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jai mitchell

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AMOC (Gulf Stream Current) Collapsing
« on: August 05, 2021, 07:18:43 PM »
Shocking, this has not been expected for a significant time.

News reports of a paper published today in Science.  https://www.nature.com/articles/s41558-021-01097-4

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation
Niklas Boers
Nature Climate Change volume 11, pages680–688 (2021)

Quote
Abstract
The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic Ocean basin. These results reveal spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.



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jai mitchell

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #1 on: August 05, 2021, 07:21:38 PM »
https://www.earthisland.org/journal/index.php/articles/entry/scientists-spot-warning-signs-of-gulf-stream-collapse/

Quote
“The signs of destabilization being visible already is something that I wouldn’t have expected and that I find scary,” said Niklas Boers, from the Potsdam Institute for Climate Impact Research in Germany, who did the research. “It’s something you just can’t [allow to] happen.”

Quote
The new research, published in the journal Nature Climate Change, is titled “Observation-based early-warning signals for a collapse of the AMOC.” Ice-core and other data from the last 100,000 years show the AMOC has two states: a fast, strong one, as seen over recent millennia, and a slow, weak one. These data show rising temperatures can make the AMOC switch abruptly between states over one to five decades.

Quote
David Thornalley, at University College London in the UK, whose work showed the AMOC is at its weakest point in 1,600 years, said: “These signs of decreasing stability are concerning. But we still don’t know if a collapse will occur, or how close we might be to it.”
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #2 on: August 05, 2021, 11:52:14 PM »
It seems to me that the most important finding of this research as indicated by the extract below is that a freshwater flux of sufficient size flows into the North Atlantic say from calving from Jakobshavn (see the attached image), melting of the Greenland ice sheet, melting Arctic sea ice, a freshwater flux from the Beaufort Gyre and/or freshwater from enhanced precipitation and river runoff.

Title: "Ocean Current System – That Influences Weather Patterns Globally – Seems To Be Approaching a Tipping Point"

https://scitechdaily.com/ocean-current-system-that-influences-weather-patterns-globally-seems-to-be-approaching-a-tipping-point/

Extract: "“Most evidence suggests that the recent AMOC weakening is caused directly by the warming of the northern Atlantic ocean. But according to our understanding, this would be unlikely to lead to an abrupt state transition. Stability loss that could result in such a transition would be expected following the inflow of substantial amounts of freshwater into the North Atlantic in response to melting of the Greenland ice sheet, melting Arctic sea ice and an overall enhanced precipitation and river runoff,” Boers explains."
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #3 on: August 06, 2021, 12:32:20 AM »
See also:

Boers, Niklas and Martin Rypdal (May 25, 2021), "Critical slowing down suggests that the western Greenland Ice Sheet is close to a tipping point", PNAS, 118, (21), e2024192118; https://doi.org/10.1073/pnas.2024192118

https://www.pnas.org/content/118/21/e2024192118

Significance
It has been suggested that, in response to anthropogenic global warming, the Greenland Ice Sheet may reach a tipping point beyond which its current configuration would become unstable. A crucial nonlinear mechanism for the existence of this tipping point is the positive melt-elevation feedback: Melting reduces ice sheet height, exposing the ice sheet surface to warmer temperatures, which further accelerates melting. We reveal early-warning signals for a forthcoming critical transition from ice-core-derived height reconstructions and infer that the western Greenland Ice Sheet has been losing stability in response to rising temperatures. We show that the melt-elevation feedback is likely to be responsible for the observed destabilization. Our results suggest substantially enhanced melting in the near future.

Abstract
The Greenland Ice Sheet (GrIS) is a potentially unstable component of the Earth system and may exhibit a critical transition under ongoing global warming. Mass reductions of the GrIS have substantial impacts on global sea level and the speed of the Atlantic Meridional Overturning Circulation, due to the additional freshwater caused by increased meltwater runoff into the northern Atlantic. The stability of the GrIS depends crucially on the positive melt-elevation feedback (MEF), by which melt rates increase as the overall ice sheet height decreases under rising temperatures. Melting rates across Greenland have accelerated nonlinearly in recent decades, and models predict a critical temperature threshold beyond which the current ice sheet state is not maintainable. Here, we investigate long-term melt rate and ice sheet height reconstructions from the central-western GrIS in combination with model simulations to quantify the stability of this part of the GrIS. We reveal significant early-warning signals (EWS) indicating that the central-western GrIS is close to a critical transition. By relating the statistical EWS to underlying physical processes, our results suggest that the MEF plays a dominant role in the observed, ongoing destabilization of the central-western GrIS. Our results suggest substantial further GrIS mass loss in the near future and call for urgent, observation-constrained stability assessments of other parts of the GrIS.

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Général de GuerreLasse

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #4 on: August 06, 2021, 12:39:43 AM »
This is all incredibly worrying. The question is what a scientist has in mind when he says: near-term or near future?    ???
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kassy

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #5 on: August 06, 2021, 08:50:04 AM »
Quote
These data show rising temperatures can make the AMOC switch abruptly between states over one to five decades.

I guess that is the time scale.

This is another hint that 1C over reference temperatures was actually the boundary for dangerous climate change.
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hst_319

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #6 on: August 06, 2021, 08:50:54 AM »
This is all incredibly worrying. The question is what a scientist has in mind when he says: near-term or near future?    ???

In the guardian article published based on the report there is some more elaboration on that.
The sad part seems to be that we don't have a clue...

https://www.theguardian.com/environment/2021/aug/05/climate-crisis-scientists-spot-warning-signs-of-gulf-stream-collapse

Quote
Levke Caesar, at Maynooth University in Ireland, who was not involved in the research, said: “The study method cannot give us an exact timing of a possible collapse, but the analysis presents evidence that the AMOC has already lost stability, which I take as a warning that we might be closer to an AMOC tipping than we think.”

David Thornalley, at University College London in the UK, whose work showed the AMOC is at its weakest point in 1,600 years, said: “These signs of decreasing stability are concerning. But we still don’t know if a collapse will occur, or how close we might be to it.”

blu_ice

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #7 on: August 06, 2021, 09:39:42 AM »
How soon would we know of AMOC state change in the North Atlantic? An immediate drop in satelliteborne SST measurements, or an unusually cold winter in N Europe?

Richard Rathbone

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #8 on: August 06, 2021, 12:15:53 PM »
How soon would we know of AMOC state change in the North Atlantic? An immediate drop in satelliteborne SST measurements, or an unusually cold winter in N Europe?

There are a couple of arrays of instruments directly measuring it. Some details and links if you want to look further into it in this carbonbrief article from last year.

https://www.carbonbrief.org/guest-post-could-the-atlantic-overturning-circulation-shut-down

Quote
Results have already changed our understanding of how the AMOC varies in time: for example, an unexpected dip in the AMOC – observed in Autumn 2009 – is thought to have played a role in the unusually cold European winters of 2009-10 and 2010-11.

The tricky part is knowing what is a fluctuation that will soon reverse and what is one that there's no coming back from. I'd expect it to be obvious within a year if it happened, but I also expect there'll be many false alarms from people trying to be the first to spot its about to happen in the years/decades its close.


Alexander555

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #9 on: August 06, 2021, 06:31:51 PM »
Where would it have the biggest impact ? Heat builds up near the equator. So if the gulfstream moves slower in the Gulf of Mexico. The water has more time to warm.  There will be much more heat moving east of the rockies. And if it moves slower to Europe, it starts with a higher heat mass.

jai mitchell

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #10 on: August 06, 2021, 06:51:05 PM »
Where would it have the biggest impact ? Heat builds up near the equator. So if the gulfstream moves slower in the Gulf of Mexico. The water has more time to warm.  There will be much more heat moving east of the rockies. And if it moves slower to Europe, it starts with a higher heat mass.

If you double the thermal gradient used to drive a heat engine, the rate of power produced increases by 8-fold.

Read: https://www.bloomsbury.com/us/storms-of-my-grandchildren-9781608195022
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TeaPotty

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #11 on: August 06, 2021, 07:58:22 PM »
Shocking, this has not been expected for a significant time.

News reports of a paper published today in Science.  https://www.nature.com/articles/s41558-021-01097-4

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation
Niklas Boers
Nature Climate Change volume 11, pages680–688 (2021)

Quote
Abstract
The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic Ocean basin. These results reveal spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.

Full Article:
Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation
https://www.nature.com/articles/s41558-021-01097-4.epdf?sharing_token=qWMQcnRcVRZQmZ_yahYbd9RgN0jAjWel9jnR3ZoTv0ODQw4Na6S4LwvIIwjZ_S3NdBoG6pi8c5NBfIwoUKp1VK_OHHszXMnB3OMoyz8L8emOhG-hoDsJyn1YMubz_IampYbIRg_8P9vjnfIPPzRQwm6m9BfwEGfoLu0JsB4E2trSfyu4r947mOz1oZQlyxQxZLxaMkEINR4Wt7XEIrPrRkahci-lKgCSTZahFzlH7wM%3D
« Last Edit: August 06, 2021, 08:17:57 PM by TeaPotty »

Alexander555

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #12 on: August 06, 2021, 09:46:34 PM »
What's the driving power behind the Gulf Stream ?

Alexander555

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #13 on: August 06, 2021, 10:16:05 PM »
I always had the idea that the warm water flowed on top of the cold water. And basically pushed it to the deep sea. And that this downward motion was the driver behind the gulf stream. But the warm water from the gulf is more salty( does that makes it more heavy ?) than the water in the arctic sea. And the other hand warm water weights less as cold water. What changed is plenty more fresh water in the arctic, from ice melt. And some more in the atlantic from snow melt in northern america the last years. Or would the north atlantic water become warmer ? As the arctic warms faster as the rest. Maybe that makes it more defficult to create that downward motion.

Alexander555

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #14 on: August 06, 2021, 10:36:58 PM »
Or maybe it's that warm flow of salty water that cools down with the salt in it. And because the extra salt it's heavier, and is pushed down by the less salty north atlantic water.  The flow is like 100 times all rivers on the planet.

kassy

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #15 on: August 07, 2021, 12:13:10 PM »
The Gulf Stream, together with its northern extension the North Atlantic Drift, is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and stretches to the tip of Florida and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean as the North Atlantic Current. The process of western intensification causes the Gulf Stream to be a northwards accelerating current off the east coast of North America. At about 40°0′N 30°0′W, it splits in two, with the northern stream, the North Atlantic Drift, crossing to Northern Europe and the southern stream, the Canary Current, recirculating off West Africa.
...
The Gulf Stream proper is a western-intensified current, driven largely by wind stress

https://en.wikipedia.org/wiki/Gulf_Stream

The Atlantic meridional overturning circulation (AMOC) is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers
...
AMOC consists of upper and lower cells. The upper cell consists of northward surface flow as well as southward return flow of North Atlantic Deep Water (NADW). The lower cell represents northward flow of dense Antarctic Bottom Water (AABW) – this bathes the abyssal ocean.[
...
The net northward heat transport in the Atlantic is unique among global oceans, and is responsible for the relative warmth of the Northern Hemisphere.[1] AMOC carries up to 25% of the northward global atmosphere-ocean heat transport in the northern hemisphere.[4] This is generally thought to ameliorate the climate of Northwest Europe, although this effect is the subject of debate.[5][6][7]

As well as acting as a heat pump and high-latitude heat sink,[8][9] AMOC is the largest carbon sink in the Northern Hemisphere, sequestering ∼0.7 PgC/year.[10] This sequestration has significant implications for evolution of anthropogenic global warming – especially with respect to the recent and projected future decline in AMOC vigour.

https://en.wikipedia.org/wiki/Atlantic_meridional_overturning_circulation
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Jim Hunt

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #16 on: August 07, 2021, 02:14:50 PM »
Carl Wunsch's 2004 letter to Nature on the difference between the Gulf Stream and the AMOC:

https://www.nature.com/articles/428601c

Quote
European readers should be reassured that the Gulf Stream's existence is a consequence of the large-scale wind system over the North Atlantic Ocean, and of the nature of fluid motion on a rotating planet. The only way to produce an ocean circulation without a Gulf Stream is either to turn off the wind system, or to stop the Earth's rotation, or both.

Real questions exist about conceivable changes in the ocean circulation and its climate consequences. However, such discussions are not helped by hyperbole and alarmism. The occurrence of a climate state without the Gulf Stream any time soon — within tens of millions of years — has a probability of little more than zero.

See also this 2021 note: http://ocean.mit.edu/~cwunsch/papersonline/amoc_shutdown_2.pdf

Quote
A deprecated device in ancient Greek drama is labelled as a deus ex machina or ‘god from a box’ (Oxford English Dictionary), a theatrical device in which a god emerges from a container at the end of the a play resolving all plot and human difficulties. In climate and paleoclimate, the idea of a shutdown in the so-called Atlantic Meridional Overturning Circulation (AMOC) is meant to explain many observations, past, present, and future, and to describe the impact on the wider observational system. An arbitrary, but recent, example is Stocker (2021)
« Last Edit: August 07, 2021, 02:34:46 PM by Jim Hunt »
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Richard Rathbone

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #17 on: August 07, 2021, 06:13:36 PM »
Thats just a parochial American complaining Europeans are parochial. "Gulf Stream" in common parlance means one thing in the US and another in Europe and he's deliberately confusing the two in order to bait Europeans. Same with translating machina as box. More bait.

There'll still be a current that could be described as a "Gulf Stream" if overturning in the Nordic and Labrador Sea stops. It won't go to the same place. For someone in Florida or Morocco, it might well not look all that different to today, but for someone in NE America or NW Europe it would look like a collapse.

There are genuine arguments about just how stable the overturning is, but this quote from Wunsch is misinformation concocted by using "Gulf Stream" in two separate ways in the same paragraph and hoping that the reader won't notice.

European readers should not be reassured that there will still be an eastwards flowing current. If it heads for Morocco rather than Norway its going to make a big difference to them, even if it doesn't to someone in Florida. The North Atlantic is unusual. The equivalents to the Gulf Stream elsewhere don't head north. Its incumbent on someone that thinks the Gulf Stream has to head north to use serious arguments not disingenuous rhetoric to support their position.
« Last Edit: August 07, 2021, 07:11:46 PM by Richard Rathbone »

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #18 on: August 07, 2021, 08:11:11 PM »
https://www.realclimate.org/index.php/archives/2005/05/gulf-stream-slowdown/

seems to endorse Wunsch's the wind driven gulf stream as separate from AMOC/North Atlantic drift which has historically and is modelled to slow down.

If it cancels out part of GHG driven global warming, will that make NW Europe into the problematic deniers of GW?

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #19 on: August 07, 2021, 09:48:00 PM »
Reposting my question from the melting season thread: My question is if the AMOC shuts down, what impacts does that have on melting and even “Atlantification” of the Arctic?

Richard Rathbone

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #20 on: August 07, 2021, 11:50:12 PM »
Realclimate is fine, but that quote from Wunsch is pretending that the North Atlantic Drift will continue because the rest of the Gulf Stream is still in a similar place.

It would change the incentives. People that are having problems because they've just got a whole lot colder aren't going to be in favor of cooling the world down when their bit of it isn't expected to warm back up again for many centuries. I'd expect pretty drastic geoengineering to appear on the agenda. Its a pump that will take a lot of priming.

...

I'd expect Arctification of the Atlantic rather than Atlantification of the Arctic.

https://en.wikipedia.org/wiki/Younger_Dryas - this sort of thing but all the extra CO2 in the atmosphere now will mean there's a warming effect on top of the temperatures then.

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #21 on: August 08, 2021, 07:20:41 AM »
"The AMOC will very likely weaken over the 21st century (high confidence), although a collapse is very unlikely (medium confidence). Nevertheless, a substantial weakening of the AMOC remains a physically plausible scenario."

https://www.ipcc.ch/srocc/chapter/chapter-6/

Jim Hunt

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #22 on: August 08, 2021, 11:24:12 AM »
Nevertheless, a substantial weakening of the AMOC remains a physically plausible scenario.

The IPCC SROCC also states that:

Quote
The heat transported by the buoyancy-loss driven AMOC, in particular, contributes to the relatively clement climate of northern Europe and the north Atlantic Basin as a whole, although the wind-driven ocean gyres also contribute to the meridional ocean heat transport (see the review by Buckley and Marshall (2015)). As a result, there is a concern that significant changes in ocean circulation could lead to localised climate changes that are much larger than the global mean. Projected and observed changes in the AMOC and the rates of formation of deep water-masses in the north Atlantic are discussed in Chapter 6.7.1, along with the possibility of abrupt or enduring changes resulting from forcing by Greenlandic meltwater. A significant reduction in AMOC would, in turn, modestly weaken the Gulf Stream transport, which also has a substantial wind driven component (Frajka-Williams et al. 2016). Most aspects of the large-scale wind-driven ocean circulation are very likely to be qualitatively similar to the circulation in the present day, with only modest changes in transports and current location.

All being well we'll discover very soon what AR6 has to say on the subject!

https://forum.arctic-sea-ice.net/index.php/topic,3603.0.html

The SROCC also mentions Carl Wunsch a few times. This for example:

"The Global General Circulation of the Ocean Estimated by the ECCO-Consortium"

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #23 on: August 08, 2021, 11:43:59 AM »
Thats just a parochial American complaining Europeans are parochial.

At least it seems to have generated some debate! I found it curious that at first glance (and unlike the SROCC!) the Wiki article on the "Gulf Stream" doesn't seem to reference Wunsch anywhere.

Quote
Its incumbent on someone that thinks the Gulf Stream has to head north to use serious arguments not disingenuous rhetoric to support their position.

Have you previously read any of Wunsch's work? If not see one or more of the links above. Here's another one, quoted with apparent approval by Stefan Rahmstorf:

https://www.nature.com/articles/35015639

Quote
As formulated in almost all models of the Earth's climate, both theoretical and numerical, the dense water sinking in the meridional overturning circulation at high latitudes then flows, close to the ocean bottom, throughout the world, returning to the surface by a uniform upwelling through the ‘interior’ ocean. Under the simple assumption that a uniform upwelling of cold water is balanced by a uniform downward mixing of warmer water throughout the water column, a steady state is achieved. Almost all numerical models of the ocean and climate systems represent this process through spatially constant vertical ‘eddy’-mixing coefficients, as do the textbook theories.

It has become evident, however, that the actual circulation is much more subtle and interesting.


P.S. An apparent quote from Niklas Boers:

https://www.tipes.dk/ocean-current-system-seems-to-be-approaching-a-tipping-point/

Quote
Freshwater inflow and especially Greenland meltwater runoff has indeed accelerated in the last decades. However, although the first sign of regional destabilization of the Greenland Ice sheet has been detected, recent Greenland runoff should not be sufficient for destabilizing the AMOC.

To understand this in-depth we need to find ways to improve the representation of the AMOC and polar ice sheets in comprehensive Earth system models and to better constrain their projections. I hope that the results presented here will help with that!

« Last Edit: August 08, 2021, 12:34:56 PM by Jim Hunt »
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Richard Rathbone

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #24 on: August 08, 2021, 03:03:37 PM »

P.S. An apparent quote from Niklas Boers:

https://www.tipes.dk/ocean-current-system-seems-to-be-approaching-a-tipping-point/

Quote
Freshwater inflow and especially Greenland meltwater runoff has indeed accelerated in the last decades. However, although the first sign of regional destabilization of the Greenland Ice sheet has been detected, recent Greenland runoff should not be sufficient for destabilizing the AMOC.

To understand this in-depth we need to find ways to improve the representation of the AMOC and polar ice sheets in comprehensive Earth system models and to better constrain their projections. I hope that the results presented here will help with that!

Thats been the general opinion for a couple of decades. Paleo says it happens, but models say it takes Aggasiz levels of meltwater and the ice that is still left can't provide it. But there's still plenty of room for improvement in the models, which may lead to that opinion changing.

Hansen snipes at ocean modelling too, but unlike Wunsch, he does it in the context of papers where he is also showing how he can do it better. Wunsch ought to at the very least to put a number on what he considers the thermohaline driving force to be, but instead he relies on rhetorical sophistry to imply to the reader that its too small to be relevant.

e.g. this quote from the same reference you quoted.
Quote
(The terminological problem with ‘thermohaline’ circulation arises because, for example, half the heat transport in the North Pacific Ocean is in the wind-driven upper circulation4.)

Its the driving force, not the transport thats important. He's using an irrelevant observation to make the reader think his argument is stronger than it actually is. The comparison that could actually support his argument is one between driving forces, he doesn't actually give an estimate for that anywhere and is distracting the reader away from noticing this omission by a rhetorical device.

He's also quoting for the North Pacific, when the relevant ocean is the North Atlantic. Did you notice that? He slipped that one passed me on my first couple of readings.

icefisher

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #25 on: August 09, 2021, 12:14:37 AM »
After reviewing several papers examining Arctic Ocean relationship with Atlantic Meridional Overturning Circulation (AMOC) there is evidence of a temporary slowdown.  However, a collapse is not imminent. 

One possibility for the slight slowdown:  wind forcing changes of the West Norwegian Sea opening up a second channel overflow supplying the Faroe Bank Channel Overflow.

"To investigate more closely the pathways excited by the different atmospheric forcing, we employ backward trajectory simulations (see Methods section) to identify the variable modeled pathways feeding the FBCO. To ensure that we are only tracing the densest overflow water, we only track water particles colder than 2 °C and only do so until they reach a latitudinal section corresponding to  ~ 65°N (Fig. 4). Backtracking water for several years is important, not only to demonstrate the robustness of the variable pathways, but also to show the significant role a multiyear atmospheric forcing regime plays in modulating the modeled FBCO pathways. In this regard, it is instructive to trace these deep dense waters during the early 1990s and 2000s (Supplementary Fig. 3), as these are periods with particularly strong and weak wind forcing and hence spin-up and spin-down of the top-to-bottom basin circulation in the Norwegian Sea21 (Fig. 3), respectively. Figure 4a demonstrates that when the atmospheric forcing, and hence the basin circulation in the Norwegian Sea is anomalously cyclonic (early 1990s), the FBCO is weaker than normal and the source of the deep water is predominantly via the western approach, the short or direct path around the Faroe Plateau into the FSC. In contrast, when the atmospheric circulation is in an anticyclonic regime (early 2000s), the FBCO is stronger than average and the path is predominantly along the eastern Norwegian Basin (Fig. 4b), the long or indirect path into the FSC. Thus, fluid is deflected from north of the Faroes over to the Norwegian slope before turning south into the FSC. There is, however, a subtle difference between the two periods: while the trajectories during the early 1990s seem to be more constrained to shallower depths, those pertaining to the early 2000s appear to trace deeper isobaths and hence a second path is opened up with water crossing from north of the Faroes over to the Norwegian slope."

https://www.nature.com/articles/s41467-020-17426-8

A second possibility:  Increased eddy activity to dissipate excess energy accumulated from melting Arctic Sea ice and Beaufort Gyre freshwater doming since 2007. 

"Implications for the changing Arctic

Arctic sea ice loss is projected to continue over the coming decades, with climate models predicting seasonally ice-free conditions (<106 km2) as early as the 2020s, but more likely towards the middle of the century7,32. Previous hypotheses suggested that the Arctic atmospheric circulation oscillates between predominantly cyclonic and anticyclonic circulation regimes over timescales of 5–7 years1, however, the Arctic has been in an anticyclonic regime since the late 1990s, coinciding with a period of increasing freshwater storage9. Were the Arctic to switch back to predominantly cyclonic atmospheric circulation we might expect a dissipation of mechanical energy by the atmosphere in summer and autumn months, as well as weaker (or reversed) net Ekman pumping and release of freshwater. Here, the results from 2012 provide a useful test case in contrast to the extreme of 2007. In 2012, as in 2007, there was an almost complete loss of sea ice in the BG region (Fig. 4), the cyclonic atmospheric circulation conditions actually dissipated energy in the summer (Fig. 2a) and were more favorable for upwelling (Fig. 2e) causing a (temporary) reduction in FWC (Fig. 2d). The difference in the BG response during extreme ice loss in 2012 compared to 2007 highlights the important interplay between atmospheric circulation and sea ice conditions that controls the state of the BG. A similar reversal of the prevailing anticyclonic atmospheric circulation was observed in wintertime 2016–17. This event was linked to increased intrusions of Atlantic cyclones due to thin ice in the Barents Sea region (and associated thermal anomalies), and a shift in the polar vortex33. Increased intrusions of cyclones into the western Arctic and further reversals in the wintertime atmospheric circulation due to declining sea ice in the Barents Sea is an intriguing, but highly uncertain, hypothesis34. These events suggest that a switch to more cyclonic circulation conditions would lead to a period of freshwater release and a spin down of the BG. However, regardless of the prevailing atmospheric circulation regime, we expect the Arctic Ocean to become more sensitive to atmospheric forcing as the sea ice cover continues to decline under climate change.

Our results show that as the BG region becomes increasingly ice-free earlier in summer and later into October and November, the current anticyclonic atmospheric circulation regime will do significantly more work on the ocean surface currents. Meanwhile, year-round dissipation of energy underneath sea ice will also increase as currents speed up (Fig. 2a, b), but our results suggest that this will not completely account for increased direct atmosphere-ocean energy input. Under this scenario, the Arctic Ocean will continue to become more energetic, and dissipation of additional energy and freshwater stabilization by eddies will be increasingly important. These are critical processes for the accurate representation of the BG system in models. However, currently, only the highest resolution numerical models are eddy-resolving in the Arctic Ocean, where the radius of deformation is 10–15 km in the deep basins and as small as 1 km in the shelf seas. Coupled climate models leave these important dissipative processes unresolved and it is unclear whether commonly used parameterizations of eddies, tuned for the global ocean, are representative in the Arctic. Increases in the eddy diffusivity and increased mixing by eddy activity in a more energetic Arctic Ocean is also expected to enhance vertical transport of warm Atlantic water, with consequences for sea ice growth and mixing of biogeochemical tracers.

https://www.nature.com/articles/s41467-020-14449-z

A third possibility:  High northern latitude subsurface water temperatures are out of phase with the temperature evolution observed in Antarctica.  There is an approximate 200 year lag between Greenland and Antarctic temperatures.  Arctic Sea Ice bottom melting started approximately 1970?  With so much CO2 entering the atmosphere it is easy to see a return to cooler conditions soon but not cold conditions like the stadials. 

Significance

Paleoclimatic proxy records from Greenland ice cores show that the last glacial interval was punctuated by abrupt climatic transitions called Dansgaard–Oeschger (DO) events. These events are characterized by temperature increases over Greenland of up to 15°
C within a few decades. The cause of these transitions and their out-of-phase relationship with corresponding records from Antarctica remains unclear. Based on earlier hypotheses, we propose a model focusing on interactions between ice shelves, sea ice, and ocean currents to explain DO events in Greenland and their Antarctic counterparts. Our model reproduces the main features of the observations. Our study provides a potential explanation of DO events and could help assess more accurately the risk of abrupt climatic transitions in the future.

https://www.pnas.org/content/115/47/E11005

Finally, responsible science takes time, energy and money.  Recognizing knowledge gaps and filling them is a good first start.

Floats in particular could help address one area of considerable interest, namely, the degree to which fresh water from the Arctic and Greenland Sea can mix with and dilute warm, saline water from the Atlantic. Such dilution could suppress deep temperature- and density-driven convection, thus weakening or shutting down the overturning in the Nordic Seas and, by extension, the deepest component of the AMOC.

https://eos.org/science-updates/rethinking-oceanic-overturning-in-the-nordic-seas   

FishOutofWater

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #26 on: August 09, 2021, 03:31:26 AM »
I have read dozens of papers, maybe a hundred or more on the AMOC & the Gulf Stream. The problem is that different definitions are used and different parts of the AMOC elephant are described. There are multiple components to the AMOC and they act independently, not in synch. Because of the independence of the components of the AMOC different observers reach different conclusions.

This new paper did not provide enough details for me or any of us to reach conclusions about what's going to happen to the AMOC in the near future. I'm not saying it's wrong. I'm not in their league. What I'm saying is that it doesn't tell us much that we didn't already know if we read the recent literature on the AMOC and Gulf Stream.

The bottom line is that Europe is not likely to go into a little ice age or Younger Dryas period. The Younger Dryas happened when ginormous amounts of meltwater flooded the north Atlantic. That's a useful precedent for seeing what happens when fresh water covers the Labrador sea and northwestern north Atlantic, but that's not happening now.

What's happening now is the destabilization of the climate and weather patterns that we depend on for modern high intensity agriculture. The intensification of subsidence high pressure areas has led to fire intensification and severe droughts.

There was a reduction in Gulf Stream heat transport in 2009-2010. The SST patterns and ocean heat patterns of high SSTs in the tropical western north Atlantic and below normal SSTs and heat content in the temperate western north Atlantic were caused by that AMOC and Gulf Stream slowdown.

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #27 on: August 09, 2021, 03:53:11 AM »
This paper has been very big in the news. But, it’s findings are built on two layers of proxy data.

We have certainly seen over the past few years studies that indicate the AMOC is slowing down.

However, if you dig into the material and methods of this paper, it is very speculative in the data that was analyzed. It has received a lot of lukewarm feedback from others in the field. 

Media likes to take a study and blow it out of proportion for clicks. 

I’m not arguing that this was not a good, important study.

But, I think it has been blown way out of proportion to the actual findings. 
« Last Edit: August 09, 2021, 04:09:48 AM by Rod »

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #28 on: August 09, 2021, 11:16:17 AM »
All being well we'll discover very soon what AR6 has to say on the subject!

According to the apparently not quite final version of the AR6 WG1 report:

Quote
The Atlantic Meridional Overturning Circulation (AMOC) will very likely decline over the 21st century for all SSP scenarios. There is medium confidence that the decline will not involve an abrupt collapse before 2100. For the 20th century, there is low confidence in reconstructed and modelled AMOC changes because of their low agreement in quantitative trends. The low confidence also arises from new observations that indicate missing key processes in both models and measurements used for formulating proxies and from new evaluations of modelled AMOC variability. This results in low confidence in quantitative projections of AMOC decline in the 21st century, despite the high confidence in the future decline as a qualitative feature based on process understanding. {9.2.3}
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Jim Hunt

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #29 on: August 09, 2021, 12:01:16 PM »
e.g. this quote from the same reference you quoted.
Quote
(The terminological problem with ‘thermohaline’ circulation arises because, for example, half the heat transport in the North Pacific Ocean is in the wind-driven upper circulation4.)

He's also quoting for the North Pacific, when the relevant ocean is the North Atlantic. Did you notice that? He slipped that one passed me on my first couple of readings.

That ancient article was about ‘thermohaline’ circulation in general rather than the AMOC in particular. The point I was endeavouring to make is that it was referenced by Stefan Rahmstorf who is the author of, for example, "Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation". Which doesn't reference Wunsch!

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #30 on: August 09, 2021, 09:11:43 PM »
There is one area that is getting colder almost all the time ,no matter what year or month you take. And that's the area south of Greenland. And that's probably meltwater from Greenland, snowmelt from North-America. Maybe it interferes the warm salty flow coming from the South. Making it less salty. That would take away the reason to dive into the deep sea. Or at a slower speed/ smaller flow. And if you don't move that mass over there. It want move all the rest either. Or at least a part of what drives the rest. A slowdown.

AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #31 on: August 09, 2021, 11:27:02 PM »

P.S. An apparent quote from Niklas Boers:

https://www.tipes.dk/ocean-current-system-seems-to-be-approaching-a-tipping-point/

Quote
Freshwater inflow and especially Greenland meltwater runoff has indeed accelerated in the last decades. However, although the first sign of regional destabilization of the Greenland Ice sheet has been detected, recent Greenland runoff should not be sufficient for destabilizing the AMOC.

To understand this in-depth we need to find ways to improve the representation of the AMOC and polar ice sheets in comprehensive Earth system models and to better constrain their projections. I hope that the results presented here will help with that!

Thats been the general opinion for a couple of decades. Paleo says it happens, but models say it takes Aggasiz levels of meltwater and the ice that is still left can't provide it.
...

In regards to Aggasiz levels of meltwater flux, the two attached panels of one table from the MCDS-BN thread cites possible sources of freshwater flux (in Sverdrup, Sv) through 2060 that could possible lead to an abrupt collapse of the AMOC (MOC) prior to 2100 (assuming a maximum credible domino scenario of Earth System chain reactions/tipping points).
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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #32 on: August 09, 2021, 11:46:48 PM »
"There is one area" That's just off center of the N. Atlantic amphidromic point, which means there's a very little pressure on it to move, but move it does. It's slowly forced west into the currents flowing south down the east coast of the U.S. In my view since it's a mix of Arctic and Greenland  meltwater with some recirculating Atl. waters it is more or less at equilibrium with it's surroundings. Once forced west and south it joins other northern waters pressed hard against continental America raising sea levels there due to it's relative inertia. Where they meets 'Gulf' waters they mix each slowing the others progress, and due to their superior volume the 'Gulf' waters prevail, and continue pushing across the Atlantic but at a subdued pace. So the more surface waters flowing from the Arctic the more energy it takes to overcome their inertia. I'm guessing a good proxy for the volume of this flow is the actual tidal/sea level data on the east coast, and there should be some relation to Gulf stream/AMOC speed.
It's worth looking at https://www7320.nrlssc.navy.mil/GLBhycomcice1-12/natspg.html to see what you think is happening.
I'd never noticed before but on the amph. gif it almost looks like tidal forces are driving waters into the north Atlantic like a slingshot.

AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #33 on: August 10, 2021, 05:27:03 PM »
Stocker (2020) '... suggests that substantial reductions or instabilities of the AMOC could also occur in a future warmer climate.'

Thomas F. Stocker ( 27 Mar 2020), "Surprises for climate stability", Science, Vol. 367, Issue 6485, pp. 1425-1426, DOI: 10.1126/science.abb3569

https://science.sciencemag.org/content/367/6485/1425

Summary
Instabilities in Earth's climate system have intrigued scientists ever since analyses from Greenland ice cores revealed climate variations over the last hundred thousand years. Abrupt changes were not singular events but a pervasive feature of the last ice age. Studies pointed to the ocean, specifically the Atlantic Meridional Overturning Circulation (AMOC), as a possible origin of these large swings. Their occurrence in the distant past of the last ice age and their absence in the past 8000 years suggested that we are living in times of relative climate stability. On page 1485 of this issue, Galaasen et al. report that over the past 500,000 years, there were disruptions in the formation of the North Atlantic Deep Water mass—an essential driver of the AMOC—during interglacial periods. This suggests that substantial reductions or instabilities of the AMOC could also occur in a future warmer climate.

Thus, in my opinion, people who have a great deal of faith that consensus climate model projections will likely closely match the actual climate behavior in coming decades are likely to experience such unpleasant surprises, no later than 2050.
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #34 on: August 10, 2021, 05:33:21 PM »
The linked reference uses paleo-evidence to confirm that the AMOC has rapidly slowed in recent decades:

Caesar, L., McCarthy, G.D., Thornalley, D.J.R. et al. Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nat. Geosci. 14, 118–120 (2021). https://doi.org/10.1038/s41561-021-00699-z

https://www.nature.com/articles/s41561-021-00699-z

Abstract: "The Atlantic Meridional Overturning Circulation (AMOC)—one of Earth’s major ocean circulation systems—redistributes heat on our planet and has a major impact on climate. Here, we compare a variety of published proxy records to reconstruct the evolution of the AMOC since about AD 400. A fairly consistent picture of the AMOC emerges: after a long and relatively stable period, there was an initial weakening starting in the nineteenth century, followed by a second, more rapid, decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades."
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #35 on: August 10, 2021, 05:34:42 PM »
Simon et al. (2020) indicates that:

"The mass and salt transport through the Indian-Atlantic Ocean Gateway, via the Agulhas leakage, can be considered as a potential controlling factor in the Southern Hemisphere impacting on the North Atlantic salt budget. Today Agulhas leakage of ~5–15 Sverdrup (Sv) is one of the dominant sources of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC), connecting the warm route around the southern tip of Africa with the North Atlantic. The advection of salt is communicated north within 2–4 decades suggesting a rather fast impact of Agulhas leakage on the AMOC."

As the ozone hole over Antarctica was accelerating westerly wind velocities no later than 1990, the associated increase in Agulhas Leakage (AL) is certainly currently contributing to the current slowing of the AMOC as indicated by the attached image [from Marino, G. and Zahn, R. (2015)].

Simon, M.H., Ziegler, M., Barker, S. et al. A late Pleistocene dataset of Agulhas Current variability. Sci Data 7, 385 (2020). https://doi.org/10.1038/s41597-020-00689-7

https://www.nature.com/articles/s41597-020-00689-7

Abstract: "The interocean transfer of thermocline water between the Indian and the Atlantic Oceans known as ‘Agulhas leakage’ is of global significance as it influences the Atlantic Meridional Overturning Circulation (AMOC) on different time scales. Variability in the Agulhas Current regime is key in shaping hydroclimate on the adjacent coastal areas of the African continent today as well as during past climates. However, the lack of long, continuous records from the proximal Agulhas Current region dating beyond the last glacial cycle prevents elucidation of its role in regional and wider global climate changes. This is the first continuous record of hydrographic variability (SST; δ18Osw) from the Agulhas Current core region spanning the past 270,000 years. The data set is analytical sound and provides a solid age model. As such, it can be used by paleoclimate scientists, archaeologists, and climate modelers to evaluate, for example, linkages between the Agulhas Current system and AMOC dynamics, as well as connections between ocean heat transport and Southern African climate change in the past and its impact on human evolution."

See also:

Marino, G. and Zahn, R. (2015), "The Agulhas Leakage: the missing link in the interhemispheric climate seesaw?", PAGES MAGAZINE, VOLUME 23, NO 1
« Last Edit: August 11, 2021, 04:22:05 PM by AbruptSLR »
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #36 on: August 10, 2021, 05:35:17 PM »
The linked reference provides evidence that the AMOC, has been, and is currently slowing in response to increasing atmosphere GHG concentrations.

Dima, M., Nichita, D.R., Lohmann, G. et al. Early-onset of Atlantic Meridional Overturning Circulation weakening in response to atmospheric CO2 concentration. npj Clim Atmos Sci 4, 27 (2021). https://doi.org/10.1038/s41612-021-00182-x

https://www.nature.com/articles/s41612-021-00182-x

Abstract: "The Atlantic Meridional Overturning Circulation (AMOC), a tipping component of the climate system, is projected to slowdown during the 21st century in response to increased atmospheric CO2 concentration. The rate and start of the weakening are associated with relatively large uncertainties. Observed sea surface temperature-based reconstructions indicate that AMOC has been weakening since the mid-20th century, but its forcing factors are not fully understood. Here we provide dynamical observational evidence that the increasing atmospheric CO2 concentration affects the North Atlantic heat fluxes and precipitation rate, and weakens AMOC, consistent with numerical simulations. The inferred weakening, starting in the late 19th century, earlier than previously suggested, is estimated at 3.7 ± 1.0 Sv over the 1854–2016 period, which is larger than it is shown in numerical simulations (1.4 ± 1.4 Sv)."
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #37 on: August 10, 2021, 05:36:26 PM »
The linked reference concludes that:

"Our results suggest that the continued decrease in anthropogenic aerosol emissions that accompany efforts to reduce air pollution will reinforce GHG-induced AMOC weakening over the next few decades – with the caveat that internal AMOC variability will also be important."

Thus, in the coming few decades not only could a domino sequence of freshwater hosing events work to significantly slow the AMOC but also so will the projected coming decrease in anthropogenic aerosol emissions.

Hassan, T., Allen, R. J., Liu, W., and Randles, C. A.: Anthropogenic aerosol forcing of the Atlantic meridional overturning circulation and the associated mechanisms in CMIP6 models, Atmos. Chem. Phys., 21, 5821–5846, https://doi.org/10.5194/acp-21-5821-2021, 2021.

https://acp.copernicus.org/articles/21/5821/2021/

Abstract
By regulating the global transport of heat, freshwater, and carbon, the Atlantic meridional overturning circulation (AMOC) serves as an important component of the climate system. During the late 20th and early 21st centuries, indirect observations and models suggest a weakening of the AMOC. Direct AMOC observations also suggest a weakening during the early 21st century but with substantial interannual variability. Long-term weakening of the AMOC has been associated with increasing greenhouse gases (GHGs), but some modeling studies suggest the build up of anthropogenic aerosols (AAs) may have offset part of the GHG-induced weakening. Here, we quantify 1900–2020 AMOC variations and assess the driving mechanisms in state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6). The CMIP6 forcing (GHGs, anthropogenic and volcanic aerosols, solar variability, and land use and land change) multi-model mean shows negligible AMOC changes up to ∼ 1950, followed by robust AMOC strengthening during the second half of the 20th century (∼ 1950–1990) and weakening afterwards (1990–2020). These multi-decadal AMOC variations are related to changes in North Atlantic atmospheric circulation, including an altered sea level pressure gradient, storm track activity, surface winds, and heat fluxes, which drive changes in the subpolar North Atlantic surface density flux. To further investigate these AMOC relationships, we perform a regression analysis and decompose these North Atlantic climate responses into an anthropogenic aerosol-forced component and a subsequent AMOC-related feedback. Similar to previous studies, CMIP6 GHG simulations yield robust AMOC weakening, particularly during the second half of the 20th century. Changes in natural forcings, including solar variability and volcanic aerosols, yield negligible AMOC changes. In contrast, CMIP6 AA simulations yield robust AMOC strengthening (weakening) in response to increasing (decreasing) anthropogenic aerosols. Moreover, the CMIP6 all-forcing AMOC variations and atmospheric circulation responses also occur in the CMIP6 AA simulations, which suggests these are largely driven by changes in anthropogenic aerosol emissions. More specifically, our results suggest that AMOC multi-decadal variability is initiated by North Atlantic aerosol optical thickness perturbations to net surface shortwave radiation and sea surface temperature (and hence sea surface density), which in turn affect sea level pressure gradient and surface wind and – via latent and sensible heat fluxes – sea surface density flux through its thermal component. AMOC-related feedbacks act to reinforce this aerosol-forced AMOC response, largely due to changes in sea surface salinity (and hence sea surface density), with temperature-related (and cloud-related) feedbacks acting to mute the initial response. Although aspects of the CMIP6 all-forcing multi-model mean response resembles observations, notable differences exist. This includes CMIP6 AMOC strengthening from ∼ 1950 to 1990, when the indirect estimates suggest AMOC weakening. The CMIP6 multi-model mean also underestimates the observed increase in North Atlantic ocean heat content, and although the CMIP6 North Atlantic atmospheric circulation responses – particularly the overall patterns – are similar to observations, the simulated responses are weaker than those observed, implying they are only partially externally forced. The possible causes of these differences include internal climate variability, observational uncertainties, and model shortcomings, including excessive aerosol forcing. A handful of CMIP6 realizations yield AMOC evolution since 1900 similar to the indirect observations, implying the inferred AMOC weakening from 1950 to 1990 (and even from 1930 to 1990) may have a significant contribution from internal (i.e., unforced) climate variability. Nonetheless, CMIP6 models yield robust, externally forced AMOC changes, the bulk of which are due to anthropogenic aerosols.

Extract: "Consistent with the recent decreases in anthropogenic aerosol emissions, nearly all of the future emission scenarios (shared socio-economic pathways, SSPs) (O'Neill et al., 2014) yield large reductions in future anthropogenic aerosol emissions, with global sulfate emissions projected to decrease by up to 80 % by 2050. Thus, anthropogenic aerosol emissions, including those around the North Atlantic, will likely continue to rapidly decline over the next few decades. Our results suggest that the continued decrease in anthropogenic aerosol emissions that accompany efforts to reduce air pollution will reinforce GHG-induced AMOC weakening over the next few decades – with the caveat that internal AMOC variability will also be important."
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #38 on: August 10, 2021, 05:41:29 PM »
While Antarctic icebergs behaved much differently during the Pleistocene glacials than they do today; nonetheless, the linked reference demonstrates that Antarctic icebergs from MICI events (implied by the rafted ice debris cited in the study) leading to the glacials can have a profound impact on the AMOC and consequently on climate state.  The associated linked article indicates that the authors believe that modern MICI events would have a different, but still significant, impact on both the AMOC and on climate state, and consequently they encourage future ESMs to evaluate the potential future impacts of such Antarctic iceberg armadas on future climate states (as Hansen et al. 2016 did).

Starr, A. et al. (2021), "Antarctic icebergs reorganize ocean circulation during Pleistocene glacials", Nature, 589, 236–241, DOI: 10.1038/s41586-020-03094-7

https://www.nature.com/articles/s41586-020-03094-7

Abstract
The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.

See also:

Title: "Melting icebergs key to sequence of an ice age, scientists find"

https://phys.org/news/2021-01-icebergs-key-sequence-ice-age.html

Extract: "However, due to the increased global temperatures resulting from anthropogenic CO2 emissions, the researchers suggest the natural rhythm of ice age cycles may be disrupted as the Southern Ocean will likely become too warm for Antarctic icebergs to travel far enough to trigger the changes in ocean circulation required for an ice age to develop.

"Likewise as we observe an increase in the mass loss from the Antarctic continent and iceberg activity in the Southern Ocean, resulting from warming associated with current human greenhouse-gas emissions, our study emphasises the importance of understanding iceberg trajectories and melt patterns in developing the most robust predictions of their future impact on ocean circulation and climate," he said."
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #39 on: August 10, 2021, 05:55:55 PM »
The linked, open access, reference concludes that:

"Our study suggests increased North African dust emissions, whether driven by AMOC changes or atmospheric circulation changes, could amplify the risk of an AMOC collapse. Therefore, it is important to include dust as an interactive component of the climate system."

This indicates that consensus climate models are once again erring on the side of least drama with regard to coming climate risk by ignoring the identified dust-climate feedback.

Murphy, L. N., M. Goes and A. C. Clement (09 November 2017), "The Role of African Dust in Atlantic Climate During Heinrich Events", Paleoceanography and Paleoclimatology, https://doi.org/10.1002/2017PA003150

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017PA003150

Abstract
Increased ice discharge in the North Atlantic is thought to cause a weakening, or collapse, of the Atlantic meridional overturning circulation (AMOC) during Heinrich events. Paleoclimate records indicate that these periods were marked by severe tropical aridity and dustiness. Although the driver of these events is still under debate, large freshwater input is necessary for climate models to simulate the magnitude, geographical extent, and abruptness of these events, indicating that they may be missing feedbacks. We hypothesize that the dust‐climate feedback is one such feedback that has not been previously considered. Here we analyze the role of dust‐climate feedbacks on the AMOC by parameterizing the dust radiative effects in an intermediate complexity model and consider uncertainties due to wind stress forcing and the magnitude of both atmospheric dust loading and freshwater hosing. We simulate both stable and unstable AMOC regimes by changing the prescribed wind stress forcing. In the unstable regime, additional dust loading during Heinrich events cools and freshens the North Atlantic and abruptly reduces the AMOC by 20% relative to a control simulation. In the stable regime, however, additional dust forcing alone does not alter the AMOC strength. Including both freshwater and dust forcing results in a cooling of the subtropical North Atlantic more comparable to proxy records than with freshwater forcing alone. We conclude that dust‐climate feedbacks may provide amplification to Heinrich cooling by further weakening AMOC and increasing North Atlantic sea ice coverage.
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #40 on: August 10, 2021, 05:57:02 PM »
The linked reference quantifies the influence that Sudden Stratospheric Warming (SSW) events can have on the Beaufort Gyre sea ice.  This information help to better understand the risk that a major SSW event in the boreal summertime could temporarily reverse the direction of rotation of the Beaufort Gyre circulation long enough to trigger a major freshwater hosing discharge from the Arctic Ocean into the North Atlantic, where the freshwater could trigger a major slowdown of the AMOC.

Lukovich, J. V. et al. (17 January 2009), "Atmospheric forcing of the Beaufort Sea ice gyre: Surface‐stratosphere coupling", JGR: Oceans, https://doi.org/10.1029/2008JC004849

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008JC004849%4010.1002/%28ISSN%292169-9291.BEAUFORTG1

Abstract: "In a companion article we examined the nature of correspondence between synoptic weather patterns and reversals in the Beaufort Sea ice gyre. In this paper we extend this analysis to examine the role of stratospheric forcing on surface phenomena. Investigated in particular is the correspondence between reversals in stratospheric winds at 10 mbar during winter as defined by stratospheric sudden warmings (SSW) and mean sea level pressure synoptic types in the Beaufort Sea region. Connections between stratospheric and surface events are characterized using relative vorticity and the square of strain computed at different pressure levels from the stratosphere to the surface in the Beaufort Sea region. We quantify the correspondence between stratospheric flow and surface phenomena through investigation of the frequency in synoptic types derived in a companion article during stratospheric sudden warming events. Investigation of stratospheric wind gradients averaged over the Beaufort Sea region demonstrates a prevalence in anticyclonic activity during SSWs that persists for approximately 20 days. Examination of the evolution in synoptic types in the Beaufort Sea region also shows an increase in the number of synoptic types associated with anticyclonic activity during SSWs.

Extract: "Examination of the evolution in wind gradient fields and their anomalies from the stratosphere to the surface during SSWs demonstrates a band of anticyclonic activity that extends from the stratosphere to the surface during SSWs. Investigation of the evolution in synoptic types during SSWs demonstrates an increase in the number of anticyclones and concomitant strengthening of the Beaufort High during SSWs, thereby providing a signature of the correspondence between stratospheric flow and surface cyclone types in the BSR. An interesting avenue for future research includes an extension of this analysis to explore the predictive skill of zonal wind anomalies, horizontal wind gradients, and anticyclone development in the BSR.
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AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #41 on: August 10, 2021, 05:58:07 PM »
The linked reference indicates that the projected decrease in anthropogenic aerosols will lead to a slowing of the AMOC.

Xiaofan Ma et al. (06 Nov 2020), "Dependence of regional ocean heat uptake on anthropogenic warming scenarios", Science Advances, Vol. 6, no. 45, eabc0303, DOI: 10.1126/sciadv.abc0303

https://advances.sciencemag.org/content/6/45/eabc0303

Abstract
The North Atlantic and Southern Ocean exhibit enhanced ocean heat uptake (OHU) during recent decades while their future OHU changes are subject to great uncertainty. Here, we show that regional OHU patterns in these two basins are highly dependent on the trajectories of aerosols and greenhouse gases (GHGs) in future scenarios. During the 21st century, North Atlantic and Southern Ocean OHU exhibit similarly positive trends under a business-as-usual scenario but respectively positive and negative trends under a mitigation scenario. The opposite centurial OHU trends in the Southern Ocean can be attributed partially to distinct GHG trajectories under the two scenarios while the common positive centurial OHU trends in the North Atlantic are mainly due to aerosol effects. Under both scenarios, projected decline of anthropogenic aerosols potentially induces a weakening of the Atlantic Meridional Overturning Circulation and a divergence of meridional oceanic heat transport, which leads to enhanced OHU in the subpolar North Atlantic.
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ritter

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #42 on: August 11, 2021, 05:09:01 AM »
Thanks, AbruptSLR. I feel much better now. :-\

AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #43 on: August 11, 2021, 04:23:02 PM »
Thanks, AbruptSLR. I feel much better now. :-\

Tough love is better than no love at all ;)
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FishOutofWater

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #44 on: August 11, 2021, 05:19:26 PM »
The linked reference quantifies the influence that Sudden Stratospheric Warming (SSW) events can have on the Beaufort Gyre sea ice.  This information help to better understand the risk that a major SSW event in the boreal summertime could temporarily reverse the direction of rotation of the Beaufort Gyre circulation long enough to trigger a major freshwater hosing discharge from the Arctic Ocean into the North Atlantic, where the freshwater could trigger a major slowdown of the AMOC.

Lukovich, J. V. et al. (17 January 2009), "Atmospheric forcing of the Beaufort Sea ice gyre: Surface‐stratosphere coupling", JGR: Oceans, https://doi.org/10.1029/2008JC004849

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008JC004849%4010.1002/%28ISSN%292169-9291.BEAUFORTG1

Abstract: "In a companion article we examined the nature of correspondence between synoptic weather patterns and reversals in the Beaufort Sea ice gyre. In this paper we extend this analysis to examine the role of stratospheric forcing on surface phenomena. Investigated in particular is the correspondence between reversals in stratospheric winds at 10 mbar during winter as defined by stratospheric sudden warmings (SSW) and mean sea level pressure synoptic types in the Beaufort Sea region. Connections between stratospheric and surface events are characterized using relative vorticity and the square of strain computed at different pressure levels from the stratosphere to the surface in the Beaufort Sea region. We quantify the correspondence between stratospheric flow and surface phenomena through investigation of the frequency in synoptic types derived in a companion article during stratospheric sudden warming events. Investigation of stratospheric wind gradients averaged over the Beaufort Sea region demonstrates a prevalence in anticyclonic activity during SSWs that persists for approximately 20 days. Examination of the evolution in synoptic types in the Beaufort Sea region also shows an increase in the number of synoptic types associated with anticyclonic activity during SSWs.

Extract: "Examination of the evolution in wind gradient fields and their anomalies from the stratosphere to the surface during SSWs demonstrates a band of anticyclonic activity that extends from the stratosphere to the surface during SSWs. Investigation of the evolution in synoptic types during SSWs demonstrates an increase in the number of anticyclones and concomitant strengthening of the Beaufort High during SSWs, thereby providing a signature of the correspondence between stratospheric flow and surface cyclone types in the BSR. An interesting avenue for future research includes an extension of this analysis to explore the predictive skill of zonal wind anomalies, horizontal wind gradients, and anticyclone development in the BSR.

You have misinterpreted the conclusions of this study and made several misstatements.

Northern hemisphere SSWs are impossible in boreal summertime because there is no stratospheric polar vortex and the troposphere is decoupled from the stratosphere in the summer. Rossby wave energy moving upwards from the troposphere is reflected back down at the tropopause in the summer.

SSWs lead to anticyclonic winds in the polar regions. Therefore, SSWs lead to the intensification of polar high pressure areas such as the Beaufort high. That leads to accumulation of fresh water under the high, not a "hosing" event.

This SSW problem is complicated and so is the AMOC. I would write more if I had the time.

-update with coffee and breakfast-

My personal opinion on the AMOC is that it will become unstable. We're starting out warm and getting warmer so it won't be exactly like the Younger-Dryas, but it's a pretty reasonable guess that there will be surges of fresh water associated with Greenland glacial melting that will be followed by multi-year AMOC slowdowns. I think that we are already seeing the climate chaos that will result from surges of warm water into the Arctic and subpolar north Atlantic followed by surges of fresh melt water from Greenland and the Arctic ocean.

Climate models are not designed to model kinetic effects and relatively short term instabilities. We cannot expect those models to capture the climate chaos that we are likely to see in coming decades. The models paint with a broad brush.

Much of what has been said in the popular media about the AMOC and climate is misleading. There are multiple components of the AMOC that are linked but on a short term basis act independently. There can be a slow down in overturning in the Labrador sea at the same time as the temperate and subtropical north Atlantic is building up heat and salinity. The southern hemispheric component of the AMOC can also be doing its own thing in the short term. Oceanographic studies have reported on the details; Google RSMAS and AOML to get started on internet resources on the north and south Atlantic components of the AMOC.

The bottom line is that climate chaos is one of the possible results of meltwater surges impacting the AMOC. This possible chaos is not well addressed by global climate models. And it is powerful chaos because regional ocean heat gradients may increase with an unstable AMOC. Those strong gradients may lock in weather patterns causing drought and fires in one region and floods in another. I think that's what's happening now.
« Last Edit: August 11, 2021, 06:33:27 PM by FishOutofWater »

AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #45 on: August 12, 2021, 12:37:08 AM »
While apparently, I got my cyclonic and anticyclonic rotational directions mixed up (w.r.t. SSW events); still the impact of a potential freshwater flux event from the Beaufort Gyre on the AMOC cannot be ignored as indicated by the linked 2021 article and associate reference.  This studied not only modeled a historical (from1983–1995) freshwater flux from the Beaufort Gyre into the North Atlantic; but also it looked at possible future larger freshwater fluxes as the freshwater content of the Beaufort Gyre has increase by over 40% from that past event.

Title: "Freshwater outflow from Beaufort Sea could alter global climate patterns"

https://www.lanl.gov/discover/news-release-archive/2021/February/0224-beaufort-sea.php

Extract: "The Beaufort Sea, the Arctic Ocean’s largest freshwater reservoir, has increased its freshwater content by 40 percent over the last two decades, putting global climate patterns at risk. A rapid release of this freshwater into the Atlantic Ocean could wreak havoc on the delicate climate balance that dictates global climate.

“A freshwater release of this size into the subpolar North Atlantic could impact a critical circulation pattern, called the Atlantic Meridional Overturning Circulation, which has a significant influence on northern-hemisphere climate,” said Wilbert Weijer, a Los Alamos National Laboratory author on the project.

If this very large volume of freshwater is released into the North Atlantic, the impacts could be correspondingly very large as well. The exact impact is still unknown. “Our study of the previous release event offers a picture into the potential impacts of a future, larger release,” Weijer said.

The model used in the study was also partly developed at Los Alamos. It is the Energy Exascale Earth System Model version 0 (E3SMv0), which incorporates LANL’s Parallel Ocean Program (POP) and its powerful sea ice model, CICE."

See also:

Zhang, J., Weijer, W., Steele, M. et al. Labrador Sea freshening linked to Beaufort Gyre freshwater release. Nat Commun 12, 1229 (2021). https://doi.org/10.1038/s41467-021-21470-3

https://www.nature.com/articles/s41467-021-21470-3

Abstract: "The Beaufort Gyre (BG), the largest Arctic Ocean freshwater reservoir, has drastically increased its liquid freshwater content by 40% in the past two decades. If released within a short period, the excess freshwater could potentially impact the large-scale ocean circulation by freshening the upper subpolar North Atlantic. Here, we track BG-sourced freshwater using passive tracers in a global ocean sea-ice model and show that this freshwater exited the Arctic mostly through the Canadian Arctic Archipelago, rather than Fram Strait, during an historical release event in 1983–1995. The Labrador Sea is the most affected region in the subpolar North Atlantic, with a freshening of 0.2 psu on the western shelves and 0.4 psu in the Labrador Current. Given that the present BG freshwater content anomaly is twice the historical analog studied here, the impact of a future rapid release on Labrador Sea salinity could be significant, easily exceeding similar fluxes from Greenland meltwater."
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FishOutofWater

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #46 on: August 12, 2021, 01:11:33 AM »
Yes, the stored relatively fresh water in the Beaufort gyre and the Arctic ocean can be driven by anomalous northerly winds into the north Atlantic by changes in synoptic weather patterns. That happened in 2012 to 2016. It's likely the cause of the temporary apparent recovery of Arctic sea ice from 2013 to 2015.

There's an excellent article published last year that you might want to read that discusses the largest freshwater pulse in 120 years into the subpolar gyre/AMOC that took place from 2012-2016.

https://www.nature.com/articles/s41467-020-14474-y

Abstract

The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater from the western boundary into the eastern basins. We find that wind-driven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic.

FishOutofWater

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #47 on: August 12, 2021, 01:23:53 AM »
Also note that the Agulhas leakage adds salinity to the AMOC and is a key positive element of the overturning circulation. Per the article you cited:

"The mass and salt transport through the Indian-Atlantic Ocean Gateway, via the Agulhas leakage, can be considered as a potential controlling factor in the Southern Hemisphere impacting on the North Atlantic salt budget. Today Agulhas leakage of ~5–15 Sverdrup (Sv) is one of the dominant sources of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC), connecting the warm route around the southern tip of Africa with the North Atlantic. The advection of salt is communicated north within 2–4 decades suggesting a rather fast impact of Agulhas leakage on the AMOC."

There is possibly an atmospheric teleconnection of an active AMOC period with winds in the southern hemisphere that tend to reduce the leakage. It is the strong AMOC that drives the weak leakage, causing a potential oscillation and potential pulses. Note that this theory has been challenged by recent papers. The physics of the leakage is still being studied and is not settled science.

What ever the case, a stronger leakage leads to a stronger AMOC in the subpolar north Atlantic but it takes a few decades for the increased salinity to impact deep water formation there.

AbruptSLR

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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #48 on: August 12, 2021, 06:36:32 PM »
...

There is possibly an atmospheric teleconnection of an active AMOC period with winds in the southern hemisphere that tend to reduce the leakage. It is the strong AMOC that drives the weak leakage, causing a potential oscillation and potential pulses. Note that this theory has been challenged by recent papers. The physics of the leakage is still being studied and is not settled science.

What ever the case, a stronger leakage leads to a stronger AMOC in the subpolar north Atlantic but it takes a few decades for the increased salinity to impact deep water formation there.

Personally, I do not share your confidence that consensus climate scientists clearly understand all of the teleconnections and bi-polar feedbacks between the AL and the AMOC when you acknowledge that there competing theories on this matter.  For instance in the second panel of the first attached image showing a weak AMOC with a strong AL the SST in the North Atlantic is cold because of freshwater fluxes from meltwater.  Thus to me, it seems conceivable that the extra salinity that you cite may cause the warm North Atlantic current to dive beneath the cold fresh surface water and penetrate deeper into the Arctic Ocean before it gets cold enough to sink to form NADW.  I think that more models should include the influence of freshwater fluxes as E3SM is progressively working to do.
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Re: AMOC (Gulf Stream Current) Collapsing
« Reply #49 on: August 15, 2021, 04:47:53 AM »
Actually, the impacts of a slow but not stopped AMOC are very serious. True, the north Pacific ocean doesn't turn into a steaming stagnant dead zone as climate models predict for the most extreme GHG increases, so that's good, but globally a slow AMOC causes rapid global warming because little heat is subducted in the subpolar gyre. Heat builds up in the subtropical and tropical Atlantic waters including the south Atlantic.

And that's the answer to the 64 trillion dollar question about why warming is accelerating now. We have a slowed AMOC and some of the warm water stored in the subtropical Atlantic has now surged north into the subpolar gyre. The cold hole south of Greenland has been covered over top. But it's not going down all that easy now because warm salty water is covering cool fresh water at mid levels.

And, as you surely know, Antarctic deep water formation has gone back into a funk. It briefly revived a few years back when sea ice extent declined around Antarctica, but it has gone back into a coma. Hansen is more concerned about the shut down in the near future of Antarctic deep water formation than complete shut down of the AMOC. He is, of course, correct.

Expect warming to accelerate even more as sulfate aerosol levels drop with the closing of coal plants. That has happened in the U.S. but not India and China where coal is still king.

We're not totally fucked but we fucked where it hurts.