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Tropical SSTs: Natural variations or Global warming?

Filed under: — group @ 11 September 2006

by Michael Mann and Gavin Schmidt

Roughly a year ago, we summarized the state of play in the ongoing scientific debate over the role of anthropogenic climate change in the observed trends in hurricane activity. This debate (as carefully outlined by Curry et al recently) revolves around a number of elements – whether the hurricane (or tropical cyclone) data show any significant variations, what those variations are linked to, and whether our understanding of the physics of tropical storms is sufficient to explain those links.

Several recent studies such as Emanuel (2005 — previously discussed here) and Hoyos et al (2006 — previously discussed here) have emphasized the role of increasing tropical sea surface temperatures (SSTs) on recent increases in hurricane intensities, both globally and for the Atlantic. The publication this week of a comprehensive paper by Santer et al provides an opportunity to assess the key middle question – to what can we attribute the relevant changes in tropical SSTs? And in particular, what can we say about Atlantic SSTs where we have the best data?

This role of SST remains pivotal in understanding long-term trends in hurricane activity, regardless of whether the SST increases are natural or anthropogenic in origin. Mann and Emanuel (2006) noted that once SST is accounted for as a factor, there is no apparent multidecadal signal in long-term hurricane numbers, regardless of what that signal might be (i.e., SST, wind sheer, or any other factors that potentially influence activity). Other studies (e.g. Bell and Chelliah, 2006) generally agree that the ‘memory’ of any multi-decadal osillation (whether forced or natural) lies in the ocean, and atmospheric simulations using observed SST data in the Atlantic reproduce many of the observed correlations (Zhang and Delworth, 2006). So understanding the origin of the warming SSTs is central to understanding changes in hurricane behavior.

The essential question, then, is what has caused the long-term SST changes? This question is sometimes reduced to the ‘straw man’ of whether tropical SSTs trends (which are very clear in the data) are internal (i.e., due to a natural oscillation of the climate system) or anthropogenic (i.e. forced by some combination of human-related causes)? Of course, this is a over-simplification since there always at least some role for internal variability. So a more useful question is to what extent SST changes can be attributed to various possible causes.

In the Atlantic, the proposition advanced by some is that a natural oscillation known as the Atlantic Multidecadal Oscillation (AMO) is responsible for the observed trends in SST (and by extension, North Atlantic hurricane activity). To examine that question requires a clear analysis of the data, and examination of what models suggest regarding the likely amplitude, spatial extent and physical mechanisms underlying this oscillation.

First of all, how is the AMO defined? It is intrinsically quite difficult to detect any multidecadal oscillation in roughly only one century of instrumental data. Simply put, separating an oscillation from trend becomes exceedingly tricky (and increasingly dependent on statistical assumptions) as the timescale of the oscillation approaches the length of the record. Unfortunately, this situation holds for the AMO, which has been attributed periodicities anywhere from 40-100 years, the latter approaching the length of available instrumental climate observations. In some earlier studies, the AMO was defined using multivariate signal detection procedures to tease oscillatory patterns apart from long-term (potentially non-linear) trends (e.g. Mann and Park, 1994) or using climate model-based estimates of forced trends to estimate a possible residual oscillatory component (e.g. Schlesinger and Ramankutty, 1994).

In some more recent studies, however, the AMO has been defined simply as the residual low-frequency pattern after linear detrending of SST observations (e.g. Goldenberg et al, 2001). The linear detrending is intended to remove any potential forced signal, under the assumption that it is linear in time. However, if the forced signal is not linear, then this procedure can produce a false apparent ‘oscillation’ purely as an artifact of the aliasing of the non-linear secular trend (Trenberth and Shea , 2006). In fact, we have very strong indications for the 20th Century that the forcings over that period have not varied in a smooth, linear fashion.

Because of the procedural difficulties in isolating the AMO signal in the instrumental record, the estimated attributes of the signal are quite sensitive to how it is defined. The earlier studies mentioned above (i.e., Mann and Park, 1994; Schlesinger and Ramankutty, 1994) found an AMO signal with a large projection onto high-latitude North Atlantic SST variations, but little projection onto tropical North Atlantic SST. This contrasts with studies using the linear detrending procedure described above, which indicate a sizeable impact of the AMO on tropical North Atlantic SST.

In a regression analysis using instrumental observations, Emanuel and Mann (2006) find that the estimated temporal history of the anthropogenic climate change signal in tropical North Atlantic SST superficially resembles the temporal pattern often ascribed to the AMO (i.e., early 20th century and 1960s-1980s cool phases, and 1930s-1950s and recent warm phase). However, they identify this irregular warming pattern with a combination of greenhouse gas warming influences and late 20th century sulphate aerosol cooling influences (which are especially large during the late boreal summer in the tropical Atlantic). It is therefore likely that the non-linear temporal history of anthropogenic tropical Atlantic warming has masquaraded as the ‘AMO’ in some studies.

Does the AMO even exist as a climate phenomenon absent the complications in detecting the signal in actual observations? Here the answer is probably yes. Within coupled models, enhanced multi-decadal variability with an apparent origin in Atlantic ocean-atmosphere dynamics, does occur. This was shown in work published in the early 1990s by Delworth and collaborators using the GFDL coupled ocean-atmosphere model (see the update and review in Delworth and Mann, 2000), and in more recent work by Knight et al (2005 and 2006) with the Hadley Centre coupled model. This AMO signal, in the model simulations, is associated with oscillatory variations in the meridional overturning circulation such that when the overturning is stronger than normal, there is a warming pattern in the North Atlantic (and vice versa). However, the warming in the model simulations is largely confined to the extratropical North Atlantic, with only a small (roughly 0.1 C maximum) projection onto the Main Development Region (MDR) for Atlantic hurricanes. The model simulation results therefore appear consistent with those analyses of observations which find that the AMO signal does not have a substantial projection onto tropical Atlantic SST. This does not mean that the AMO could not in principle influence tropical Atlantic Hurricane activity. In fact, detailed analyses of both the GFDL and Hadley Centre simulations indicate that the AMO is associated with moderate changes in wind sheer in the tropical Atlantic, which could potentially influence Atlantic hurricane activity. However, as discussed earlier, a number of studies find that it is the SSTs that have played the primary role in the observed increases in hurricane intensities in recent decades.

An alternative approach to the problem is a formal ‘detection and attribution’ analysis which seeks to establish the role of a potentially forced signal in the midst of climate ‘noise’. This is where the new Santer et al paper comes in. Here, the authors examine the model simulations for the 20th Century that were coordinated for the IPCC AR4 and which now form a very valuable database that can be used in addressing issues such as those which concern us here. For each of the models, the trends in key Atlantic and Pacific regions can be compared in the runs with and without forcing. Assuming for the moment that the models produce a reasonable approximation for the naturally occurring decadal variability, it can easily be seen whether a) the trends in the models are similar to those in the real world, and b) to what extent they can be explained by the forcings. In the Santer et al study, they find that the model trends when driven with the 20th Century forcings do match the observations, and moreover, are clearly larger than can be explained by internal variations (see the figure extracted from Figure 2 of their paper). Interestingly, the study also supports the observation-based finding of Emanuel and Mann (2006) that sulphate aerosols are likely to have masked a significant component of the late 20th century tropical Atlantic greenhouse warming.

But do the models produce a reasonable amplitude of internal variability? This is a difficult question to answer because it can’t be easily deduced from the climate record (since there are many forcings, some natural, some anthropogenic) that are potentially obscuring the internal signal. However, over the period when we have good data, we can certainly check whether the models amplitude of variability is in the ball park of the observations. Santer et al did this as well, and find that indeed, there is no reason to think that models as a whole are systematically underestimate the internal component. One of the advantages of the IPCC AR4 data is that with so many models participating (22 models here), there will be a range of results – some models have more variability than observed, others less. A robust conclusion can therefore be drawn if the signals are clear regardless of the magnitude of any one models’ representation of the internal variability. (Santer et al have posted an illuminating Q&A on their study that discusses this point further.)

This result (and an associated paper by Knutson et al who look in more detail at the GFDL simulations) is particularly notable because among the models they look at at precisely the GFDL and HadCM3 models known to generate the ‘AMO’ in control simulations. Thus even in those models which exhibit oscillatory ‘AMO’ behavior, the observed tropical SST trends can only be explained when anthropogenic forcing is included.

In total, at least four studies, two based entirely on analyses of observations, and the other two based on climate model simulations, independently come to the conclusion that warming tropical Atlantic and Pacific SSTs cannot be purely attributed to any natural oscillation. These studies do not conclusively show a hurricane/global warming link, let alone determine what it’s magnitude might be, but they do strengthen one pillar of that linkage.

57 Responses to “Tropical SSTs: Natural variations or Global warming?”

  1. 51
    John Lederer says:

    This is from left field, but I would appreciate learning.

    Commonly, climate models assume an energy balance of some sort between incoming energy (solar radiation) and radiation out. The difference affects global temperature.

    However, these are not the only sources of energy that could affect earth’s climate. The earth has substantial internal energy, both as a relict of its formation and from nuclear processes of radioactive decay. As we go into the interior of the earth,for instance, temperatures rise about 25 degrees C a kilometer, a fact easily seen in deep mines.

    Since the earth is cooling over a large time scale this internal energy must be transferred to the surface and radiated out ( disturbing the simplicity of our “solar in = radiation out” description).

    Morever, the evidence of geology, and known history, suggests that the transfer of this energy to the atmosphere is not uniform over time. Oceans offer an obvious method of transferring this energy. We know that oceans are quite deep in places. We know there is geologic activity at the bottom of the ocean, We know that water is a very good heat transfer medium.

    So my questions are:

    1, Is this second source of energy considered in climate models, or is it known to be insignificant; and
    2. Is there any impression of the degree to which internal energy affects ocean temperature?


    [Response: The geothermal flux is around ~0.005 W/m2 on average (though bigger in certain local regions). That compares to the solar absorbed at the top of the atmosphere of about 240 W/m2 – thus the geothermal flux at the global scale is completely negligible. Over land, it determines the temperature gradient at depth below the surface (look up borehole temperatures), but for climate purposes it is too small to matter mostly. In the ocean, you see very localised hot spots (black smokers etc.), but the large scale temperatures do not seem to be much affected. Thus climate models tend not to inlcude this flux, though some ocean modellers have started to – but with mixed results. – gavin]

  2. 52

    Re #31:

    Another paper has been published that does not find support for the MOC slow-down results published by Bryden et al. (2005). This one by Schott et al. in GRL. They write: “Although the water mass characteristics show interannual to decadal variations at those locations (Figure 2c), there is no sign of any MOC ‘slowdown’ trend over the past decade, contrary to some recent suggestions [Bryden et al., 2005].”

    It is funny how the Byden et al. results that the MOC is slowing down are Nature-worthy, while the results that don’t indicate that it is, are published in GRL. Hmmm.

    -Chip Knappenberger
    to some degree, funded by the fossil fuels industry since 1992

    [Response: Funny that magazines which have a news focus report news? Who’d have thunk it? To be specific, ‘something big might be happening’ is news, ‘nothing big might be happening’ is not. That isn’t too say that sometimes context gets lost in the coverage (as we discussed at the time), but looking for some political agenda in decisions about scientific news-worthiness is only worthwhile for unredeemable conspiracy theorists. These are the same magazines that published Fan et al, Friis-Christensen and Lassen, Solanki et al after all…. – gavin]

  3. 53


    Are you suggesting that less rigorous, but more newsworthy (or at least considered to be by the editors) (e.g., some aspect of anthropogenic climate change is worse than we thought) papers are more likely to appear in Nature or Science than are more rigorous but less newsworthy (e.g., some aspect of anthropogenic climate change is not as bad as we thought) papers? In other words, there is editorial bias in the bent of the articles that appear in these journals which may impact the scientific standards by which the articles are judged?

    to some degree, funded by the fossil fuels industry since 1992
    (but definately not an “unredeemable conspiracy theorist”! :^) )

    [Response: No. I don’t think there is any bias in rigorousness in what appears in Nature or Science compared to GRL or JGR. But papers that go counter to the grain or are somehow surprising are more newsworthy than those that agree with other work. Thus Bryden et al was a surprising result and it gets in. But then so was the methane from plants story, or the Solanki study. However, most ‘surprising’ results are surprsing because they go against what is generally accepted – and thus are often not accpeted and sometimes prove to be over-interpreted. That’s how things work. – gavin]

  4. 54
    Tom Fiddaman says:

    Is there any evidence that the article was submitted to Nature first and rejected? Given publication lags, that the data runs well into 2005, and that the article was submitted in early 2006, it seems likely that it went straight to GRL. As far as I know, Nature doesn’t get to cherry-pick articles out of other journals’ queues. A conspiracy theory seems premature.

  5. 55


    From a scientific standpoint, “worse than we expected” is no more likely than “not as bad as we expected” if we accept the fact that expectations are based upon our best understanding of things. Thus, each is equally surprising and scientifically interesting. You are suggesting that to sell subscriptions (I assume), Nature and Science are more likely to promote one side of the issue than another as being newsworthy. As such, they are not an unbiased source for gathing scientific background on any particular issue. And, since these journals are a major source for the press, the press only gets what the editors of Nature or Science deem to be “surprising.”

    Thus, sites exist like our World Climate Report must exist to bring some of these other, equally surprising, scientific findings to light. Such as is the case with our story of the Meinen et al. and Schott et al. results.


    [Response: Well, one could debate why such sites exist, but I would be of the opinion that agenda-driven ‘analyses’ are not useful in improving the state of knowledge, and in fact are rather antithetical to the whole idea of doing science. On your specific point, the null hypothesis in these cases was nothing much is happening. Bryden comes along with a surprising result – which deserved to be widely known even though it was often mis-interpreted – and that gets a high profile publication. We said at the time that the result was unlikely to stand, and so it’s not surprising that follow up papers are not supportive. But see, it’s not surprising, and therefore not going to get into Nature. Look, I have almost certainly had more papers rejected from Nature and Science than you, and mostly I (obviously) didn’t agree with their decisions. But given the nature of Nature, that too is not surprising. It might be comforting to think of yourselves as victims persecuted by the establishment, but you confuse the nature of news-worthiness with your own take on the science. They aren’t the same. – gavin]

  6. 56

    Thanks for your time and comments, Gavin. I think that you and I pretty much agree with what is going on and why. The shame of it is, regarding global warming or any issue really, is that when a big “surprising” finding is announced in Nature or Science (and thus the press as well), and it is later not supported by “not surprising” research published in less popular journals, the public usually isn’t made aware of this and is thus left with the wrong impression as to the current state of scientific understanding on that particular topic. I guess, through our efforts, we (you and I) seek to rectify the situation as we see fit (and each of us sees fit to cover different topics for different reasons).

    I didn’t intend the issue to reflect any personal experience that I have or haven’t had with Science or Nature.


  7. 57
    John L. McCormick says:

    RE # 55

    Chip, you said:

    [From a scientific standpoint, “worse than we expected” is no more likely than “not as bad as we expected” if we accept the fact that expectations are based upon our best understanding of things.]

    Then, you implored us to link to the World Climate Report link because, as you said, [World Climate Report must exist to bring some of these other, equally surprising, scientific findings to light.]

    So, I did link to the site and found the supposed important article by Konstantinos Andreadis and Dennis Lettenmaier of the University of Washington in Geophysical Research Letters entitled “Trends in 20th century drought over the continental United States”.

    I read World Climate summary of their paper which included the graphic of Annual trends in drought duration. Blue triangles showing significant downward trends, red triangles showing significant upward trends.

    The longer duration droughts appear to coincide with US grain-growing regions of the US and particularly the Northern Plains. In fact, this is the general area below which is the depleting Ogallala Aquifer.

    Now, I am not sure why their report is considered important because it neither references nor reflects upon the recent Arctic ice melt minima post-1995 and implications of the lost albedo and effects upon surface temperature, sub-arctic air flow patterns or transport of tropical moisture to the US regions experiencing significant upward trends of drought duration.

    Their report is not complete without some consideration of the melt back of Arctic ice and impact on Western North American temperature and precip patterns.

    Bulletin: All passengers survived the plane crash; all victims died on the way to the emergency room.