Thermohaline Circulation Shutdown = Business As Usual?

[trevw]

Found this rather interesting article (apologies if it's already been discussed) - apparently the reason we are unusually warm for our longitude has very little to do with the gulf stream and thermohaline circulation and everything to do with a 'standing wave' caused by the rockies..

btw - I'm not a GW skeptic, however if what is described in the article is true (and it seems genuine) then I feel it's things like this that undermine the AGW 'cause' it seems to me it's often presented as fact in the UK that a shutdown would plunge us into arctic conditions and this is held up as a reason to act against climate change, a sort of 'trump card', whereas it sounds like it is more likely to affect storm generation etc rather than general temps and if this became accepted general knowledge - it could appear to be a body blow against the AGW lobby (as well as dissapointing the odd poster on here ) whereas in my humble opinion there are still plenty of reasons to take AGW seriously.

http://www.americanscientist.org/template/...print=yes#52131

[johnholmes]

an interesting read, but I wonder what other equally perhaps even more emminent scientists have made of this paper?

John

[John Cox]

I had a good read of this article.

Although interesting, all the ideas in it just seem at most to say that a NAD shutdown would not have as much effect as the media say it will.

One point of interest is the fact that they say that the fact that europe does get prevailing westerlies compared to the opposite on the East coast of America would keep our temperatures higher than them even if there was not the influence of the NAD. I do wonder if the NAD did shutdown would we still get as must westerlies.

Look at last winter . A much decrease in the westerlies and their strength.

Me thinks perhaps it is shutting down

[The PIT]

Good points John C.

Shows how careful one has to be when making deductions. At present there's so many arguments and counter arguments it's very hard to tell whats FUD and whats not FUD

[trevw]

I would also be interested in the opinion of other experts in the field. It all seems plausible but who knows? I picked it up from a discussion thread on realclimate where it seemed to get lost amongst other points..

Although interesting, all the ideas in it just seem at most to say that a NAD shutdown would not have as much effect as the media say it will.

Isn't that in itself quite significant though? Various magazines/TV programs have taken this as a theme and as far as I remember all have claimed the majority of our additional warming is due to the gulf stream etc. If instead of freezing winters and damp gloomy summers, year round ice forming glaciers in scotland etc. we are just looking at a 1 deg ish fall in CET (and a reduction in cyglogenisis caused by warm currents?) the effects are presumeably going to be very different.

Presumeably the prevailing weather direction would still be from the west? But there would be less in the way of 'activity' generated by the atlantic?

Had to look 'FUD' up handy word. I had always thought that the 'shutdown scenario' and it's supposed effects if it occurred were beyond doubt.. now it looks like the more extreme scenarios could well be FUD..? (is that correct usage?)

trevw

[John Cox]

Hi trevw,

Well we know that the media always hype things up as it helps to sell newspaers and advertising on TV.

I agree that it would be great to hear from some experts in the field and i for one would love to see if these views could be discussed by "experts" or indeed by anyone with an interest in this field.

Anything that "takes the blinkers off", mine included, cannot but be a good thing.

Anyone on NW with some thoughts on this subject? You would need to read the article carefully first to get a good feel for what he is talking about

[summer blizzard]

If the NAD were to shut down, thic cap would grow, that is a fact, this would in the beggining give the chance of very blocked weather with hot summers and cold winters however without heat being imported to the Arctic, there would be nothing to stop the ice cap growing and the albeido effect would take over, so while there would be a decrease in westerlies after five to ten years, the albeado effect would take over causing a massive drop in temperature.

[Dawlish]

Hi trevw,

What an interesting article that is and no, I, for one, haven't come across it before. When I'm led to publications from organisations I come across for the first time, especially American ones, I always go first for the provenance and who funds them. The American Scientist checks out very well and actively supports the teaching of evolution in American schools, against the wave of far right, fundamentalist creationsism that is threatening some mid-West and deep South communities. Looks good to me.

The article is so thought provoking....he says, going back to read it again. Thanks trev! As John says, it would be nice to find some opposing views to judge it against.

Paul

The effects of a shutdown scenario have been discussed ad nauseam, of course and we all know the "Day After Tomorrow" outcomes, thanks to the studio special effects teams....the only trouble is that there is no evidence of any shutdown happening or is about to happen. "May happen" is the crucial phrase. Media and hollywood hype has soooo much to answer for here! I wouldn't worry about it in any of our lifetimes.... or our children's.... or their children's (possibly).

Edited July 12, 2006 by Dawlish

[Gray-Wolf]

The papers from earlier this year that show how it is predominantly the atmosphere that moves heat to the poles and cold to the equator marries in nicely with the 'standing wave' theory.

The sooner folk accept that all mother nature wants is to maintain an equilibrium then the sooner we can move on from the tosh as 'ice age now ' and co. pedal.

Our planet is warming and that heat will spread polewards by one means or another and the changes this warming will bring to our finely balenced 'human world' is what we should all be focusing on , surely?

Edited July 12, 2006 by Gray-Wolf

[Thundersquall]

I haven't read the article however it sounds rather like a load of bull to me.

We are much warmer at our latitude compared to New York due to the Gulf Stream.

It's the Gulf STream that keeps NW Scotland very much milder than it 'should' be.

Maybe the Government is trying to 'hide' the fact that our actions are disturbing the Gulf Stream?

Edited July 12, 2006 by Thundersquall

[trevw]

If the NAD were to shut down, thic cap would grow, that is a fact, this would in the beggining give the chance of very blocked weather with hot summers and cold winters however without heat being imported to the Arctic, there would be nothing to stop the ice cap growing and the albeido effect would take over, so while there would be a decrease in westerlies after five to ten years, the albeado effect would take over causing a massive drop in temperature.

would it actually grow.. or given global increases in temperature would it just stabilize around the current levels and stop shrinking? Read somewhere that by the time the warmer water has reached the poles it has lost most of it's heat so am not sure how much difference it would make to ice formation.. (would be really interested to know)

Presumably if the ice did grow this would be a fairly slow process - steady build up of multi-year ice over decades giving a better base for each subsequent winters freeze?

[Dawlish]

I haven't read the article however it sounds rather like a load of bull to me.

If you read the article, it might just not seem like a load of bull to you any more.

Paul

[summer blizzard]

http://www.thewe.cc/contents/more/archive2...hange_event.htm

This is an artcle relating to the current situation if anybody would like to read it...

http://www.realclimate.org/index.php/archi...tream-slowdown/

This is a discussion about the weakening of the Gulf Stream/NAD

[trevw]

http://www.thewe.cc/contents/more/archive2...hange_event.htm

This is an artcle relating to the current situation if anybody would like to read it...

http://www.realclimate.org/index.php/archi...tream-slowdown/

This is a discussion about the weakening of the Gulf Stream/NAD

thanks, that's very interesting! in the posts below the realclimate article there is a discussion of the source for the topic article... sounds like the conclusion is that our climate would be very much like that of british columbia but a trifle warmer rather than anything like labrador etc. and that the greatest heating effect caused by the TH circulation is further north. Will quote the two (Edit: make that 3) bits below.. (hope it's not too long)

Trevw

3 This is a good overview of the Gulf Stream - thermohaline circulation (THC) story, but in addressing the question in this post I fear you have left out a discussion of the major factor that keeps northern Europe warm compared to similar latitudes in eastern North America. Atmospheric circulation patterns, specifically the predominant shape of the Jet Stream, controlled largely by orographic forcing of the Rocky Mountains, have more to do with keeping northern Europe warm than does the Gulf Stream or THC. The shape and location of the Rockies help to cause a persistent wave in the Jet Stream, with a trough in over eastern North America which brings cold continental air, via northwesterly winds, to northeastern North America, and a crest over the eastern north Atlantic and western Europe which brings warm maritime air, via southwesterly winds, to northern Europe. The ocean plays a role in that the seasonal storage and release of heat by the Atlantic Ocean mixed layer, coupled with wind advection, lend northern Europe a maritime climate with mild winters, but this is true whether the Gulf Stream operates or not.

This is argued very clearly by one of your colleagues from across the Hudson River at LDEO, Richard Seager.

See Seager et al., 2002. Is the Gulf Stream responsible for Europe's mild winters?, Quarterly Journal of the Royal Meteorlogical Society, v. 128, 2563-2586.

Also see this news release from Columbia U: http://www.columbia.edu/cu/news/03/02/rich...r_research.html

I understand that this post is meant to address variation in one side of the story (the Gulf Stream), but if we are to educate the public away from the Gulf Stream = warm Europe idea, we must give atmospheric circulation its due as the dominant forcing. I agree that THC largely determines the amount of ocean heat transport to the North Atlantic, but this affects both sides of the Atlantic, not just Europe. According to Seager et al. (2002) THC appears to account for the North America vs. Europe winter temperature difference only in the highest latitudes, north of about 60 degrees N, due to the fact that the heat transport limits sea ice cover there.

[Response:You are partly right. Two issues are sometimes confused:

(1) What keeps the eastern side of the Atlantic (say, Britain) warmer than the western side (say, Labrador)? That is largely atmospheric circulation and the general presence of an ocean upstream, i.e. maritime vs. continental air masses, as was affirmed in the paper by Seager et al. (2002). (But note they have a mistake in calculating heat storage vs. heat transport contributions, underestimating the latter, as explained by Rhines and Häkkinen (2003))

(2) What keeps the northern Atlantic and surroundings warmer than the northern Pacific and surroundings? That is indeed largely the ocean heat transport associated with the THC, in lay-persons terminology often referred to as the Gulf Stream.

Hence, traditionally the effect of the ocean heat transport is illustrated by comparing temperatures on the eastern side of both oceans: e.g. the classic review paper by Weaver and Hughes (1992) compares Bodö (Norway, 67°N) with average January temperature of -2°C to Nome (Alaska, 65°N) with average January temperature of -15°C. Another way to look at data is to plot the deviation of temperatures from the zonal average, as in Fig. 3 here.

A good way to estimate the effect of the thermohaline part of the heat transport is to shut it down by dumping a lot of freshwater into the north Atlantic in a climate model, which stops deep water formation there. Two examples are shown in the figures in Gavin's post; it leads to several degrees cooling over Europe but also on the western side of the Atlantic. Hence, the Gulf Stream does contribute a lot to Europe's warmth, and I see no reason why anybody would want to "educate the public away" from this idea. - Stefan]

14 Thanks for the reply, Stefan (comment #3). I have downloaded and skimmed the Rhines and Hakkinen (2003) response to Seager et al. (2002), but it will take a more carfeul reading for me to digest its contents. On first glance, it does seem to present meaningful challenges to certain areas of Seager et al.'s approach. I'd be interested to hear if Seager has responded in kind - anybody know?

Anyway, to address a couple points in your response to my previous comment ...

1) The data you cite from Weaver and Hughes (1992) merely supports my last statement (paraphrased from Seager et al., 2002), that THC (or more specifically, the North Atlantic Drift) keeps European latitudes north of 60 deg N warmer than other land areas at those same latitudes. So, while it is instructive to say that the North Atlantic Drift, which is indeed a remote branch of the Gulf Stream, has a dominant influence on keeping Scandinavia warm, I'm not convinced that the same can be said for it in terms of keeping Britain (between about 50-60 deg N) warm, as Gavin implies in the original post. Comparing temperatures in Britain to temperatures in British Columbia, Canada would be more meaningful.

[Response: That would be more meaningful, and it does give the same qualitative answer. Britain is around 5 degrees warmer than the mean for it's latitude, BC is not. Look at Stefan's excellent fact sheet, and as he points out above figure 3 in particular. -gavin]

[Further Response: It is true that the effect is strongest in high latitudes. Have a look at Fig. 10 of Seager et al. 2002, which shows the January cooling in their model after switching off ocean heat transport. This cooling exceeds 20 ºC in northern Norway, and it is "only" 3-6 ºC in Britain, Germany and more generally in central and southern Europe. Clearly much less than in the far north, but clearly not peanuts. Other models get similar results. So I think one should say: the Atlantic ocean currents keep the whole of Europe warmer than it would otherwise be by several degrees, with the effect getting stronger the further north you go. - Stefan]

2) In terms of "educating the public away" from misleading ideas, well, I think that is part of our job, of pursuing scientific truth and honestly reporting our findings about the natural environment. That said, however, I could have chosen my words more carefully in my previous comment. In more precise terms, I don't think it is necessary to completely disavow the public of the notion that the Gulf Stream contributes to western Europe's mild climate; rather I would argue that we need to emphasize "contributes" over "determines" and include the atmospheric side of the story, which is at least as important as the oceanic side.

[Response: Part of the education has to be differentiating the factors that contribute to current climate conditions (i.e. the 33 C greenhouse effect, the atmospheric poleward heat transport), and the things that are most likely to contribute to variations. For instance, there is no evidence that, with the current configuration, atmospheric heat transports have vastly different modes of behaviour - and so they are unlikely to suddenly flip into a new state. However, such changes have been modelled (and observed in the climate record) for the ocean transports. - gavin]

32 Thanks Gavin, I get the point (in your response to my comment #14) that your intention here is to discuss changes in the ocean/atmosphere system that could cause a cooling of European climate, and that both observational and model evidence point to a weakening of THC as the most likely candidate. Actually, as an ocean/climate interactions student, I am happy to say that I basically agree with you on this. My point, which I hope you would likewise concede, is that even with a weakened or shut-off THC, western Europe would still remain warm relative to other land masses at the same latitudes (possibly even warmer than British Columbia, as it is now), based primarily on atmospheric circulation patterns. I am afraid that these news broadcasts, movies, etc. may give the public the impression that if the Gulf Stream weakens, western Europe will equal Siberia (climatologically speaking), and that is simply not true. After all, as you say, the Rockies aren't going anywhere anytime soon, so regardless of what happens to THC, the persistent crest in the Jet Stream over the eastern North Atlantic and western Europe should continue to bring warm (though perhaps not as warm) maritime air to the northwestern part of the continent.

[Response: I am happy to concede that. In fact these two media stories were the first I have ever seen in which Siberia is quoted as potential outcome. That is not on the cards. -gavin]

Edited July 14, 2006 by trevw