"Explaining" a Positive NAO

Trouet et 2009 posit a positive NAO as the “explanation” of the Medieval Climate Anomaly, pausing only briefly to ask what might have caused a centuries long (“temporally pervasive”) positive NAO, falling back on an arm-waving attribution to a stronger Atlantic meridional overturning circulation:

The persistently strong winter MCA NAO and its weakening during the LIA raise question about the mechanism responsible for producing such a temporally pervasive atmospheric state over the North Atlantic, as well as MCA-LIA climate anomalies elsewhere … Stronger westerlies associated with a prolonged positive NAO phase may have enhanced the Atlantic meridional overturning circulation (AMOC) (27), which, in turn, generated crossequatorial salinity and SST anomalies in the tropical Atlantic and a related northward migration of the intertropical convergence zone (28). An enhanced AMOC may have accommodated a constructive feedback mechanism, proxy evidence for which is provided by North Atlantic records (Fig. 4B and table S2), that reinforced the La Niña–like conditions in the tropical Pacific (22).

Trouet et al 2007 here recycle similar language from Esper the Non-Archiver 2007:

Association of a strengthened THC with a poleward shift of westerly storm tracks was recently suggested [Seager et al., 2007] to be teleconnected with persistent La Nina-like conditions in the tropical Pacific [Cobb et al., 2003], the major driver for medieval drought in N America [Cook et al., 2007; Graham et al., 2007]. Similar considerations might also account for the persistently drier conditions now reconstructed for NW Africa.

Anyway, let me compare excerpts from two recent diagrams. On the right is Trouet’s diagram of a positive NAO phase, showing strong westerlies, Azores high and dry Morocco. On the left is a similar diagram that I located in a non-peer reviewed Internet publication (though by a scientist with a longer resume than Trouet). Both diagrams clearly associate dry Morocco (and a wrm MWP) with positive NAO.

However, the author of the left diagram did not attribute a persistently positive NAO to a strong Atlantic circulation, but to a weaker Atlantic circulation.

A weaker Atlantic Ocean Thermohaline Circulation (THC) and weaker Surrounding Antarctic Subsidence (SAS) or Meridional Overturning Circulation (MOC = THC + SAS) for the period of a decade or a decade-and-a-half can bring about a gradual global upper ocean warming. When a weaker MOC occurs, the Southern Hemisphere tropical oceans upwell less cold water into the thermocline, and there is generally less global rainfall. With a lag of 5-10 years a modest globe warming ensues.

Here’s the rest of the left diagram, this time associating a positive NAO and weaker Atlantic Ocean Thermohaline Circulation with El Nino instead of La Nina.

I think that it’s worth noting that the argument in favor of a strong Atlantic MOC as presented by Trouet et al 2009 in Reader’s Digest for Scientist Science contains no levels of proof greater than the internet article attributing the same phenomenon to a weak Atlantic MOC. Even the arm-waving in Trouet is pretty perfunctory, no elaborate choreography.

At this time, I have no views on which theory is more strongly supported – whether persistently positive NAO due to strong Atlantic MOC; or whether it is due to weak Atlantic MOC. HOwever, it’s interesting that both camps seem content with the idea that changes in ocean circulation underlie the Medieval Climatic Anomaly and both camps associate it with a positive NAO and dry Morocco.

To the extent that Cobb’s Pacific corals – (22) in the first quote – are relied on as evidence for a “cool” medieval Pacific, it seems to me that they are a very thin reed, given that it appears that they may also be explainable by a slight northward migration of the ITCZ (see my post on this) – and such slight northward migration of the ITCZ is already contemplated in the Trouet “explanation”.

The diagram on the left occurs in William Gray 2009, online here. Given that Gray and Trouet agree on key elements – positive NAO, dry Morocco,…- I’m a little puzzled as to how Trouet et al purport to refute any of Gray’s views. Just another climate mystery, I guess.

Reblog this post [with Zemanta]


  1. RGM
    Posted Apr 17, 2009 at 11:13 AM | Permalink

    Dear Steve,
    It has well explained what causes positive or negative NAO phases in the book «Global Warming: Myth or Reality?», by Marcel Leroux. North Pole (including North Greenland) situation is the primary cause. In positive NAO phase the NP is in cold situation (more ice mass). In negative one the NP is warm (less ice). The second cause is the rapid or slow mode of the Mobile Polar High (MPH is the base of Leroux theory). MPH is strong (rapid mode) with positive NAO phase and weak (slow mode) with negative NAO phase. Strong MPHs transport more cold air (high pressure), in their meridional changes, to Azores islands. In this case the pressure atmospheric air is higher on Azores. When there is a high gradient between Poles-Tropics the NAO is in positive phase and the MPH is strongest. It is the actual situation. Then we are observing a cold scenario and not a warm scenario. Excuse me; my English is not good enough.

    • Steve McIntyre
      Posted Apr 17, 2009 at 12:12 PM | Permalink

      Re: RGM (#1),

      I’m not in a position to say whether one point of view or the other is correct. I’m just observing the disagreement over a seemingly innocuous issue.

      #3,4. Please – no piling on with complaining. I understand the frustration, but please remember that there are non-angry readers as well.

  2. Dave Dardinger
    Posted Apr 17, 2009 at 11:26 AM | Permalink


    Didn’t you use that Gray drawing in another post a while back? It looks familiar.

  3. Hank
    Posted Apr 17, 2009 at 11:34 AM | Permalink

    I happened to see William Gray on C-SPAN a few or couple years ago. His take seemed to be that things started going downhill for climate/weather studies when the federal agencies started budgeting the big bucks for computer modeling and neglecting his style of analysis. It would be interesting to know how decisions happen within the agencies. I believe John Sununu indicated (unwittingly) that he was partly responsible. He stated at the Heartland Institute conference that he was instrumental in raising the budget, circa 1989, for climate change research from a couple hundred million to 1.5 billion. The reason seemed to be that computer models needed more hardware speed. This is not to say that Sununu didn’t have good insights into the shortcomings of modeling, but nonetheless it seems like it probably moved whole agencies in a bad direction.

    • Andrew
      Posted Apr 17, 2009 at 5:42 PM | Permalink

      Re: Hank (#3), Gray is a guy you sees it as very observation analysis versus modelers. However, raising the money for improving models was not inherently a bad idea. But, well, its Government, ya know? Which is why I like one thing Ross says about the “T3 Tax” idea-that it would encourage private sector modeling an research of climate, trying to maximize accuracy.

      This post is hysterical! When Gray makes these arguments, he’s being misleading, but when Mann and pals make these arguments…Well, hey, it’s…

  4. theduke
    Posted Apr 17, 2009 at 11:49 AM | Permalink

    Steve: the only regrettable thing about your website is that it was not around in the 80s and 90s when these studies were beginning to flourish and being bandied about as “conclusive evidence” of AGW.

  5. TAG
    Posted Apr 17, 2009 at 1:11 PM | Permalink

    Is this arm waving really the depth of analysis that is acceptable in current scientific discourse. I wonder if this would be considered acceptable for a structural engineer constructing a building.

  6. JFD
    Posted Apr 17, 2009 at 1:24 PM | Permalink

    snip – nothing to do with this thread

  7. PhilH
    Posted Apr 17, 2009 at 1:37 PM | Permalink

    I wish I may (twice),
    I wish I might (once),
    Maybe someday I’ll get it right.

  8. rt
    Posted Apr 17, 2009 at 2:22 PM | Permalink

    Where’s the evidence for increased AMOC in table S2?

  9. MartinGAtkins
    Posted Apr 17, 2009 at 4:35 PM | Permalink


    Is anyone aware of a problem with the WUWT server? There has been increasing difficulty with wrg access over the last five days or so.

    Although I’m in Thailand, this is not due to any local problems.


  10. Posted Apr 17, 2009 at 4:46 PM | Permalink

    Steve, are Trouet et al arguing that Pacific La Nina cooling or something like it was involved in helping to actively maintain the MWP/MCA? Or are they only saying that a La Nina dominated MCA is a passive by-product driven by the NAO and AMOC?

    Either way, La Nina type conditions should then hold for the duration of the MWP, i.e. centuries. The La Nina we Australians know today does not seem to behave at all like this. The El Nino Southern Oscillation (ENSO) flips between El Nino (warming) and La Nina (cooling) every few years. This problem parallels the question you and others raised as to how the NAO could stay in a warm phase for the several centuries of the MWP.

    A couple of further points about La Nina may possibly be relevant to all this:

    (1) It has recently been suggested that the ENSO is subject to a longer-period multidecadal modulation by the Pacific Decadal Oscillation (PDO). This PDO cycle would cause La Nina to predominate for about 30 years, then El Nino to predominate for another 30 years or so. But this multidecadal cycle would still be far too brief to cause La Nina predominance for the whole MWP.

    (2) On the other hand, the PDO switchings between pro-El Nino (warming) mode and pro-La Nina (cooling) mode correlate rather well with the global temperature rises and falls during the 20th century, including the global warming of 1977-1998 and its recent cessation.

  11. srp
    Posted Apr 17, 2009 at 4:50 PM | Permalink

    What often puzzles me as a layman in reading these climate “explanations” (not just the AGW ones) is how one labeled phenomenon (say, Specified Sort-of-Periodic Oscillation Over Norway or SSPOON) is treated as the exogenous cause of another (say, Dry Air and Cold Summers Over Large Islands or DACSOLI) without much of a story. Perhaps the experts are almost as confused and that’s why they can tell stories that point in different directions.

    How do we know if SSPOON and DACSOLI are even physically coherent entities rather than cherry-picked statistical coincidences? Assuming they are coherent, how do we know whether SSPOON causes DACSOLI, DACSOLI causes SSPOOBN, some third factor causes both of them, or the correlation is chance? Does it even makes sense to think about a direction of causality if these entities are part of some feedback loop like a thermostat, a heater, and the temperature of the room on a cold day where each causes the other?

    The one good thing about the CO2 hypothesis I can see is that it starts from clear physically coherent entities (CO2, sunlight, atmospheric temperature) and gives a clear theoretical direction of cause and effect (modulo the various feedbacks). The same would be true for a solar theory, a volcanic theory, or any other exogenous “forcing” idea. Of course, if the world really isn’t like any of that then I guess I’ll have to be satisfied with these all-endogenous daisy-chain explanations. But that doesn’t mean I have to like it.

  12. kuhnkat
    Posted Apr 17, 2009 at 5:48 PM | Permalink

    Dear RGM,

    you simply described a series of EFFECTS which knock on to other effects. Care to take a stab at what CAUSES those effects??

  13. Willem Kernkamp
    Posted Apr 17, 2009 at 5:55 PM | Permalink

    I thought that Professor Gray’s article gave a nice qualitative explanation of the sub-grid modeling problems of the current GCM’s that may explain the observed differences with vertical temperature. However, his argument against using GCM’s for climate prediction I believe to be incorrect. Specifically, it is not true that an inability to predict weather for more than a few days implies that climate prediction with GCM’s is impractical. The fallacy is that while small differences in initial conditions can cause large variations in weather, they do not cause proportionally large variations in predicted climate. This is so because weather is inherently unstable, while climate is inherently stable. These properties translate directly into the numerical realm. It would be a mistake for the community at climate audit to dismiss the GCM tools because their current state of the art leaves something to be desired.

    A rational approach would be to run the models based on a water vapor distribution that better matches the observations.

    Furthermore, the problem of grid cell size will eventually be conquered, because the top speed of super computers continues to increase at an exponential rate.

    • Andrew
      Posted Apr 17, 2009 at 6:33 PM | Permalink

      Re: Willem Kernkamp (#15), Is it really so unreasonable to suggest that there is some dependence of the evolution of future climate states on those in the past and future? And I suspect that, whether we manage to shrink are grid scales or not, many other problems will still remain.

    • Ron Cram
      Posted Apr 17, 2009 at 7:54 PM | Permalink

      Re: Willem Kernkamp (#15),

      Willem, are you familiar with the book “Useless Arithmetic” by Orrin Pilkey of Duke University? It is an interesting read. He argues that computer modeling of coastal areas have NEVER proven out and that nature is inherently chaotic. He is not a huge opponent of the IPCC’s conclusions, he just thinks the uncertainty should be more publicized to the public and policymakers.

      The GCMs are “tuned,” not dependent on the first principles of geophysics as some would like you to think. You might be interested to see this PowerPoint presentation on building GCMs. Or, you might be better served by googling “GCM tuning.” It could be a real eye opener for you. I have some experience with computer models of the stock market. It was amazing how I could tune them to hindcast passed market behavior almost perfectly, yet they never had any predictive power. If they had, I would be much wealthier than I am today.

      Also, your belief that climate is “inherently stable” is a bit misleading. I think this is pretty well understood by most of the regular readers of this blog. If you believe the story about “We’ve got to get rid of the MWP!” is true, then it is seems part of Mann’s goal with MBH98/9 was to prove climate changed within a very narrow band prior to the industrial age. Mann’s methodological errors, intentional or not, are now exposed. The only firm ground we are left with is the clear evidence ecologically that climate has changed dramatically, both up and down, over the last 1,000 years. Evidence includes farms in Greenland, much higher treelines than today and on and on. Since natural climate variability is obviously greater than accepted by the alarmists, how can you determine how much of the recent warming is natural and how much is from CO2? The entire theoretical basis of the GCMs has crumbled.

  14. TAG
    Posted Apr 17, 2009 at 6:52 PM | Permalink

    In regard to the issue of scientific arm waving, I have read of an albedo effect in the Arctic that is linked to a tipping point in climate change. To be specific, increased warmth will decrease snow over which will decrease albedo causing increased warmth. Hence a positive feedback effect driving more warmth will occur.

    However, during the time of worry in the 1970’s about the coming ice age, I heard of another albedo effect that would drive the world into a period of extreme cold. Decreased warmth would cause an increase in snow cover both is spatial and temporal extent. This would cause an increase in albedo which could cause a decrease in warmth and an increase in snow cover … Hence, a positive feedback loop from albedo change driving less warmth will occur.

    In the 1970s’ this was linked with the growing period of food crops and how a famine was inevitable when food crops could not be grown. There was even a TV special on Canadian television, hosted by the then future Governor General Adrienne Clarkson – on this inconvenient truth.

    So we are living on the razor’s edge. One cold or one warm winter could make all of the difference. It appears to me that arm waving has a long history in the scientific discourse about climate.

    • Craig Loehle
      Posted Apr 18, 2009 at 3:52 PM | Permalink

      Re: TAG (#17), I feel really deprived–there was no course in arm-waving at my univesity…

      • Kenneth Fritsch
        Posted Apr 18, 2009 at 4:34 PM | Permalink

        Re: Craig Loehle (#24),

        I feel really deprived–there was no course in arm-waving at my univesity…

        I thought universities taught identification of arm waving – and spelling.

        • Andrew
          Posted Apr 18, 2009 at 5:04 PM | Permalink

          Re: Kenneth Fritsch (#25), Craig is speaking with a sophisticat’ univuhsity graduate! But now adays I reckon they teach you how to arm wave, not identify it. But hey, its…education. Re: Micky C (MC) (#23), Um, what exactly are you proposing needs Razoring? I don’t quite understand what you are saying.

        • Micky C (MC)
          Posted Apr 19, 2009 at 7:35 AM | Permalink

          Re: Andrew (#26), It’s quite a normal acitivity in building a theory. You can’t have it both ways. Either there is or was a complex process that lead to the MWP that is not detectable today, and this is superseded by the CO2 forcing model, or the interaction is still taking place and that this has a hand in actually driving current temps.
          Or there never was a complex interaction and CO2 caused the MWP.
          If you start with a theory you must apply it to all situations; the more it gets convoluted and ‘special circumstances’ appear, then you cut the theory. Occams Razor: Cut away the more improbable and over-complicated explanations. The subtle point is that the theory can be complex but universally complex; it doesn’t have to be a simple linear cause and effect type theory. THis MWP/MCA explanation is interesting and by logic it must apply today. So where does that leave the other forcing model?

        • Andrew
          Posted Apr 19, 2009 at 8:28 AM | Permalink

          Re: Micky C (MC) (#28), I know what Occams Razor is. I just didn’t get what you thought needed cutting away. Thanks for clarifying.

        • Kenneth Fritsch
          Posted Apr 19, 2009 at 10:45 AM | Permalink

          Re: Micky C (MC) (#28),

          Mickey, you definitely describe a frustration I have with the current treatment of the MWP/MCA. With the evidence presented it seems to remain rather fuzzy – until a summary comment is made where the term MCA is used along with all that that term implies.

          Re: Craig Loehle (#27),

          Craig, please continue bringing on the science and we will interpret the spelling.

        • Craig Loehle
          Posted Apr 18, 2009 at 7:26 PM | Permalink

          Re: Kenneth Fritsch (#25), I studied the sciences, no spelling classes either.

  15. Geoff Sherrington
    Posted Apr 17, 2009 at 7:38 PM | Permalink

    Insofaras it might or might not be important, the question of whether events like the MWA was global or regional is being argued. The available data are not good.

    I suspect that some light could be thrown on the regionality problem by a study of the 1998 temperature peak. There are so many versions of this (satellite versus surface, for example) but basically good data for study. Until the global presence/absence of this peak in various places is explained to the satisfaction of all, I think that the presence/absence of a global MWA will stay an elusive and unsettled topic.

    Is there value in starting with simple examples and good data then pressing the frontier?

  16. Posted Apr 17, 2009 at 8:54 PM | Permalink

    Here’s a graph from Latif et al (2006) which may help the conversation:

    The black line is a measure of the strength of the thermohaline circulation (THC). The shaded curve is a smoothed version of the NAO.

    Basically, changes in the NAO lead changes in the THC by 5 to 20 years. (See Latif for a physical explanation and for the basis of the 5-to-20 year statement.)

    I guess that both Trouet and Gray could be “correct” in their views, depending on the time scale. Trouet may well be right over a century while Gray’s data may support his view over the shorter periods of his hurricane-related studies.

    Whether the NAO could get “stuck” on positive is an entirely different matter. My layman’s conjecture is that the NAO and THC interact in ways which contribute to each other’s cyclic behavior, such that a “stuck” NAO is unlikely.

  17. Posted Apr 17, 2009 at 9:51 PM | Permalink

    Latif’s conjecture on a Pacific/Atlantic connection can be found here . It also speculates on how a strong THC eventually leads to a weakened NAO.

    There is also a relationship between NAO strength and Arctic ice (high winter NAO values tend to flush ice out of the Arctic, I believe). If that is correct then Latif’s Figure 4 may benefit Arctic ice cover in the coming decades.

  18. Ian
    Posted Apr 18, 2009 at 12:33 AM | Permalink

    Interesting new article considering AMO and Sub-Saharan Africa in this context

    BBC summary

    UA Version

    Sorry no link to Reader’s Digest Science article, I don’t go there anymore for scientific info

  19. Micky C (MC)
    Posted Apr 18, 2009 at 10:57 AM | Permalink

    Though Trouet and Gray may be correct there is a more pressing concern. On one hand the appearance of an MWP/MCA is explained by a complex interaction of thermohaline and North Atlantic Oscillations, where as with regards to today these interactions are superseded by a simple “increase in CO2 causes increase in forcings” relationship and temp goes up. So unless they can explain the context of both types of behaviours with regards to universality, at least one of them is not correct. Sounds like a philosophical argument hence Occams Razor time.

%d bloggers like this: