and β. As said before, the flow strength between the boxes is set by the density difference between the boxes:

ψ = k ( ρ 1 − ρ 2 ) {\displaystyle \psi =k(\rho _{1}-\rho _{2})} , where k is a hydraulic pumping constant. Each box exchanges heat with the atmosphere. The atmospheric temperatures (T1a,T2a) are fixed in this model. The evaporated water (η2 ≥ 0) in the equatorial box is precipitated, via the atmosphere, in the high-latitude box. The governing differential equations for the temperatures and salinities in the Stommel box model are:

d T 1 d t = | ψ | Δ T + λ T ( T 1 a − T 1 ) {\displaystyle {dT_{1} \over dt}=|\psi |\Delta T+\lambda _{T}(T_{1}^{a}-T_{1})}

… excerpt ends here. Continue reading the full article.

Illustrations

Multiple equilibria in the Atlantic meridional overturning circulation: The Dansgaard-Oeschger cycle. Dividing the 50-10 kyr before present period in boxes of 1470 years clearly shows the periodicity of the DO-cycle. The DO-events are numbered.
The Dansgaard-Oeschger cycle. Dividing the 50-10 kyr before present period in boxes of 1470 years clearly shows the periodicity of the DO-cycle. The DO-events are numbered.
Multiple equilibria in the Atlantic meridional overturning circulation: Schematic of the Stommel two-box model.
Schematic of the Stommel two-box model.
Multiple equilibria in the Atlantic meridional overturning circulation: Solution of the Stommel two-box model. Here the dimensionless overturning rate ψ ∗ {\displaystyle \psi ^{*}} is plotted against the dimensionless salinity forcing F ∗ {\displaystyle F^{*}} . For F ∗ {\displaystyle F^{*}} smaller than 0.25, the overturning is bistable. The solution with the solid line is stable and the solution with the dashed line is unstable.
Solution of the Stommel two-box model. Here the dimensionless overturning rate ψ ∗ {\displaystyle \psi ^{*}} is plotted against the dimensionless salinity forcing F ∗ {\displaystyle F^{*}} . For F ∗ {\displaystyle F^{*}} smaller than 0.25, the overturning is bistable. The solution with the solid line is stable and the solution with the dashed line is unstable.
Multiple equilibria in the Atlantic meridional overturning circulation: Hosing experiment on the Stommel two-box model. The filled dots are two possible equilibria for the same salinity forcing. An instantaneous surface forcing is applied to the red dot. This brings the system to the stable branch of the negative ψ ∗ {\displaystyle \psi ^{*}} . Next, the perturbation is removed. This puts the system back into the bi-stable regime of F ∗ {\displaystyle F^{*}} , but on the other branch (the blue dot). The red lines indicate the instantaneous perturbation. The blue lines indicate the removed perturbation.
Hosing experiment on the Stommel two-box model. The filled dots are two possible equilibria for the same salinity forcing. An instantaneous surface forcing is applied to the red dot. This brings the system to the stable branch of the negative ψ ∗ {\displaystyle \psi ^{*}} . Next, the perturbation is removed. This puts the system back into the bi-stable regime of F ∗ {\displaystyle F^{*}} , but on the other branch (the blue dot). The red lines indicate the instantaneous perturbation. The blue lines indicate the removed perturbation.

Worked examples

Example 1 — a first encounter with Multiple equilibria in the Atlantic meridional overturning circulation

Start with the simplest possible case. Write down what Multiple equilibria in the Atlantic meridional overturning circulation claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Multiple equilibria in the Atlantic meridional overturning circulation before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Multiple equilibria in the Atlantic meridional overturning circulation ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Multiple equilibria in the Atlantic meridional overturning circulation

In research
Multiple equilibria in the Atlantic meridional overturning circulation appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Multiple equilibria in the Atlantic meridional overturning circulation in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Multiple equilibria in the Atlantic meridional overturning circulation is common in secondary-school and first-year university syllabi. It links to neighbouring topics Currents of the Atlantic Ocean, so understanding it makes those chapters shorter.
In everyday life
Look for Multiple equilibria in the Atlantic meridional overturning circulation outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Multiple equilibria in the Atlantic meridional overturning circulation in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Multiple equilibria in the Atlantic meridional overturning circulation means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Multiple equilibria in the Atlantic meridional overturning circulation out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Multiple equilibria in the Atlantic meridional overturning circulation in simple terms?

The Atlantic meridional overturning circulation (AMOC) is a large system of ocean currents, like a conveyor belt. It is driven by differences in temperature and salt content and it is an important component of the climate system.

Why does Multiple equilibria in the Atlantic meridional overturning circulation matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Multiple equilibria in the Atlantic meridional overturning circulation?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Multiple equilibria in the Atlantic meridional overturning circulation.

Tags

  • Currents of the Atlantic Ocean

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