Atlantic Conveyor - A Description

[crimsone]

Contained within the text is a pretty good (if simplified) description of what the Atlantic Conveyor is, how it works, and it's role in amongst the other major thermoline circulations in the earths oceans and indeed the weather itself.

Also in it is a theory on what would happen if it stopped. Given a recent thread, I thought I'd post it and see what people think about it's accuracy, or anything that it perhaps misses...

http://www.dhushara.com/book/diversit/extr...aoclim/chao.htm

[Stratos Ferric]

Mmmm,

That's a first Crimsone; a direct link from a (rather dodgy IMHO) page on the THC to a site covering the Sexual Paradox. In the Bizarre stakes, and N-W strikes me as generally playing a face card laden hand, that's prety good, even by, say, PP's standards.

[crimsone]

Oops! I didn't check the link itself. I merely googled and found it, posting it after looking through the text. I merely scan-read the page text, which appears to be a reproduction of a genuine article or two. None the less, it's pretty bizarre looking I must admit.

I've just had a look, and I can't actually find the original... "Chaotic Climate", Wallace Broecker, Scientific American Nov 95 - if anybody can find a copy of this on the web for direct comparison to the link I posted I'd appreciate it.

Anyhow, having read the whole page, regardless of what site it happens to be on, what of the accuracy of the information it contains? I'm not talking about first impressions, but of the actual scientific accuracy of the information.

Edited August 28, 2006 by crimsone

[parmenides3]

Oops! I didn't check the link itself. I merely googled and found it, posting it after looking through the text. I merely scan-read the page text, which is a reproduction of a genuine article or two. None the less, it's pretty bizarre looking I must admit.

I've just had a look, and I can't actually find the original... "Chaotic Climate", Wallace Broecker, Scientific American Nov 95 - if anybody can find a copy of this on the web for direct comparison to the link I posted I'd appreciate it.

Anyhow, having read the whole page, regardless of what site it happens to be on, what of the accuracy of the information it contains? I'm not talking about first impressions, but of the actual scientific accuracy of the information.

Friendly, Hi, crimsone. Please read my other post. I'll post a link to a commentary on Wally Broecker's latest findings when I get back from walking the dog.

P

[crimsone]

Thanks for the (future) posting of the link. It would be much appreciated. I shall read your post momentarily.

Edited August 28, 2006 by crimsone

[johnholmes]

when I set msn search to this topic it came up with over 15,000 reports. That should be enough to keep most on here occupied for a day or two!

jh

[rob-lancs]

It's definitely a well known theory, the effects of the gulf stream shutting down.

http://en.wikipedia.org/wiki/Thermohaline_circulation

http://en.wikipedia.org/wiki/Shutdown_of_t...ine_circulation

Basically, as the current water in the gulf stream heads towards the poles, it becomes cooler and evaporation increases the salinity, this makes the water dense and it falls, this tends to rise again in the Indian and Pacific Oceans, probably down to the amount of fresh water input, for example prevailing winds that hit the Rockies etc. and the precipitation or snow (which will melt) will run into the Pacific and the higher freshwater will dilute the salinity. The heat in the Pacific will then cause the bottom water to rise to the surface and head back towards the Gulf. The idea of temperature (thermo) and salinity (haline) driving this circulation gives rise to its name.

I've yet to find agreement though that the gulf stream shutting down would have a significant effect, I've seen it quoted that it only contributes to 3C of the heat transport to here, 9C is from the atmosphere and 8C is heat released from the oceans. The question is whether this is enough to tip the balance. Oceanographers tend to think it will, meteorologists are more sceptical thinking the heat in Pressure systems. Someone better qualified as a meteorologist might be able to expand on this.

Thermohaline Circulation is pretty much one of the basic concepts of my University course, so I thought I'd put this forward.

Edited August 28, 2006 by rob-lancs

[parmenides3]

when I set msn search to this topic it came up with over 15,000 reports. That should be enough to keep most on here occupied for a day or two!

jh

That the web is full of such stuff is something I can vouch for, having browsed the subject for some weeks now. What is interesting ( to me) is that very many of them are derivative of a relatively small number of sources. Having spent time looking at the same graphs, arguments and references time and time again, I think it is fair to say that there are a great many scientific papers relevant to the subject, a vastly greater number of less scientific opinion pieces, but only a few key papers and websites. These sites are useful to me because they allow access to hundreds of Scientific journals from varying disciplines, most of which I would normally have to pay to subscribe to. They also include debate by a fair number of individuals, eminent in their own fields, on the implications of new papers and new findings.

Sorry, Crimsone, can't find the reference right now, but basically, a new paper (August 2006) by Broecker et. al. is said to revise his earlier ideas about the Laurentide 'hosing' event & the causes of the Younger Dryas, so it will supercede the findings of the earlier research.

I'll cover the Atlantic Conveyor a little later.

P

Edit: Rob - what was your specialism? P

Edited August 28, 2006 by parmenides3

[rob-lancs]

Officially it's "Ocean Science"

Edited August 28, 2006 by rob-lancs

[parmenides3]

Officially it's "Ocean Science"

In that case, you'll appreciate this:

http://ocean.mit.edu/~cwunsch/papersonline...cean_19july.pdf

There is also a copy of 'Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation', by Schlesinger et.al. (2005), on http://www.metoffice.gov.uk/corporate/pres...kCh5Jan2006.pdf

The first paper is from the perspective of Oceanography, and it suggests that the simple models we are used to seeing in the public domain are so simplified as to be wrong. It also points out that the models used in GCMs are also much too simple, especially with respect to mesoscale ocean eddies and vertical heat flux.

The second paper does what it says on the tin. These are two of the recent papers I have read on the subject. if you would like more, please ask.

[Stratos Ferric]

It's definitely a well known theory, the effects of the gulf stream shutting down.

http://en.wikipedia.org/wiki/Thermohaline_circulation

http://en.wikipedia.org/wiki/Shutdown_of_t...ine_circulation

...

I've yet to find agreement though that the gulf stream shutting down would have a significant effect, I've seen it quoted that it only contributes to 3C of the heat transport to here, 9C is from the atmosphere and 8C is heat released from the oceans. The question is whether this is enough to tip the balance. Oceanographers tend to think it will, meteorologists are more sceptical thinking the heat in Pressure systems. Someone better qualified as a meteorologist might be able to expand on this.

Thermohaline Circulation is pretty much one of the basic concepts of my University course, so I thought I'd put this forward.

Rob, you need to go and see the Hadley Centre's own modelling (links from UKMO pages) if you've yet to find [external' agreement re impact. Their modelling shows a dramatic im,pact year round, but particularly in winter. As to the contribution of sources to ambien temperature - I'm assuming in the UK - then the figure you quote way understate the thermal impact of the NAD. The surface differential in the ocean around Iceland can be 6-7C between the warm drift and the colder surface return that hugs the Greenland coast and the cold current off Norway. In addition, water, being denser than air, holds huge amounts of energy for the same temperature. Finally, heat in the atmosphere, certainly in our winter, is largely derived from ocean passage anyway.

The initial impact of a NAD shut down, would, in winter, be 5-10C on average I suspect; as I recall the Hadley models quickly had the annual mean dropping by more than 5, and minima in winter often falling to -15 to -20C after 2-3 years.

[parmenides3]

Rob, you need to go and see the Hadley Centre's own modelling (links from UKMO pages) if you've yet to find [external' agreement re impact. Their modelling shows a dramatic im,pact year round, but particularly in winter. As to the contribution of sources to ambien temperature - I'm assuming in the UK - then the figure you quote way understate the thermal impact of the NAD. The surface differential in the ocean around Iceland can be 6-7C between the warm drift and the colder surface return that hugs the Greenland coast and the cold current off Norway. In addition, water, being denser than air, holds huge amounts of energy for the same temperature. Finally, heat in the atmosphere, certainly in our winter, is largely derived from ocean passage anyway.

The initial impact of a NAD shut down, would, in winter, be 5-10C on average I suspect; as I recall the Hadley models quickly had the annual mean dropping by more than 5, and minima in winter often falling to -15 to -20C after 2-3 years.

SF: Rob's numbers are from a 2005 paper which he has referred to on another strand - can't find it. RealClimate are due to publish a critique shortly. It strikes me that there may be some confusion between surface heat transport and ocean heat transport, both of which, of course, would contribute to our climate.

P

[parmenides3]

Sorry for the double post. Below is a (rather lengthy) extract, summarising the state of scientific opinion on this subject as of January this year. I do not have access to the original (Nature), as I am not a subscriber. Hope it hepls.

19 Jan 2006

Atlantic circulation change summary

Filed under: Climate Science Paleoclimate Oceans— gavin @ 11:00 pm - ()

Nature this week has an excellent summary of the state of the science with regards to possible changes in the ocean thermohaline (or meridional) circulation in the Atlantic and its impact on climate. Even though it quotes a couple of us, it's still worth reading if you want to understand how results like the Bryden et al paper - that suggested that the Atlantic overturning had reduced by 30% in recent decades - are assimilated into the scientific picture.

The [bryden et al]result came as a surprise to those in the field. Few scientists had thought that such dramatic slowing of the thermohaline circulation could happen so soon. Models suggest that the increase in fresh water needed for a conveyor shutdown would not be expected without a global warming of 4–5 C; warming in the twentieth century is currently put at 0.6 C (ref. 3). The most complex computer models of the climate and oceans, the sort used to make climate predictions for the Intergovernmental Panel on Climate Change (IPCC), suggest that the flow might be expected to slow by an average of 25% by the end of the twenty-first century, but not to shut down completely.

The reason for this surprise is that other data of relevance, such as the changes in salinity or deep convection or lack of observed cooling in the North Atlantic, aren't obviously consistent with such a large change:

[Ruth Curry's] recent analysis of 1950 to 2005 salinity data suggests that 4,000 cubic kilometres — eight times the annual outflow of the Mississippi river — of fresh water have accumulated in the upper ocean layers since the 1960s. “The extra freshwater input is beginning to affect density,” she says. But the amount of fresh water needed to shut down the thermohaline circulation in Rahmstorf ’s comparisons is an order of magnitude greater than the flux reported by Curry, and she agrees that the circulation will not be unduly affected this century. Peter Wadhams, an oceanographer at the University of Cambridge, UK, last year reported a substantial weakening of convection ‘chimneys’ down which surface water flows in the Greenland sea, but it is unknown how much of the observed effect is due to natural variability.

It is possible that the Bryden result was an unlucky statistical artefact: “The results are based, after all, on just five snapshots of an extremely noisy and under-sampled system,” says Carl Wunsch. His own analysis doesn't suggest any significant change over the last decade or so (though it's probably worth pointing out that Wunsch's opinions of the relative (un)importance of the Atlantic circulation for climate change are probably not (yet?) mainstream in the community).

Everyone quoted is however agreed on one thing: "the notion that [a future change in the themohaline circulation] may trigger a mini ice age is a myth”. The evidence of previous changes for instance at the Younger Dryas or during the 8.2 kyr event is quite strong, and significant coolings were observed particular around the North Atlantic, but even such localised coolings are not predicted to occur if the circulation slows as an effect of global warming.

It is however a complicated business, and the stability of this circulation depends on many aspects of climate that are poorly observed and uncertainly modelled. So it may yet be some time before new observations (such as the permanant monitoring array recently installed along a section of the ocean), better modelling, and a better appreciation of the paleo-climatic data add up to a coherent understanding of this interestingly counter-intuitive aspect of climate change.

Source: RealClimate.org

[Stratos Ferric]

The most telling line in that is the point about a dearth of samples (of the NAD flow). At best it is a very diffuse and slow flow, actually measuring and comparing volumes year to year cannot be easy. Satellites can help at the surface but won't tell you much about what's going on below.

As to impact of a shutdown, it beggars belief that we wouldn't have some localised cooling. Our climatic anomaly cannot be attributed to anything else that I can think of. Shut down the driver of that anomaly and, ipso facto, you should lose the anomaly. What I could accept is that if background warming is the driver behind any shut-off, then the degree of warming required to effect complete shut-off may more than compensate for the loss of adjustment caused by closing down the driver. I.e. the warming to close NAD is bigger than the cooling effect of closing it down.

Finally, even if you close down NAD, it's hard to believe there would be no oceanic flow. The spin of the earth and air movement at the surface provide energy and surface vector for ocean flows. There might be some realignment, but my simple mind suggests that it would be hard ever to escape from a basic poleward flux of energy, if only because of the shape of the North Atlantic bowl.

[parmenides3]

The most telling line in that is the point about a dearth of samples (of the NAD flow). At best it is a very diffuse and slow flow, actually measuring and comparing volumes year to year cannot be easy. Satellites can help at the surface but won't tell you much about what's going on below.

As to impact of a shutdown, it beggars belief that we wouldn't have some localised cooling. Our climatic anomaly cannot be attributed to anything else that I can think of. Shut down the driver of that anomaly and, ipso facto, you should lose the anomaly. What I could accept is that if background warming is the driver behind any shut-off, then the degree of warming required to effect complete shut-off may more than compensate for the loss of adjustment caused by closing down the driver. I.e. the warming to close NAD is bigger than the cooling effect of closing it down.

Finally, even if you close down NAD, it's hard to believe there would be no oceanic flow. The spin of the earth and air movement at the surface provide energy and surface vector for ocean flows. There might be some realignment, but my simple mind suggests that it would be hard ever to escape from a basic poleward flux of energy, if only because of the shape of the North Atlantic bowl.

I think the last two paragraphs are most informative: 'the stability of this circulation depends on many aspects of climate that are poorly observed and uncertainly modelled', is a telling comment.

One problem we have is the idea of a 'shutdown' itself. I think it is clear that salinity changes and vertical flux changes are ongoing. It is probably also fair to say that these changes are both a reflection of changes in the climate and influences on (our) regional weather patterns. But the term is, in itself, provocative, and as such, might be deceptive. As I understand it, what these scientists are broadly saying is that they don't know enough about how the ocean circulation works, but what they do know doesn't point, at the moment, to a phase change in the N. Atlantic, though this is a possibility, given GW, at some time in the next 200 years.

The Schlesinger paper cited in an earlier post deals with your last point, as does Curry & Mauritsen, Woods Hole, 2005. What is proposed is not and end to circulation, but a shift of pattern, so that the Gulf Stream no longer passes the UK with the same force, heat or salinity as at the moment. Experiments with AOGCMs indicate that there would be a 'reversal' of the flow, so the warm water would head to Labrador first, before cooling and heading our way. This, if it is correct, would undoubtedly cool our climate, no question. The models also showed, though, that such a change was temporary; when the salinity levels readjusted, the flow 'switched back' to the current pattern.

I think the conclusion was that regionalised cooling caused by circulation pattern shifts would be compensated for, over a 100 year period, by Global Warming, which implies, to me, that should such an event occur, we might well suffer some decades of colder weather before returning to the current pattern.

I do have some concern about what I have found out recently about salinity anomalies in the Atlantic and the Arctic. The kinds of changes which have been measured recently all point to extremes in the patterns of one kind of another; this, in turn, feeds the notion of a 'point of no return', but my concern is not scientific, merely intuitive, and I do tend to trust the experts; more, perhaps, than some.

The point I wanted to make, though, is in the article; the notion that a change in the thermohaline circulation may trigger a mini-ice age is a myth.

P

[rob-lancs]

Cheers for all this going to print a bit of stuff out before I head south, it'll probably be a while before I'm back on the net so it should help pass a few hours in the meantime.

[tugmistress]

An article i think one of two of you may find interesting ....

A new analysis of 50 years of changes in freshwater inputs to the Arctic Ocean and North Atlantic may help shed light on what's behind the recently observed freshening of the North Atlantic Ocean. In a report, published in the August 25, 2006 issue of the journal, Science, MBL (Marine Biological Laboratory) senior scientist Bruce J. Peterson and his colleagues describe a first-of-its-kind effort to create a big-picture view of hydrologic trends in the Arctic. Their analysis reveals that freshwater increases from Arctic Ocean sources appear to be highly linked to a fresher North Atlantic.

"The high-latitude freshwater cycle is one of the most sensitive barometers of the impact of changes in climate and broad-scale atmospheric dynamics because of the polar amplification of the global warming signal," says Peterson. "It's easiest to measure these changes in the Arctic and the better we understand this system, the sooner we will know what is happening to the global hydrologic cycle."

The multi-disciplinary team of scientists led by Peterson calculated annual and cumulative freshwater input anomalies (deviations from expected levels) from net precipitation on the ocean surface, river discharge, net attrition of glaciers, and Arctic Ocean sea ice melt and export for the latter half of the 20th century. The scientists compared the fluxes to measured rates of freshwater accumulation in the North Atlantic during the same time period.

Endless Pools

Their analysis showed that increasing river discharge and excess net precipitation on the ocean contributed the most freshwater (~20,000 cubic kilometers) to the Arctic and high-latitude North Atlantic. Sea ice reduction provided another ~15,000 cubic kilometers of freshwater, followed by ~2,000 cubic kilometers from melting glaciers. Together, the sum of anomalous inputs from all of the freshwater sources analyzed matched the amount and rate at which fresh water accumulated in the North Atlantic during much of the period from 1965 through 1995.

"This synthesis allows us to judge which freshwater sources are the largest, but more importantly shows how the significance of different sources have changed over the past decades and what has caused the changes," says Peterson. "It prompts us to realize that the relative importance of different sources will change in future decades. Creating a big-picture or synoptic view of the changes in various components of the high-latitude freshwater cycle puts the parts in a perspective where we can judge their individual and collective impact on ocean freshening and circulation."

In recent years, much attention has been given to the observed freshening of Arctic Ocean and North Atlantic and the potential impacts it may have on the earth's climate. Scientists contend that a significant increase of freshwater flow to the Arctic Ocean could slow or halt the Atlantic Deep Water formation, a driving factor behind the great "conveyor belt" current that is responsible for redistributing salt and thermal energy around the globe, influencing the planet's climate. One of the potential effects of altered global ocean circulation could be a cooling of Northern Europe within this century.

The team's comparison of freshwater sources and ocean sink records revealed that over the last half century changes in freshwater inputs and ocean storage occurred not only in conjunction with one another, but in synchrony with rising air temperatures and an amplifying North Atlantic Oscillation (NAO), a climatic phenomenon that has strong impacts on weather and climate in the North Atlantic region and surrounding continents, and the associated Northern Annular Mode (NAM) index.

Peterson and his colleagues contend that the interplay between the NAO and NAM, and continued rising temperatures from global greenhouse warming, will likely determine whether the Arctic and North Atlantic Oceans will continue to freshen. But the scientists caution that the difficultly in predicting fluctuations in atmospheric circulation makes it impossible to know where we might be headed.

"Atmospheric modes of circulation such as the NAO and NAM exert a great deal of control on net precipitation in the ocean and even on regional temperatures, and hence ice melt as well," says Peterson. "But what drives the NAO is the $64,000 question. Our inability to predict trends in the NAO/NAM means that, even if we could predict global warming very well, a large degree of uncertainty will remain in any forecasts of the decadal-centennial trajectories of the Arctic freshwater balance.

from Underwater Times

[parmenides3]

Nice find there, Tugmistress: thanks. This is similar to the report from Woods Hole Oceanographic Institute, which attempted to quantify the net Arctic-Atlantic freshwater flux. The numbers they came up with do not sound greatly different in scale to the ones mentioned above.

One problem with the idea of a freshwater influx causing a rapid change in ocean circulation is that of scale. Some recent experiments using GCMs and known fluid theory estimated that the amount of influx would have to be about 0.6 Sv (1 Sv = 10 to the 6 square metres per second). The amounts mentioned here are at least an order of magnitude too small - less than 0.05 Sv; this is why the papers concluded that a collapse was unlikely in the next 100 years.

The other problem is with this quote:

In recent years, much attention has been given to the observed freshening of Arctic Ocean and North Atlantic and the potential impacts it may have on the earth's climate. Scientists contend that a significant increase of freshwater flow to the Arctic Ocean could slow or halt the Atlantic Deep Water formation, a driving factor behind the great "conveyor belt" current that is responsible for redistributing salt and thermal energy around the globe, influencing the planet's climate. One of the potential effects of altered global ocean circulation could be a cooling of Northern Europe within this century.

This is correct as far as it goes, but the scientist principally responsible for this theory, Wally Broecker, published a paper in January of this year, in which he draws away from the conclusions he made in 1995. If you are interested, I might be able to pull across the relevant material on this from whichever thread I posted it on originally.

There is also a long post about the idea of the 'Atlantic Conveyor Belt' (problems with it) on page 13 or 14 of one of the Global Warming threads.

I can understand the concern about the freshwater anomaly - to some extent, I share this concern - but, because of the above, I tend to think that, on its own, this would not be sufficient to cause a THC-induced cooling.

:lol:

PS: I haven't forgotten you, Blast; but I want to take my time over phrasing my response. P