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The alternative Mannian oscillation

Nic Lewis has a new, long and rather technical post up at Climate Audit. He's looking at Michael Mann's latest paper in GRL which claims that the standard way of calculating the Atlantic Multidecadal Oscillation is flawed. In short, Nic has shown that Mann is wrong. What follows here is an attempt at a layman's summary for those who fear to negotiate the longer version.

The Atlantic Multidecadal Oscillation (AMO) is an ongoing series fluctuations in the mean sea surface temperature of the North Atlantic ocean. Both northern hemisphere and global temperatures are correlated with the AMO, and around a third of the global warming since the mid 1970s might be due to the strengthening AMO rather than to the increase in greenhouse gases.

Mann's new paper in GRL claims that the normal way of calculating the AMO - by removing the trend from the sea temperature records - is flawed. But his results turn out to depend on a series of clever tricks. He defines the AMO as, in essence, the smoothed difference between model simulations and observations. Of course this isn’t true, because we know that the models aren’t perfect and the difference must therefore also be a function of all the things the models don’t simulate correctly as well. But by ignoring these factors Mann is able to explain away a multitude of sins. So the hiatus in surface temperature rises since the end of the last century is explained by the AMO going negative (in Mann’s estimation at least). And in the final decades of the twentieth century, when surface temperature rises were relatively rapid, in line with the models, the AMO was, by Mann’s definition, broadly neutral. The possibility that the models were wrong but that this was hidden by a positive AMO is brushed under the carpet.

Mann’s main illustrations use a simplified climate model called an EBM, which encapsulates a rather high climate sensitivity of 3°C and a very high transient climate response of 2.8°C. In other words it would be expected to produce very rapid warming. However, Mann still manages to get it to match the observations (he uses the average northern hemisphere temperatures, not global ones) quite closely, by using  values for things like the effect of greenhouse gases other than CO2 that are very different to current best estimates and therefore highly implausible. This close fit, particularly after the data has been smoothed, means that Mann's AMO is relatively small from peak to trough. Moreover it is declining in magnitude after 2000, in sharp contrast to the standard AMO graph, which is still rising over this period.

Mann then tries to show that his method is the correct one. He does this by doing a series of simulations, adding a dummy AMO signal (in the shape of red noise) to the EBM-simulated temperature series. He shows that when he processes each of these through his smoothed-difference algorithm he gets out what is in essence a random signal – in other words the dummy AMO but smoothed.

Next he processes the same set of simulated-NH-temperature-plus-red-noise series through his version of the standard AMO methodology (which is based on average NH, not just North Atlantic, temperatures). He shows that after smoothing you then always get the same thing out: something that looks just like the smoothed AMO you get from the observations. Aha! he seems to say, the standard AMO is an artefact – it doesn’t matter what’s in there, you always get the same signal out.

Nic Lewis observes a problem with this, however. There are two keys to understanding what is happening. Firstly, under the standard AMO methodology and with only a couple of cycles of AMO under consideration, the amount of noise that Mann is adding is too small to show up much in the final reckoning. Secondly recall that, after smoothing, there is a very close match between Mann’s simulated temperature series and the observations. So when you process the simulated temperatures plus a bit of noise that is too small have much impact through Mann's version of the standard AMO algorithm you do indeed get something that looks like the AMO calculated on the observations. To all intents and purposes the simulation and observations are the same after smoothing; the noise is a red herring.

As Nic points out, an experiment described in Mann’s own SI makes this point clear. Here Mann shows an alternative set of results in which he simulates the temperatures using a different set of assumptions on volcanic aerosols. This makes the simulated temperatures rather different to the observations. If you then do the experiment adding noise as before, you can see the problem. Using the standard algorithm, the AMO signal extracted for simulated-temperature-plus-dummy-AMO-noise looks very much like that extracted for simulated temperature alone; but the signal extracted from the observations is very different. Clearly, Mann’s argument that the standard AMO signal is an artefact is wrong.

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Reader Comments (40)

Flaws or errors..... meh ....I suspect the passage through peer-review to publication was smooth as silk.

May 19, 2014 at 2:13 PM | Unregistered CommenterJack Savage

"We have to find a way to get rid of the AMO"

May 19, 2014 at 2:22 PM | Unregistered CommenterDaveO

Is Mann taking over from Hansen as the "Reverse Cassandra" of choice?


May 19, 2014 at 2:26 PM | Unregistered CommenterTom O'Connor

Of course Mann is wrong. If loving global warming is wrong, he don't wanna be right.

May 19, 2014 at 2:32 PM | Unregistered Commenterstan

Mann tortures data. So what's new?

May 19, 2014 at 2:34 PM | Unregistered CommenterJeremy Poynton

I wonder who peer reviewed the paper in GRL? His mate Gerald North? Or?

The Hockey Stick treatment guaranteed to give you what you want!

May 19, 2014 at 2:38 PM | Unregistered CommenterCharmingQuark

EBM.... Extreme Bowel Movement....obviously a huge bunch of crap ! Like most of Mikeys "work".

May 19, 2014 at 3:02 PM | Unregistered CommenterSweet Old Bob

"Both northern hemisphere and global temperatures are correlated with the AMO, and around a third of the global warming since the mid 1970s might be due to the strengthening AMO rather than to the increase in greenhouse gases."

Southern hemisphere too.


The so-called Atlantic Multidecadal Oscillation (AMO) is an ongoing series fluctuations in the mean sea surface temperature of the EARTH and should be called Global Climatic Oscillation (GCO). The 'secular' linear trend is just another longer quasi-cycle, so it's not only multidecadal. ALL of the global warming since the mid 1970s is due to the strengthening GCO rather than to the increase in greenhouse gases. GCO is solar/orbital.

May 19, 2014 at 3:04 PM | Unregistered CommenterEdim

I'll wait for Nick Stokes to give the definitive answer ;<)

May 19, 2014 at 3:09 PM | Registered CommenterPhillip Bratby


Indeed it would be interesting to see who the peers were that reviewed the paper. Dare I say it but I suspect the "team" has had some involvement in getting the paper through to publication.

Climate science must come clean. Daylight is the best disinfectant therefore if like to start seeing names named because only then will junk science stop being published (because no one will want to be associated with poor work!).

Oh and start archiving data AND make it available!


May 19, 2014 at 3:11 PM | Unregistered CommenterMailman

Summing up, it appears that:
(A) Mann accepts the reality of the pause (by 'showing' that it is easily explained)
(B) Mann shows (unlike Dame Slingo) that models and observations do match (and he's done it)
(C) Mann demonstrates that the ten or so alternative explanations for the current 'hiatus' brought to the game by his fellow team members must be moot (and hence rebutted)
(D) He really does deserve that Nobel Prize that he never got!

May 19, 2014 at 3:18 PM | Unregistered CommenterRoyFOMR

Indeed Philip, Nick Stokes will be along any minute now to defend the garbage.

May 19, 2014 at 3:24 PM | Unregistered CommenterJohn B

For what it’s worth, this Wood for Trees graph of Hadcrut4 versus AMO since 1860 shows a fairly remarkable correlation:

May 19, 2014 at 3:37 PM | Unregistered CommenterDoug Brodie

But his results turn out to depend on a series of clever tricks.
There's a surprise.

May 19, 2014 at 3:56 PM | Registered CommenterMike Jackson

So does this mean the heat never really was hiding in the deep ocean and was just circulating in the Atlantic at different times than we thought? Is the natural conspiracy over and can Gavin and Santer now go back to using their old solar and aerosol forcings? Were Kosaka and Ping Xie looking in the wrong ocean?

May 19, 2014 at 4:01 PM | Unregistered CommenterDaveO

The smoothed difference between obs and mods

Wait amoment? I thought "innovative" warmish
Insights indicated these were UNrelated

And now we can,like, add and subtract them
From one another

Anyway if the Penn dunc-ariat says , then we must
Believe it(i don't wNt to be sued for denying their
IP rights or something)

May 19, 2014 at 4:09 PM | Unregistered CommenterPtw

Mike Jackson,

Clever tricks AND making your own assumptions and changing the meaning of various things to fit your theory. Something Mann and co ARE world experts in.


May 19, 2014 at 4:32 PM | Unregistered CommenterMailman

Nice summary of Mann's statistical trick. But it omits the most important thing Nic found.
The alternative forcing results (Crowley aerosols) that show Mann's methods and conclusions are wrong are published in the SI, and exist in the published MatLab code. They ( what may become the infamous blue lines) were removed only from the figures in the main paper. Here we have Mann hiding the fail rather than the decline, and again doing so incompetently. Based on this alone, the paper should be revised or retracted. A pure, clear, self contained violation of Feynmanns dictum about good science.

May 19, 2014 at 4:38 PM | Unregistered CommenterRud Istvan

Perhaps it's wishful thinking, but surely at some point, more and more climate scientists have to realise that having to endlessly manipulating data to try and get observations to fit the theory must mean there is something wrong?

This is the fourth or fifth paper recently which adds nothing new except in the way the data is added, subtracted, smoothed etc. Nothing new is being added here.

May 19, 2014 at 4:41 PM | Unregistered CommenterTim Hammond

I did something like this in my PhD thesis, using a least squares fit to prove a slope I had introduced earlier on. It didn't matter but it was picked up by the external examiner, who was a numbers' expert.

Mann appears to be thrashing about, trying to create a new scientific reputation. Thank the Lord I never published my first mistake in a major subject, then become famous because of it, then had to eat humble pie in public.

Eat the humble pie early on, in private, and double check everything that follows!

May 19, 2014 at 4:46 PM | Unregistered CommenterTurnedoutnice

This work by Lewis looks really 'unhelpful', the new word in climatism for 'excellent, on-the-ball, timely, and important'.

May 19, 2014 at 4:52 PM | Registered CommenterJohn Shade

Edim and Doug Brodie

Spot on

Curious how your comments appear to have generated no interest

Presumably this means there is no point in trying to estimate climate sensitivity, unless you believe CO2 was influencing AMO from at least 1860

May 19, 2014 at 5:24 PM | Unregistered CommenterJeremy Shiers

May 19, 2014 at 3:37 PM | Unregistered CommenterDoug Brodie

Doug or anyone, is there any good theory or explanation of what might cause the AMO and the frequencies it appears to 'oscillate' at??

May 19, 2014 at 6:39 PM | Unregistered CommenterRob Burton

Jeremy Shiers, Rob Burton:
I’m surprised myself that the very close correlation between global temperatures and the AMO since records began in 1860 has apparently not permeated public awareness. I chanced upon it when arguing with AGW believers on the very poor correlation between global temperatures and atmospheric CO2, which shows only a paltry 20 years of positive correlation over the last 70 years. I’ve penned a few such thoughts in my paper “20 reasons to object to a wind farm”, accessible at As for the actual science, I have no idea what causes the AMO or how it actually affects global temperature. I defer to commenter Edim on that score.

May 19, 2014 at 7:39 PM | Unregistered CommenterDoug Brodie

But his results turn out to depend on a series of clever tricks.

One gets a sense of déjà vu with Dr Mann. From 18 months ago.

May 19, 2014 at 9:51 PM | Unregistered CommenterKevin Marshall

"Doug or anyone, is there any good theory or explanation of what might cause the AMO and the frequencies it appears to 'oscillate' at??" --Rob Burton

Climate god angry. That explain everything.

How you like Mike's GRL trick, Rob?

May 19, 2014 at 10:56 PM | Unregistered Commenterjorgekafkazar

Rob burton


A short discussion of AMO.

May 19, 2014 at 11:23 PM | Unregistered CommenterEntropic man

Damn. I must be thick. I can't even understand the simplified explanation that Bish gives.

For example, Bish says:

"Mann's new paper in GRL claims that the normal way of calculating the AMO - by removing the trend from the sea temperature records - is flawed."

What does it mean, "removing the trend from the sea temperature records?" I can understand the words of course, but what do they mean? What is "the trend" referring to? Why does removing it allow calculation of the AMO?

You very rarely get an explanation of things that people take for granted you will understand, perhaps because it's so obvious to them. That's no criticism of Bish, you understand; just a frank observation.

Can what Nic says be distilled in language that assumes no prior understanding? My bet is, it can, but I wonder if anyone who does understand it can identify with the mindset of someone like me and anticipate what difficulties we might have. Now and then, Willis Eschenbach manages this trick, but hardly anyone else I can think of.

May 20, 2014 at 12:10 AM | Unregistered CommenterMichael Larkin

The possibility that the models were wrong but that this was hidden by a positive AMO is brushed under the carpet.

History will brush Mann under the carpet of deceipt.

May 20, 2014 at 12:29 AM | Unregistered CommenterStreetcred

Is this the 15th or 16th explanation for the pause that some of the truly faithful still can't bring themselves to admit even exists?

May 20, 2014 at 12:53 AM | Unregistered CommenterJamesG

If, as Nic and Rud indicate, Mann removed clear contradictory evidence to his hypothesis in the final version of the paper then I suspect Mark Steyn's lawyers will be very interested in a contemporaneous example of how Mann operates. It will need to be carefully verified, but if true Mann's suit is toast.

May 20, 2014 at 2:04 AM | Unregistered Commenterbernie1815

Many thanks to the Bishop for providing a Nic Lewis for dummies service. Hats off to Mann for providing the code and data on which his paper was based.

The Mann approach seems doubly backward in that AMO is directly measured so estimating via model is a step to the rear. The second step backwards is using the ensemble of models instead of a specific model for his calculation. You can improve an estimate by adding independent sensors (key word "independent") but a climate model is not a sensor and the models within an ensemble are not independent.

Climate science needs to focus more on direct measurements and experimentation and less on modeling.

May 20, 2014 at 2:53 AM | Unregistered CommenterSteve koch

Models are a useful tool, a set of logic and data that encapsulates what you think you know about whatever you are modeling. By comparing reality to the models predictions, you understand where you need to improve your understanding and knowledge of the modeled domain and can modify your models accordingly. The problem with using an ensemble for this purpose is that an ensemble is not a cohesive, coherent encapsulation of knowledge, it is just a grab bag, hodgepodge of different ideas that will almost certainly contain self contradictory elements. No person understands the details of how the ensemble models work. It would not be surprising if even within individual models, there are parts of the model software that nobody on the team supporting that model understands.

IIRC, there are over 100 climate models in the ensemble. Can all those models be first rate? Maybe there needs to be a Premier League of climate models. Maybe a better approach would be to pick an all star team of modeling (people like James Annan) and have those people work together on the best model in the world.

May 20, 2014 at 5:39 AM | Unregistered CommenterSteve koch

I was wrong about Nick Stokes. Over at WUWT, his only comment about the Nic Lewis article was to try to divert attention to criticsm of the Bengtsson paper.

May 20, 2014 at 6:51 AM | Registered CommenterPhillip Bratby

Did Mann coin the term "AMO"?

an aside, while Michael Mann has claimed priority for coining the name "AMO" this claim is contradicted by several sources/considerations:

[h/t DCA at Climate, Etc.]

May 20, 2014 at 7:19 AM | Registered CommenterSkiphil


it seems that in this paper there are other examples of "how Mann operates" ... he never seems to learn. As noted by Nic, the code doesn't actually produce the figures in the paper. Moreover, he is using different end-point filtering (smoothing method) for the differenced and detrended AMO signals. For the first he's using his "standard" Mann08 method, but for the latter he's introduced a modified version ("minimum roughness" option removed). Having seen dozens of cases like this in his papers, I'm confident this is yet-another-case where he's "improved" his results by an odd post-hoc "fix". Conveniently, in his code all variable names for differenced AMO contain the word "good" and the detrended ones the word "bad". :)

May 20, 2014 at 7:59 AM | Unregistered CommenterJean S

Doug Brodie mused: “For what it’s worth, this Wood for Trees graph of Hadcrut4 versus AMO since 1860 shows a fairly remarkable correlation:”

The AMO is usually just a de-trended version of a temperature plot, so the correlation isn't a coincidence, here being detrended HadCRUT4 in comparison:

May 20, 2014 at 8:38 AM | Unregistered CommenterNikFromNYC

A sense of proper perspective is required here to put the entire concept of ocean “oscillations” in perspective. I don't see any regular cycle in the Greenland ice core temperature plot based on oxygen isotope ratios that are affected by temperature, just little glitches up and down huge chaotic peaks in an overall cooling world:

Calling a couple of likely chaotic glitches in a ramp up before a likely plunge hardly passes the laugh test especially when that oscillation is merely a detrended temperature plot over a mere two potential cycles. However, a damn good proxy for the global average temperature is the 350 year long Central England Temperature highly influenced by the Atlantic Ocean itself, but this plot kills the simplistic idea of an AMO dead, since there is no periodicity:

This should obviously be noted in any paper on the so-called AMO. It seems to be a case of word games, merely, verbal categories for ill defined oscillations.

May 20, 2014 at 8:53 AM | Unregistered CommenterNikFromNYC

A few points about the Greenland temperature graph that you posted;

1) ~1350 to 1150 BC was a very cold period in the temperate zone that caused the collapse of many civilisations.
2) The period ~380 to 540 AD was the Dark Ages cold period in Europe.
3) Medieval Period warmth in Europe was well evident in the 8th century.
4) The lack of variation from 1300 to 1800 AD is rather suspect in light of the temperature variability through the LIA.

As for the AMO:
From the premise that a warm AMO is directly associated with more negative NAO/AO conditions, there is no rational basis to any claim that increased forcing of any kind will give a warm AMO, as increased forcing produces more positive NAO/AO conditions.

May 20, 2014 at 5:17 PM | Unregistered CommenterUlric Lyons

It seems obvious that efficiently heating the earth is mostly about heating the oceans efficiently, which is mostly about maximizing the amount of direct sunlight that hits the oceans. This is cuz direct sunlight can penetrate deep into the ocean where the heat can be "captured" while IR warms only the surface of the ocean so that heat can easily escape back to the atmosphere.

May 22, 2014 at 8:22 PM | Unregistered CommenterSteve koch

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