Learning About The AMO

Guest Post by Willis Eschenbach

I must admit to being greatly bemused by Michael Mann’s new (and sadly, paywalled) opus magnum about the Atlantic Multidecadal Oscillation (AMO), sometimes called Atlantic Multidecadal Variability (AMV). Here are a couple of quotes from our boy on the subject, emphasis mine:

Mann 2011:

“The AMO, defined as a 40-60 year timescale oscillation originating in coupled North Atlantic ocean-atmosphere processes, is almost certainly real“

and

“This is a key finding of Knight et al (2005) (of which I was a co-author) as well as Delworth and Mann (2000) [the origin of the term ‘Atlantic Multidecadal Oscillation’ (AMO) which I coined in a 2000 interview about Delworth and Mann w/ Dick Kerr of Science].”

followed by Mann 2021

“Two decades ago, in an interview with science journalist Richard Kerr for the journal Science, I coined the term the “Atlantic Multidecadal Oscillation” (AMO) to describe an internal oscillation in the climate system resulting from interactions between North Atlantic ocean currents and wind patterns.  … Today, in a research article published in the same journal Science, my colleagues and I have provided what we consider to be the most definitive evidence yet that the AMO doesn’t actually exist.”

I do enjoy Mann’s implication that he was the discoverer of the AMO phenomenon, when in fact it had been described in detail in 1994 by Schlesinger and Ramankutty, six years before the publication of Delworth and Mann. Also, in the linked Kerr article in Science that Mann refers to above, despite discussing the name “Atlantic Multidecadal Oscillation” in detail, Kerr never says that Mann named the phenomenon … but I digress.

First, what is the AMO? It is a slow temperature swing of the Atlantic, most visible in the North Atlantic. Here’s a graphic of the oscillation.

Figure 1. Long AMO, from NOAA. This shows a period of about 65 years. There are various instrumental versions of the AMO data. This is the longest instrumental version of the AMO held by NOAA, starting in 1856.

Since the first description of the AMO in 1994, the phenomenon has been extensively studied by any number of scientists. A search on Google Scholar shows 31,300 web pages discussing the AMO. So why does Michael Mann now claim it’s not a natural variation of the Atlantic?

Because “state-of-the-art” climate models say so … his study starts like this:

An analysis of state-of-the-art climate model simulations spanning the past millennium provides no evidence for an internally generated, multidecadal oscillatory Atlantic Multidecadal Oscillation (AMO) signal in the climate system and instead suggests the presence of a 50- to 70-year “AMO-like” signal driven by episodes of high-amplitude explosive volcanism with multidecadal pacing

(Protip—any time someone starts out by talking about “state-of-the-art climate models” you can safely ignore their claims … but again I digress.)

Mann’s claim in his new paper, “Multidecadal climate oscillations during the past millennium driven by volcanic forcing“ (paywalled), is that in preindustrial times what people have been calling the “AMO” was actually a stable Atlantic that was being forced by sporadic volcanic eruptions that just happen to have the same frequency as the AMO. But then that volcanic forcing has died out in modern times, and just in the nick of time volcanic forcing has been replaced by anthropogenic forcing … funny how that works. In M. Mann’s world, it’s always the humans who are to blame.

In any case, I thought I’d see what I could learn from the data in both the instrumental and proxy AMO records, along with the volcanic records discussed by Mann. To start with, here’s the Amman et al. dataset that Mann et al. used of 61 tropical eruptions that they say drove the AMO before modern times. I’ve shown the eruptions as vertical lines. On top of these volcano lines, I’ve overlaid several of the empirical modes of a Complete Ensemble Empirical Mode Decomposition (CEEMD) analysis of the eruptions, showing the various longer-term cycles in the data.

Figure 2. Tropical volcanic eruptions, and various CEEMD modes.

Here’s the thing about signals. As the brilliant mathematician Joe Fourier showed way back a couple of centuries ago, any signal can be decomposed as the sum of underlying signals of various periods. CEEMD is like Fourier analysis, except it doesn’t break a signal down into regular sine waves. It breaks a signal down into underlying signals that can change over time, as you can see above.

Now, is there a cycle in the eruption data similar to the ~ 65 year period of the AMO? Well … kinda. But since each and every signal can be broken down into underlying signals, it may just be by chance. The underlying signals have to have some period, and it might just be fifty to sixty years, as in the volcanos.

So that’s the volcanos. How about the proxy records of the AMO? The main one that is discussed by Mann is the Wang et al study, “Internal and external forcing of multidecadal Atlantic climate variability over the past 1,200 years“. The data is available here. It’s based on “a network of annually resolved terrestrial proxy records from the circum-North Atlantic region.” In that study, Wang et al. distinguished between what they called “AMV”, Atlantic Multidecadal Variability”, and the AMO. They said that something like 30% of the variability of the AMV was from volcanoes, and when that’s taken out we’re left with the AMO. Me, I doubt that, because modern volcanoes show little effect on the AMO. I also wanted to see how well the eruptions matched up with their data, so I’ve used their raw “AMV” data.

First I looked at how well the Wang proxy records matched the instrumental records shown in Figure 1. I’ve also added in the 50-60 year empirical mode of the CEEMD analysis of the Amman eruption records shown above in Figure 2.

Figure 3. Two AMO records and one eruption record, 1856 to present.

We see a couple of things in Figure 3. First, the Wang paleo proxy AMV (red) is very close to the modern instrumental AMO (blue).

However, the Amman eruption data is a quite poor match to the modern AMO data. This is no surprise. Look at Figure 1. If you don’t know which year the huge Pinatubo eruption occurred, you couldn’t tell it from Figure 1.

Next, I looked at the longer term view of that same data. Figure 4 shows that result.

Figure 4. Two AMO records and one eruption record, 800 to present.

Again, some interesting things in Figure 4. First, the average length of the cycles in the Wang paleo AMV is 65 years, which matches the modern data.

However, as in the modern period, there’s a very poor fit between the Amman eruption data and Wang paleo data. Among other things, the period of the eruption data averages 55 years, not the 65 years of either the Wang paleo data or the modern instrumental data. So although at times it matches up with the Wang data, it goes into and out of sync with both the instrumental AMO and the Wang AMV data.

So … how did Mann et al. come to their conclusions? As mentioned above, computer models …

The CMIP5 Last Millennium multimodel experiments provide a pseudo-ensemble of N = 16 simulations driven with estimated natural forcing (volcanic and solar, with minor additional contributions from astronomical, greenhouse gases, and land-use change) over the preindustrial period (the interval 1000 to 1835 CE is common to all simulations). We estimate the forced-only component of temperature variation by averaging over the ensemble, based on the principle that independent noise realizations cancel in an ensemble mean.

(In passing, let me note that it is certainly not always true that averaging a number of model outputs means that the “noise realizations cancel”. But again I digress …)

I rather did like the idea of a “pseudo-ensemble”, however … is that a bunch of random computer models hanging out on a street corner smoking cigarettes and pretending to be an ensemble? But I digress …

And what were their conclusions (emphasis mine)?

The collective available evidence from instrumental and proxy observations and control and forced historical and Last Millennium climate model simulations points toward the existence of externally forced multidecadal oscillations that are a consequence of competing anthropogenic forcings during the historical era and the coincidental multidecadal pacing of explosive tropical volcanic activity in past centuries. There is no compelling evidence for a purely internal multidecadal AMO-like cycle.

His claim is that for about eleven centuries, “explosive tropical volcanic activity” made it look like there is an AMO. And coincidentally, just when the volcanic forces left off, a competition between CO2 and sulfate forcings caused the AMO swings.

You’ll forgive me if, given what I see in the Figures above, I don’t find that argument even slightly compelling.

Finally, this is what I love about studying the climate. The science is far from settled, and that gives me the opportunity to learn something new from every paper that comes out.

Here on our dry northern California coastal hillside, rain is forecast starting tomorrow morning and lasting two days. However, around here, rain forecasts even twelve hours out are sometimes way wrong, and it’s generally true for rain forecasts three or four days out. Funny thing about chaotic systems. They tend to be … well … chaotic.

[NOTE: It’s now “tomorrow morning” when the rain was supposed to start … bright sunlight and not a cloud in the sky. Gotta love chaotic systems.]

Seems like out here in the real world, the modelers don’t have that whole “noise realizations cancel” deal completely worked out … but I digress.

My best regards to all, skeptics and mainstream folks alike,

w.

PS—I sign everything I write with my initial, “w.”, and for the same reason I choose my words very carefully—because I wrote them, I take ownership of them, and I know that it is always possible I will be called upon to defend them. However, I can’t defend your interpretation of my words. So when you comment, please quote the exact words that you are discussing. This avoids endless misunderstandings.