Atlantic Circular: The Gulf Stream's implications for climate (Part 3 of 3)

This is part three of a three-part series on Atlantic Circular. You can find part one here and part two here.

If the AMOC slows down

In the film “The Day After Tomorrow,” a climate catastrophe takes place in a matter of days: warming leads to rapid melting of ice, which stops circulation in the ocean, which in turn results in a sharp cooling.

The film plays on one of the theories of the formation of the so-called Dansgaard-Oeschger oscillations and individual Heinrich cold events against the background of these oscillations — rather sharp temperature changes during the last ice age. These events and fluctuations are visible both in cores from Greenland and in the bottom sediments of the subtropical Atlantic. Moreover, the climate changes were abrupt: the warm phases began with rapid warming — the maximum occurred in the Greenland region, which warmed up by 5–10 degrees over several decades — then a temperature plateau set in. Then a slow cooling began. Temperature changes were observed not only in the North Atlantic, but also in other regions, and in the South Atlantic, temperature changes occurred in antiphase!

Proxy data for the temperature of the subtropical Atlantic (green line, bottom sediments) and the North Atlantic (blue line, Greenland ice core data). The numbers show the warm Dansgaard-Oeschger events and the red squares show the Heinrich events. Stefan Rahmstorf/Nature, 2002

Having seen the nature of temperature changes, namely something similar to fluctuations (about 1500 years), scientists suggested the presence of stochastic resonance — amplification of a weak periodic signal with white noise. Important conditions for this are the fundamental nonlinearity of the system (and the climate system is such) and the presence of several stable states in it.

The idea of two stable positions of thermohaline circulation was expressed by Stommel and Broecker. The broker also put forward the idea of a “salinity oscillator”: the AMOC balances the export of fresh water from the Atlantic to the continents, its weakening leads to a weakening of this export and an increase in salinity, and an increase in salinity increases the circulation, and so on in a circle. These AMOC fluctuations affect ice sheets and sea ice in the Arctic. Their melting determines the shift of convection from high latitudes of the Atlantic (warm phase of the Dansgaard-Oeschger oscillations) to low latitudes (cold phase) — the so-called “warm” and “cold” AMOC regimes are formed.

At certain moments during the cold phase, extreme Heinrich events occurred — on the seabed, these events correspond to large-sized sedimentary rocks that could only be brought by icebergs. This allowed scientists to assume that the cover glaciers (most likely the Laurentian) grew to a critical size and then dumped part of the ice into the North Atlantic, which for a certain time “turned off” the AMOC. The North Atlantic became abnormally cold, while in Antarctica, on the contrary, it was abnormally warm.

Diagram of three AMOC modes (from top to bottom): warm, cold, and off. The red arrow shows the overturning of warm water in the North Atlantic, the blue arrow shows the deep Antarctic waters. Also diagrammatically depicts the rise of the ocean floor between Greenland and Scotland. Stefan Rahmstorf/Nature, 2002

However, the most recent studies (using more detailed paleo-data and more advanced climate models) turn the picture on its head. This AMOC first strengthened or weakened, which entailed changes in the area and mass of glaciers. A large body of work shows that the AMOC, depending on the concentration of greenhouse gases and the presence/absence of ice sheets, can be in only one of its states. Thus, with a high concentration of CO2 (as of now) and the absence of the Laurentian shield, only a warm AMOC regime is possible. In contrast, with low atmospheric CO2 concentrations (below 185 ppm) and the presence of the Laurentian Shield, only a cold or off AMOC mode is possible. The reasons for the transition between the cold and off modes are still being clarified — apparently, the slowdown of the AMOC was subsequently enhanced by the flow of freshwater from the Scandinavian ice sheet — but it is already clear that a large discharge of icebergs from the Laurentian shield occurred after a sharp stop of the AMOC and was not the cause, but a consequence of its stopping. However, it is still difficult to say whether this was the case in all the events of Heinrich in the history of the Earth.

The most interesting thing happens in conditions when ice sheets and CO2 concentrations are at average levels — it is at such moments that transitions from warm to cold phases and back are possible. Model calculations show that the causes of these transitions can be both changes in the mass of glaciers and changes in CO2 concentration. In particular, changes in the concentration of greenhouse gases led to a restructuring of the atmospheric circulation in the tropics and increased transport of moisture through Central America to the Pacific Ocean, which increased the salinity of waters in the Atlantic and strengthened the AMOC. The fluctuations of greenhouse gases in the atmosphere during ice ages were modulated by the AMOC itself, thus triggering its transitions from cold to warm phases.

However, all this applies to the conditions of ice ages, where the ocean level is low, the continents are covered with glaciers, and the concentration of CO2 in the atmosphere is low. In the modern climate, stopping the AMOC is extremely unlikely, although weakening is quite possible. How can this come back to haunt us against the backdrop of global warming?

Global warming vs. weakening of the AMOC

Modern climate models reproduce the global ocean conveyor quite well and confidently predict a weakening of the AMOC in the 21st century: if strong anthropogenic influence on the climate continues, it can reach 50 percent; in a mild scenario, it is unlikely to exceed 25 percent, but still will not go away. Models predict that the cold anomaly in the North Atlantic (the same warming hole) will persist in the coming decades due to weakening convection in the subpolar gyre (9 models out of 40 predict a fairly sharp cooling, the remaining 31 are more gradual). Will this affect the climate of Europe? To answer this question, it is necessary to isolate the effect of the AMOC weakening on air temperature.

In 1988, Manabe and Stouffer showed that in an ocean-atmosphere climate model, two stable states can form — with and without thermohaline circulation in the Atlantic (continuing the Stommel-Broecker hypothesis). Without circulation, the North Atlantic becomes 7–9 degrees colder. This cooling is also affecting Europe. Later experiments (1, 2, 3) tested the extent of cooling for a scenario of a markedly weakened (but not stopped) AMOC. It was 5–8 degrees Celsius.

The difference between the average annual surface air temperature in experiments with the AMOC turned off (top) or weakened (bottom) and the control experiment. S. Manabe et al. / Journal of Climate, 1988. L. C. Jackson et al. / Climate Dynamics, 2015

These scenarios look impressive, but there is one important “but”: the AMOC in these experiments was weakened by adding freshwater flow to the model. The results of the experiments were compared with control experiments in which the greenhouse effect corresponded to pre-industrial levels. But now the concentration of CO2 in the atmosphere is growing! So we need to conduct a reverse experiment, which scientists from the USA and France recently did.

They took a climate projection for the 21st century, taking into account anthropogenic influence, taking the most aggressive scenario — the Atlantic desalinated, the AMOC weakened by 30 percent. They compared this scenario with a situation in which the AMOC does not weaken during warming (for this, fresh water from the North Atlantic was removed from the model).

What is the result? The weakening of the AMOC means that warming due to global climate change will not be felt as strongly in Europe. The main effect of “non-warming” will manifest itself to the south of Greenland — in the area of ​​that same warming hole.

Change in average annual surface air temperature in the 21st century (in 2061–2080 compared to 1961–1980) with the expected weakening of the AMOC (top left) and with desalinization turned off and a strong AMOC (top right). The difference between the experiments is shown below. Wei Liu et al. / Science Advances, 2020

But a weakening of the AMOC, itself caused by warming, will not reverse this warming. You shouldn’t count on cooling in Europe, certainly not in the 21st century. Murmansk will not freeze either. Moreover, many new data suggest that the flow of heat into the Arctic may only increase.

Recently, a statistically significant increase in ocean kinetic energy was discovered since the early 1990s, leading to an acceleration of ocean circulation, and at greater depths. The main reason is an increase in wind in the surface layer (and, to a lesser extent, a change in its direction), especially in the tropics of the Southern Hemisphere of the Pacific Ocean. It is not yet clear how this strengthening will affect the global ocean conveyor and the AMOC.

Changes in wind work at the ocean surface (red line) and global ocean kinetic energy (blue line). Shijian Hu et al. / Science Advances, 2020

The atmosphere can also help: scientists looked at a large ensemble of modern models (from the glacial maximum to the quadrupling of CO2) and showed that the overall meridional heat flow from the equator to the poles changes little (except that at the glacial maximum, it was 4 percent more), however, which way it goes — in the atmosphere or the ocean — depends significantly on external conditions. When CO2 quadruples, the weakening of the AMOC will be more than compensated by the heat flux in the atmosphere.

The so-called Bjerknes compensation works: in the approximation of weak changes in the radiation balance at the upper boundary of the atmosphere, the climate system will continue to deliver heat from the overheated tropics to the cold poles in one way or another, which means that if one flow weakens (in the ocean or the atmosphere), then another will strengthen. Compensation by the atmosphere for the weakening of the ocean flow due to the AMOC has been shown in several modeling studies (1, 2).

However, as the greenhouse effect intensifies, the flow into the Arctic Ocean only intensifies. Thus, model experiments with different contents of greenhouse gases (from one-quarter to four times the concentration of CO2) show that heat transfer by the ocean to the Arctic increases with increasing CO2 concentration, mainly through the northeastern seas of the Atlantic. Scientists have shown that oceanic heat transfer is enhanced by wind action and heat transfer by surface currents and normal heat transfer, but the AMOC fades into the background. Perhaps this can be compared to a hydromassage bath: in one case, the bathtub is filled with cold water, and jets of very warm water flow from the sides, in the other, the jets are no longer so warm, but then all the rest of the water in the bath is no longer so cold.

And the water in this bath, that is, in the world’s oceans, becomes warmer due to anthropogenic activity. Humanity, increasing the concentration of greenhouse gases in the atmosphere, now lives in an era of imbalance of radiation fluxes at the upper boundary of the atmosphere: about 340 watts per square meter still reaches our planet, but about 339 watts go into space. As a result, in the earth’s climate system excess heat accumulates. Moreover, about 90 percent of excess heat goes into the ocean: every year about 9 zettajoules (1021 joules) are added here — this is about 15 times more than all the energy that humanity produces in a year. The results of observations and reanalyses show that the ocean is becoming warmer.

Trends in water temperature in the upper 2 km layer of the oceans in 1960–2019. Lijing Cheng et al. / Advances in Atmospheric Sciences, 2020

Warming and salinization in the upper kilometer layer have been occurring in the North Atlantic since at least the mid-20th century (but at depth, the water becomes colder and fresher, due to increased melting of Greenland ice and sea ice in the Arctic). Paleodata shows that ocean surface temperatures in the North Atlantic are now the warmest they have been in 3,000 years. The exception is the warming hole. But in the end, everything is not so simple with him.

Reconstruction of North Atlantic surface temperature at annual resolution (black), with red showing the 30-year average and gray showing the uncertainty range. Historical periods of regional and global cooling and warming are also indicated. Francois Lapointe et al. / PNAS, 2020

For example, in 2015, cooling in the North Atlantic was caused primarily by atmospheric processes that led to abnormal heat loss from the ocean. A recent study by European climatologists has shown that several players are involved in the formation of such cold anomalies: the cooling effect of clouds, the weakening of the influx of heat from low latitudes (precisely the weakening of the AMOC), and, most importantly, the increasing outflow of heat from the subpolar circulation to the polar latitudes, towards the Norwegian Sea. Scientists quite confidently attributed this increase in flow to the anthropogenic increase in the greenhouse effect.

In addition, in 2018, two independent groups of scientists showed (1, 2) that the climate response to the weakening of the AMOC, which is caused by internal variability and external forcing (increased greenhouse effect), differs significantly. In experiments without external forcing, the strengthening of the AMOC correlates well with the heat influx into the Arctic (due to heat convergence, that is, due to narrow warm jets) and the increase in temperature in Northern Europe. And in experiments with anthropogenic impact, a simultaneous weakening of the AMOC and an increase in heat influx into the Arctic is observed — due to the advection of heated surface waters, that is, due to the heating of the entire “bathtub”.

The influx of warm water into the Arctic is only growing — scientists talk about an increase in the influx of water into the Barents Sea by one sverdrup. The incoming water is about a degree warmer than before. The real “Atlanticization” of the Arctic is taking place.

Water temperature on the Kola meridian (average for a profile of 0–200 meters along the meridional section across the Barents Sea). Source: folk.uib.no/ngfhd

So, will Europe freeze? Modeling shows that strong cold anomalies in the warming hole area lead to a peculiar fixation of the position of jet streams and blocking anticyclones. Thus, on the contrary, abnormally strong heat waves occur over Europe. It is about the hot summer, as a consequence of the slowdown of the AMOC, that Stefan Rahmstorf, whose latest research stirred up the public in February 2021, speaks in his interview with the Zeit newspaper. And for the European climate to freeze, the entire thermohaline circulation must stop. Based on all modern ideas, including the whole galaxy of feedback that supports the AMOC, this is an extremely unlikely event.

The Gulf Stream is in no hurry to stop either. A few centuries ago, he helped white sailors export gold from America, but with heat, everything is not as simple as Matthew Morey thought. The Gulf Stream is just the tip of the iceberg: the climate of Europe is not simply dependent on the heat of the Gulf of Mexico, or sea currents in general. The warm winters of the Old World are one of the results of the entire climate system of our planet. So we shouldn’t count on the Gulf Stream’s help a second time: we will have to cope with the consequences of global warming ourselves.

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