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Hurricane Spin

Filed under: — group @ 24 April 2007

Michael Mann and Gavin Schmidt

A recent paper by Vecchi and Soden (preprint) published in the journal Geophysical Research Letters has been widely touted in the news (and some egregiously bad editorials), and the blogosphere as suggesting that increased vertical wind shear associated with tropical circulation changes may offset any tendencies for increased hurricane activity in the tropical Atlantic due to warming oceans. Some have even gone so far as to state that this study proves that recent trends in hurricane activity are part of a natural cycle. Most of this is just ‘spin’ (pun intended), but as usual, the real story is a little more nuanced.

We have commented on the connections between hurricanes and climate change frequently in the past (see e.g. here, here, here, and here). The bottom line conclusion has consistently remained that, while our knowledge of likely future changes in hurricanes or tropical cyclones (TCs) remains an uncertain area of science, the observed relationship between increased intensity of TCs and rising ocean temperatures appears to be robust (Figure 1). There is nothing in this latest article that changes that.


Figure 1. Measure of total power dissipated annually by tropical cyclones in the North Atlantic (the power dissipation index “PDI”) compared to Aug-Oct tropical North Atlantic SST (from Emanuel, 2005; data)

The Vecchi and Soden (V+S) study suggests that increased ‘vertical wind shear’ in the tropical Atlantic might overcome this effect. Wind shear is related to the rate at which different layers in the atmosphere move – zero shear means that the layers all move together, large shear means that the upper layers are moving very differently to those below – and is inimical to hurricane formation and intensification. The well-known impact of El Niño on reducing Atlantic hurricane activity is in fact due to increased shear from the associated atmospheric circulation changes. The V+S results come from analysing the results of 18 different model simulations that were done for the IPCC AR4 and which now provide a superlative database for assessing what models do and do not project. It’s important to be clear that these models do not resolve hurricane processes and that the analysis is related to the large scale ‘background’ environment in which hurricanes form. Nonetheless the idea of looking at these simulations to see what happens to that large scale environment, as V+S have done, is certainly interesting and worthwhile.

V+S find that the IPCC AR4 models produce an decrease in shear near the equator and an increase in the subtropics. Over the ‘Main Development Region’ for Atlantic hurricanes, the results are mixed and, to our eyes at least (see Figure 2), don’t provide a compelling argument for hurricane activity reductions. However, the conclusions rest heavily on something that is not robust at present; the prediction of mean changes in the Walker circulation. As we have discussed in some detail, this latter issue rests upon considerations that take us to the heart of where the models are currently at their weakest–getting marine stratocumulus clouds right, producing a realistic intertropical convergence zone in the tropical Pacific, producing realistic Kelvin wave behavior in the tropical Pacific ocean–things that are all critical for an accurate representation of the Bjerknes feedbacks which are, in turn, so central to the mean state and variability of the Walker circulation (and El Niño). It is conceivable that the various simulations in the AR4 ensemble analyzed by V+S are at the same time mostly in agreement, and yet wrong, in what they predict for future Walker circulation changes. The prediction of increased wind shear in the tropical Atlantic is no better than the underlying predictions in the models of Walker circulation changes.


Figure 2. Average of 18 models changes in Genesis Potential Index (GPI) of hurricanes for 2081-2100 (IPCC A1B scenario). GPI includes the effects of heating and wind shear. (Update: figure updated to fig 4d from the paper.)

Furthermore, the fact is (as shown in Figure 1) that hurricane intensity has increased in recent decades as SST has risen (at least in the North Atlantic for which trends are most reliable) and this prediction is based on fairly fundamental and robust thermodynamic arguments explored by Emanuel and others for decades now. Emanuel (2005) makes a compelling case that the warming ocean temperatures (and associated changes in atmospheric temperature and humidity profiles) are behind the increased TC intensity in the Atlantic. Independent analyses, such as those described in the Santer et al PNAS article, show that this warming is inconsistent with natural variability, i.e. it is likely only explainable in terms of anthropogenic forcing. That would seem to close the loop on the argument that anthropogenic forcing is likely behind a substantial component of the observed increased intensity of Atlantic TCs. So the observational evidence thus far is not in favour of increased shear preventing this increase in intensity.

This view is echoed by Kerry Emanuel in comments on the paper in the Washington Post, where he suggests that the impact of wind shear changes relative to warming SSTs in the real world, as diagnosed from trends observed thus far, may be overstated by the V+S study:

Emanuel, who was not involved in this research, said he published a study last year that calculated that increasing the potential intensity of a storm via warming by 10 percent increases hurricane power by 65 percent, whereas increasing shear by 10 percent decreases hurricane power by only 12 percent.

In the same WaPo article, Chris Landsea discusses these projections into the future as if they had relevance to the attribution of past change. While this mistake is often made, it is nonetheless incorrect. Attribution can only be done using simulations and observation of the period in question.

Finally, a cautionary note seems warranted. Suppose that the V+S findings are in fact correct, and that increased wind shear will play a substantial role in future changes in TC behavior. This could be a mixed blessing. Wind shear in the tropical Atlantic will remain highly variable from year to year, changing at the whim of individual El Niño and La Niña events which influence the Walker Circulation. Temperature trends, on the other hand, are far more steady over time, and every simulation examined by the Vecchi and Soden predicts substantial warming in the main development region for TCs in the tropical Atlantic in the decades ahead. While increases in wind shear could offset the impact of tropical temperatures in some — maybe even the majority — of storm seasons, one might worry about what happens during those seasons where there is anomalously low shear (e.g., a very strong La Niña event). The warm ocean will still be sitting there, waiting to produce tropical cyclones and Hurricanes–and the prospects for destructive Hurricane activity during those seasons could be especially grim. In short, the V+S results could presage a future where there is increased interannual variability in TC behavior, and where the worst Hurricane seasons are considerably more destructive than today.

The findings of V+S represent an important contribution to the ongoing scientific discourse on the issue of climate change impacts on tropical cyclones, and the study should spur additional work looking at the complicated issues involved in greater detail. It remains the case that the modeling of Hurricane-climate change interactions is still at a relatively primitive stage and this study is very unlikely to be the last word. We will of course follow the future developments closely.


100 Responses to “Hurricane Spin”

  1. 1
    Aaron Lewis says:

    Did Vecchi and Soden consider the possibility that if there were fewer hurricanes, there might be more of other types of storms that maybe just as destructive to human infrastructure?

  2. 2
    Fergus Brown says:

    Dim question: if the models show an increase in ENSO activity as a result of warming, would this not, in itself, imply that wind shear would, indeed, be increased more often, and Atlantic cyclone activity thereby decreased, on average, whilst intensity may or may not be affected at all?
    Regards,

  3. 3
    Nils Simon says:

    Thanks for this informative post! I found a PDF version of Vecchi and Soden 2007: http://www.gfdl.noaa.gov/~gav/REPRINTS/VS_07_SHEARS.final.pdf

  4. 4
    Thom says:

    How odd that Chris Landsea would be allowed to post his personal views on a blog when other government scientists have been having problems interacting with the media. Hopefully this is an indication that NOAA is no longer maintaing a tight leash on its scientists, but it could also be further evidence that Chris Lansea’s views continue to get special consideration from political minders at NOAA.

  5. 5
    Ray Ladbury says:

    A nice summary, Michael and Gavin, and very timely. The predictions of effects of climate change is, I think the next frontier in the debate, and all but the most bitter of bitter-enders seem to be moving in that direction (while never admitting they were wrong of course). I suspect that there will be considerably more uncertainty attached to this activity than there was to the attribution of climate change to anthropogenic activity–in part because the only guides we really have are the models and paleoclimate studies, both of which are subject to significant uncertainties. Moreover, while there will assuredly be winners and losers as the climate changes, we need to keep in mind that while global climate has had epochs of severe change in the past, human civilization has never experienced these changes. Identifying and assessing the potential threats is essential to efficient mobilization of resources to confront them, so debates along these lines will become increasingly common in the future. Again, thanks for the summary.

  6. 6
    Thom says:

    Gavin makes an appearance on the “Daily Show.” Dave Roberts at Grist has the link.

    As with prior media appearances, Gavin provides further evidence that scientists can be smart and well-dressed.

  7. 7
    Larry Coleman says:

    Where can I get a higher-res version of Fig. 1? And why does the graph start with 1972, given that earlier versions – including the one in Emanuel’s 2005 paper – start at 1950 (showing both power and temperature graphs)? By truncating at 1972, the correlation looks much better because the lack of correlation between 1958 and 1968 is removed.

    [Response: I made the figure from the data provided at Emanuel’s website (follow the link and download the Excel file). I didn’t want to use the 2005 figure because of the changes in the figure due to the various correction terms and subsequent seasons, but I couldn’t find a figure that already had that. I plotted all the data that was there. – gavin]

  8. 8

    Guys,

    Thanks for the even-handed review of Vecchi and Soden. While we may quibble about particulars and their interpretations in terms of their impacts on current and/or future storms, I think, in general, based upon your write-up, that we see eye-to-eye on the general implications of V+S, including, by the way, the possibility for even worse storm seasons in the future…when the SST is up there, and everything else falls into place, things can (and do) get nasty.

    A question–as best as I can make my way through the math describing Emanuel’s maximum potential intensity (described here ), I can’t find where wind shear is included (as you indicate in your Figure 2 caption). But, I easily could be wrong about that…

    -Chip

    [Response: Whoops, my mistake. Slightly wrong caption/figure. The GPI (figure 4d) contains both the shear effect and SST and humidity effects. The GPI index is positive (indicating increased hurricane genesis potential over most of the Atlantic MDR. – gavin]

    [Response: Thanks for the kind works Chip, and for pointing out the mistake. I see Gavin has already addressed above. – mike]

  9. 9
    Steve Bloom says:

    Re #6: Smart, well-dressed and, just to make sure this is never forgotten, too sexy for his model. :)

  10. 10
    Steve Horstmeyer says:

    To me the most salient point of the comment on Vecchi and Soden is made by Mann and Schmidt in the second to last paragraph. No matter what the vertical wind shear does in the future the addional heat is still there.
    If tropical cyclone occurrence decreases, less of the heat is dissipated, and unless ocean circulation in some way compensates by transporting the additional thermal energy elsewhere (i.e. for example out of the “main development region” of the Atlantic) some day a storm will tap the enhanced energy source.
    This represents a positive feedback loop – greater shear >> fewer hurricanes >> less thermal energy dissipation, a feedback loop that sets the stage for more intense storms and the potential for greater devestation.
    Because wind shear cannot have absolute control over hurricane occurrence does anyone know of a study that relates shallow water heat content and wind shear with hurricane occurrence? What I mean is that for a given amount of wind shear what heat content must be present above the thermocline to overcome the shear and permit hurricane formation? Thanks to Mann and Schmidt for calling atention to this article.

  11. 11
    Thomas says:

    Well, I am only a meteorologist in tiny switzerland. But when I first heard of that study let’s say 4 or 5 days ago one question arose quite fast: As Hurricanes are quite a good way for the atmosphere to move energy near the tropics to the mid-latitudes or, or to put it in another way: get rid of the ‘too much energy’ in the tropics…. well where does this energy go if there were no more hurricanes? Would all be compensated by other means?

    [Response: Excellent question. Actually, there is some interesting work being done by Matt Huber of Purdue, following up on some earlier ideas of Emanuel’s, suggesting that the role of TCs in transporting heat from equator towards the poles may be more significant than previously thought–it also allows for some interesting, though admittedly somewhat exotic, mechanisms for explaining the “cool tropics paradox” and “equable climate problem” of the early Paleogene and Cretaceous periods, i.e. the problem of how to make the higher latitudes warm without warming the tropics much, something that appears to have happened during some past warm epochs in Earth’s history. Feedbacks involving the interaction between greenhouse warming, TC behavior, and its associated impact on poleward heat transport in the ocean, could potentially do the trick. -mike]

  12. 12
    Richard Ordway says:

    re 2. Errr, did you actually read Gavin’s post? Gavin already addressed your question:

    [...and Atlantic cyclone activity thereby decreased, on average, whilst intensity may or may not be affected at all?]

    Seems pretty clear to me: Gavin wrote:

    “The warm ocean will still be sitting there, waiting to produce tropical cyclones and Hurricanes–and the prospects for destructive Hurricane activity during those seasons could be especially grim. In short, the V+S results could presage a future where there is increased interannual variability in TC behavior, and where the worst Hurricane seasons are considerably more destructive than today.”

    [Response: Actually, that part was mostly “Mike” :) -mike]

  13. 13
    Alan K says:

    it seems that your conclusion: ” the observed relationship between increased intensity of TCs and rising ocean temperatures appears to be robust” is in direct contradiction with your conclusion “our knowledge of likely future changes in hurricanes or tropical cyclones (TCs) remains an uncertain area of science”.

    Which is it?

    Or are you trying to imply or infer something the “area of science” doesn’t support?

    [Response: The uncertainties are in the magnitude of the effects, and in the countervailing pressures (El Nino changes, or wind shear as described above) not in the basic sign of the reponse. Even Bill Gray uses SST in his hurricane forecasts… -gavin]

    [Response: If you read the past discussions on this site, you’ll see that there is an important distinction between Hurricane maximum intensity and power dissipation (which it is strongly believed will continue to increase with warming SSTs), and the other various attributes (e.g. number of TCs, number of storms days, etc) for which predicted responses to anthropogenic forcing are far more uncertain. These latter attributes all belong to the more general category of “future changes in hurricanes or tropical cyclones”, hence our deliberate and careful word choice. -mike]

  14. 14
    Jeff says:

    This is slightly off-topic, but I want to say how excited I am that Dr. Trenberth is coming to speak about climate change in Rochester, NY. I encourage anyone who lives in the upstate NY area to drop by tomorrow evening at 7:30 PM. Details can be found on http://www.climateobservation.com.

  15. 15
    Fergus Brown says:

    Re #12; Richard. I get that bit. I’m trying to make sense of two apparently contradictory bits of model output: one which says that warming is likely to produce more El Nino-type events, which would apparently suppress hurricane formation, and another which implies that either frequency or intensity or both are likely to increase under the same scenario. The Vecchi & Soden seems to follow, or relate to the first idea, but not the second (or at least, not so clearly),
    Anyway, I said it was a dim question, but I don’t think your reply actually answered the question I was trying to ask.
    Any suggestions?

  16. 16

    Mike and Gavin,

    I like the MPI better than the GPI at least when it comes to trying to make sense out of intensity (so would have leaned toward showing Figure 4c instead of 4d like you all did initially). It seems like the GPI in the models is still a little squirrelly (for example, see here ). I guess your new Figure 2 (V+S Figure 4d) indicates a tendency for more storms in the Atlantic in the future (and most everywhere else except the eastern Pacific) but doesn’t really get at intensity expectations. Those come from some combination of V+S Fig 4a and 4c (I think the rh changes in V+S Fig 4b are likely captured in the MPI). I don’t know the appropriate weights in the combination (I presume the MPI should be weighted more than shear changes), but in any case it seems like V+S’s results indicate a relative minimum (likely even a negative impact compared to the present) to storms which have to pass from the eastern Atlantic to get to any land areas. So maybe our most dangerous future storms will be those that blow up in the Gulf or right along the Atlantic seaboard (where V+S Figs 4a and 4c are both positive). However, these storms are often (but not always) limited by the amount of time they have over the ocean before encountering land.

    -Chip

    [Response: Indeed, all 4 panels of the figure are instructive. I would encourage folks who are interested to click on the link at the top to the preprint version of the paper to see these and the other figures in the paper. -mike]

  17. 17
    Steve Bloom says:

    Re #16: Is it reasonable to assume that increasing shear will be consistently distributed across the entire hurricane season, which itself presumably will be lengthening? Also, it appears possible that Western Europe will become more vulnerable to hurricanes since those waters will become capable of supporting stronger storms and the effect of the shear is much reduced in that area.

  18. 18
    joel says:

    I am eager to invest in the coming global catastrophe. There will be plenty of money to be made with all these hurricanes getting worse.

    Are there any investment funds which have been set up to profit off the coming awful and expensive climate events?

  19. 19
    Hank Roberts says:

    The main article says:
    > impact of El Niño on reducing Atlantic hurricane activity is in fact
    > due to increased shear from the associated atmospheric circulation

    Looking back at last summer, with satellite imagery, is it possible to see events where clouds developed and had their tops sheared off, and say ‘this could have become a hurricane’? Or do events proceed far enough to see before that happens?

    [Response: Hank–yes, you can see this in the satellite images. As you may remember, the beginning of the season was active, not “2005″ active, but well above normal. But as we got into autumn, the upper level westerlies really picked up in strength, and you could see the tops of incipient TCs getting strongly sheared in the satellite images, with the water vapor blown aloft to the east of the cyclone resembling the trial left behind a comet. If forecasts of La Nina conditions for this coming winter are correct, we will see much less of that. And if SSTs shape up to be as warm as it appears they will be, we can expect a very active season. It will be hard to match 2005, but I wouldn’t be surprised to see as many as 18 named storms. -mike]

  20. 20
    john says:

    If we’re considering the risk of hurricane damages, and not just overall basin activity, then the effect of increased vertical wind shear would seem to be (at least) twofold–it not only reduces potential intensity, but it also influences the steering of hurricanes (since hurricanes are basically steered by the background flow plus a beta drift). If the vertical wind shear over the Atlantic MDR is expected to increase, wouldn’t we expect hurricanes to be steered farther to the east, climatologically, and therefore to pose less of a threat to most of the eastern seaboard? It seems the media is sometimes a little quick to conflate hurricane activity and hurricane risk, without considering where the hurricanes are going.

  21. 21
    Hank Roberts says:

    Thanks Mike!

    Another question about wind shear, I found answered–
    In case anyone wondered:

    Q: Why does wind shear tend to strengthen thunderstorms but weaken hurricanes?

    A: Wind shear, changes in wind speed and direction with elevation, gives a thunderstorm its spin in the form of rotating updrafts called mesocyclones. Wind shear also helps to keep a thunderstorm’s updraft separated from its downdraft, increasing the storm’s strength and duration. A hurricane, on the other hand, is a large collection of thunderstorms and occurs on a much larger scale than do individual thunderstorms. A hurricane gets its spin from the rotation of the Earth rather than from wind shear. Hurricanes rapidly intensify when the thunderstorms form tightly around the center of circulation. Like pushing over a spinning figure skater, wind shear tends to disrupt the balance of the storm.

    (Answered by meteorologist Bob Swanson, USA TODAY’s assistant weather editor, August 7, 2006)
    http://www.usatoday.com/weather/resources/askjack/archives-hurricane-science.htm

  22. 22
    Joe Fran says:

    I don’t know why anyone is building homes in Florida. This year’s hurricanes and the sea level rise in the next few years will wipe out any investment.

    Just the chart shown here by Michael and Gavin implies that even Miami will be wiped off the map in the next ten years tops.

  23. 23
    Chris Shaw says:

    Does the chart above show an increase of 2.5C in Tropical Atlantic SST’s ?

    [Response: No. The temperature changes are scaled – the raw numbers are much smaller – check out the Emanuel spreadsheet at the data link. – gavin]

  24. 24
    Ike Solem says:

    To quote Soden & Vecchi (from ScienceDaily):

    “This study does not, in any way, undermine the widespread consensus in the scientific community about the reality of global warming,” said Soden. “In fact, the wind shear changes are driven by global warming.”

    The authors also note that additional research will be required to fully understand how the increased wind shear affects hurricane activity more specifically. “This doesn’t settle the issue; this is one piece of the puzzle that will contribute to an incredibly active field of research,” Vecchi said.”

    It seems that climate scientists are doing a better job of communicating with the media, or the science journalists are doing a better job of asking questions.

    It is a little surprising is that a paper that addresses the Walker circulation and wind shear doesn’t specifically make any predictions about the future of El Ninos in a warming world (more frequent? more intense?). They do address the issue: “the extent to which El Nino serves as a useful analogue for the mechanisms behind the projected shear changes should be further examined” and they also note that Atlantic wind shear is infueneced by other factors besides the Walker circulation. (I’m now stuck with the image of farmers throwing buckets of water over pigs every time “Walker circulation” is mentioned.)

    There’s nothing in the paper that indicates that the observed changes in hurricane intensity are due to natural cycles, regardless of Landsea’s attempts to spin the article on Pielke’s website. In fact, the whole AMO hypothesis seems less and less believable; the idea is that the Atlantic thermohaline circulation speeds up and slows down in a semi-cyclic manner, and that a sped up THC results in more hurricanes… but all the evidence points to a weakening THC, not a strengthening one.

    RE#11 The role of hurricanes in the poleward heat transport immediately leads to the question, how is the poleward heat transport divided between atmospheric and oceanic routes? For example, see The partitioning between atmospheric and oceanic heat transport is not trivial. It seems that predictions are for less oceanic heat transport and more atmospheric (latent/moist) heat transport.

    Wind shear can chop off a hurricane, but is not the driving force behind TC formation. Given that the other important variables (sea surface temps, depth of the warm layer, and atmospheric moisture) are all predicted to increase, it seems hard to make the claim that tropical cyclones will be unchanged, just as it seemed unwise to claim that Lyman et al’s “Recent cooling of the upper oceans” meant that climate models had fatal flaws.

  25. 25

    Question: what effect does this have on what I assumed was the consensus that global warming will mean on average gradually stronger hurricanes, not more hurricanes? and to the idea that you have more hurricanes after El Nino has come and gone?

    Ouais, c’est ma commentaire, M. Robot.

  26. 26
    Laura V says:

    There is a study going on at LSU regarding hurricanes in the Atlantic. Liu, in his studies, believes that the Bermuda high has something to do with the strength of hurricanes and how much sediment they may deposit according to wind strength. He went on to say if the Bermuda high moved away from North American that hurricanes would move towards the Atlantic Coast of North America. If the Bermuda high was closer to North American hurricanes would move over the Gulf of Mexico. He believes that there is a cycle with the Bermuda high. This could explain some of our paleohurricane history.

    http://geology.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=geology&cdn=education&tm=3&gps=261_977_1276_570&f=11&tt=2&bt=1&bts=0&zu=http%3A//www.ga.lsu.edu/paleotempsitemap.htm

  27. 27
    Hank Roberts says:

    Say a bit more about wind shear, someone knowledgeable? The tornado-chaser sites talk about
    “directional shear and speed shear”
    http://www.tornadochaser.net/windshear.html
    Is either of these the same as “vertical shear” — are they talking about the same thing or same scale as the hurricane people?

    I recall Al Gore getting pilloried (wait, I’m framing, must…stop…)
    (ahem)
    I recall critics of his book writing such comments as
    (from http://www.wunderground.com/education/gore.asp )
    “the IPCC has not found any evidence that climate change has increased tornado frequency, or is likely to. Gore doesn’t mention the unusually quiet tornado season of 2005, when for the first time ever, no tornadoes were reported in Oklahoma in the month of May….”

    Aside: I don’t know the print time for the book, was he still writing in 2005?

    Regardless, I’m wondering — any relation, same kinds of shear or not?

    Is it coincidence there was a normal hurricane season and a quiet tornado season in 2005, then a quiet [Atlantic] hurricane season and a normal tornado season in 2006? I haven’t found any reports of measurements of wind shear over the years, let alone of correlation with storms. Would there be a past history to find?

    Pointer welcome, this digression should be somewhere else, direct me?

  28. 28

    Gavin, Mike, I’m glad you found our paper interesting. My main comment would be with respect to the statement that the weakening of the Walker circulation is “something that is not robust at present”. I’m not sure what your intended definition of robust is, but to us (Brian Soden and I) it qualifies as a robust response of the model projections to the 21st Century – and we have a paper in press at J. Climate arguing this (preprint here). One needs to contrast the long-term weakening of the Walker circulation (which is robust) with the change in the models’ El Niño (which is not robust – there’s a series of papers describing this for the current IPCC models: e.g. van Oldenborgh et al 2005 Ocean Sci., Merryfield 2005 J. Clim., Capotondi et al 2005 J. Clim., Guilyard 2005 Clim. Dyn.).

    The weakening of the Walker circulation arises in these models from processes that are fundamentally different from those of El Niño – and is present in both mixed-layer and full-ocean coupled models, so is not dependent on the models’ ability to represent Kelvin waves (by the way, most of the IPCC-AR4 models have sufficient oceanic resolution to represent Kelvin waves and the physics behind them is quite simple – so of all the model deficiencies to focus on this one seems a little odd). So even though El Niño may serve as an analogue for some aspects of the influence of the weakening Walker circulation on climate, it does not serve as a dynamical analogue nor is the sensitivity to model details the same.

    In these models, the weakening of the Walker circulation is related to the overall weakening of the tropical atmospheric circulation, which can be understood in terms of global thermodynamic arguments (e.g. Betts and Ridway 1989, J.Atmos. Sci., Knutson and Manabe 1998, J. Clim., Betts 1998 Climatic Change, and Held and Soden, 2006, J. Climate). The overall weakening of atmospheric circulation does not appear tied to details in the model formulation, and is present in every one of the IPCC-AR4 models (e.g. Held and Soden (2006, J. Climate) and Vecchi and Soden (2007, J. Clim in press). These models have a tendency to weaken preferentially in the zonally asymmetric part of the flow, a large part of which is the Pacific Walker circulation.

    The issue of variability in the Walker circulation or shear changes (shear can change for a whole host of reasons, most having little to do with the Pacific) I think remains a big one as we go forward (re: commentary’s 2nd to last paragraph). But I’m not sure if it is true that the large-scale thermodynamic conditions in the Atlantic are necessarily less variable than those of shear. There is more to the increased thermodynamic potential of tropical storms than SST (as Kerry Emanuel, Greg Holland and others have very clearly shown), an example of that is the multi-model projections of MPI response (which shows a big swath of Maximum Potential Intensity (MPI) decrease in the Atlantic, even though the entire tropics are warming). There is likely interannual variability of thermodynamic conditions in the Atlantic, as argued by Tang and Neelin (2004, Geophys. Res. Lett.), who showed that MPI changes arising from remotely forced changes to upper tropospheric temperatures are behind some of the tropical storm response to El Niño; and we find evidence for something similar going on in the models on longer timescales.

    I don’t think we know what the natural variability of MPI is in the Atlantic (or shear and relative humidity, for that matter). I can only pass on that in some of these models MPI has much variability (on many timescales) as shear does. I’m not saying that the effect of the variability of shear will be less than that of other factors, I’m only saying that we don’t know whether it will be more.

    Also, while the main focus of our study was the impact of shear on the Atlantic – because our work on the Walker circulation led us there – we did note that the models project changes in the other relevant parameters we looked at (MPI and midtropospheric relative humidity), as well as changes to shear, around the tropics. The processes behind the structure of the MPI and relative humidity changes in these models are interesting, and considering their effect is important in projecting tropical storm response to global warming. Also, even though we focus on the ensemble-mean response, the range of model responses is also interesting and important to understand; and the climate model response of large-scale environmental conditions needs to be more explicitly connected to the response of tropical storms. So there is plenty still to be done.

    [Response: Gabriel, thanks for your comment. Perhaps we should have been clearer about what we meant by “not robust at present”. We were thinking e.g. of the extensive review by Matt Collins and the CMIP group [El Niño- or La Niña-like climate change?, Climate Dynamics, 2005], the conclusion of which is well summarized by the first sentence of their abstract: The potential for the mean climate of the tropical Pacific to shift to more El Niño-like conditions as a result of human induced climate change is subject to a considerable degree of uncertainty.. So our point is, of course, that regardless of what mechanisms are responsible for a mean change in the Walker circulation in the AR4 simulations you’ve analyzed, if the simulations are not getting the dynamical mechanisms discussed in our post right, it is quite conceivable that at least the amplitude and perhaps the sign of the change in the Walker circulation is wrong, regardless of what mechanisms are responsible for the changes in the simulations analyzed. I hope this point is clearer now. – mike]

  29. 29
    Denise says:

    Hurricans are only going to get stronger as Global Warming continues.

  30. 30

    #19 Mike. the recent vagaries of La-Nina—El-Nino ( http://www.cpc.noaa.gov/products/analysis_monitoring/enso_update/sstanim.shtml ) like musical chairs renders it so difficult to say I have a lot of confidence in any hurricane forecast, especially in the wake of last year’s blunders by that the most popular hurricane forecasters in the field. I follow SST anomalies every day and it never seems so consistent off the Coast of Peru since last Spring. Right now La-Nina like conditions are a lot weaker than a few weeks ago. Being certain that a strong La-Nina is in the works is a hard scenario to accept at this time. We shall see it soon enough. But looking back at 2005, http://www.bom.gov.au/climate/current/soi2.shtml , you will see a remarkable coincidence, a warming similar to the link preceding.

  31. 31
  32. 32
    Chris S says:

    This is my first post on your site. I have been reading now for quite sometime as it seemed one to many “governmental” sites were extremely biased in the discussion of Global Warming and Global Climate Change. I find this webiste extremely resourceful and throughly backed by evidence, which is why I frequent this community often.

    I was actually suprised that NOAA was quick to release this report, where there are qoutes by Vechai (sp) that actually state Global Warming’s existence. What interests me is the fact that although these findings about the Walker Circulation and the Wind Shear may be prove true (as only time will tell) how does this effect the “track” of Hurricanes. Media outlets were quick to bring up the fact with the exception of 06′s “Ernesto” no TS or Hurricanes did much damage to the US, but that didn’t conicide with the fact that a few hurricanes were actually severly strong in strength, and extremely large in size.

    I remeber Hurricane Florence being huge, and at one point a model suggested a northely track that would go towards the Northeast due to the Wind Shear being discussed here that was strong last season. If I remeber correctly and someone correct me If I am wrong as I’m only a entry level student in Atmospheric Science (Starting Physics Calc & Chem in the fall) but I believe it was a low jet stream that prevented Florence from moving north enough to come towards the northeast. In the end it moved out towards Newfoundland and became extra-tropical, but this topic brings two questions to mind. 1) Could an increased wind shear drive stronger storm systems over the US? and 2) Could an increased Wind Shear allow further track of Hurricanes along the eastern seaboard and northeast. Is there any correlation between increased wind shear and NW movement (into the North East of US) in past years when Hurricanes have hit NYC, Conn., and Rhode Island?

    Excuse me if my questions seem a bit silly, everything I know about Meteorology and Climatology have been from learning and reading online (at the most credible places I can find, this being on of them)

  33. 33
    Daniel C. Goodwin says:

    The thought is far off-topic, but I’m reminded of a Russian study on breeding foxes for tameness, which resulted in numerous unexpected features (like border-collie coloring) within a decade. No matter how often the data shows that Nature doesn’t work in linear order, causes on this side, effects on this side, people still demand answers to questions which are phrased in distorted Cartesian terms, such as “Does global warming cause hurricanes?” You do an excellent job of explaining that our knowledge from models and observations is a little more complicated than that, and the response is “So does global warming cause hurricanes?”

  34. 34

    I’ve scanned the many thoughtful comments by learned readers, but apparently none noticed or wanted to point out the minor little typo in the first paragraph of Gavin’s notes: it’s wind shear, not wind sheer. It is a pretty trivial thing, yes, but as my 8th grade math teacher used to say back in 1962, “In this scientific age we must be accurate!”

    [Response: Cough… embarrassed grin… fixed. -gavin]

  35. 35
    Tonya says:

    This is not related to the current thread, but there is a huge need for input by you experts to a new forum http://www.climatebrains.com

    It was started by the ABC 33/40 news weather team (Birmingham, AL) comprised in part by the most notable climate change denier and weather personality, James Spann, a few days ago.

    The participants are mostly Alabamians, and unfortunately, there are many people in need of a climate education. Many bring up the fact that 30 yrs ago scientists were predicting an ice age. Many say they haven’t seen “proof” that warming is man-made.

    Will any of you be willing to provide your input and shine some light on this issue for the deep south?

    [Response: Thanks for the heads up Tonya. Unfortunately, between our day jobs and RC, most of us don’t have much time left. What works best is when the larger group of loyal RC followers, commenters, lurkers, etc. go down into the trenches, as it were, and use us as a resource at their disposal, e.g. through the strategic insertion of a hyperlink or two. -mike]

  36. 36
    Lynn Vincentnathan says:

    I read about the study somewhere (maybe ClimateArk), and thought maybe there’s this non-linear thing going on. Increasing temps cause greater hurricane intensity, then further increases decrease that intensity (maybe due to wind shear), then?? Sort like the way an unbalance wheel will make the car vibrate when it reaches a certain speed, such as 55, then not vibrate much after that.

    OTOH, energy coming into the earth system without an equal amount going back out means it’s either going to be manifested in heat or kinetic energy (e.g., fierce storms). It doen’t just go pouff, gone.

  37. 37
    Zeke Hausfather says:

    Tonya:
    A brief glance at that forum did yield the best correlation between mean surface temperature and pirates I have seen to date:
    http://i102.photobucket.com/albums/m111/farkmerunnin/Fark/piratesarecool4.jpg

    As a general rule of thumb for engaging in internet arguments on these matters, choose your battles wisely. Sometimes people are looking for rational arguments; other times they are just looking to reinforce their own preconceptions. In the latter case, the best arguments you can muster might not be able to make much of a difference. The politicization of climate science is probably the single largest impediment to substantive action on GHG abatement, and while sites like Realclimate do their best to objectivize the topic, you can’t expect to convince everyone.

  38. 38
    Deech56 says:

    Well, Tonya, I’ll take a stab at it. Thanks for the link. People at my local newspaper site seem to be tired of me arguing. Armed with Coby Beck’s “How to Talk to a Climate Skeptic” and our friends at RealClimate, and with the support of the scientific literature I will try to represent the side of truth and justice.

    BTW, this is an excellent discussion of the various forces that affect tropical storms.

  39. 39
    tamino says:

    Re: #35, #37

    I agree with Zeke. I looked at the site, and it seemed to be a place for deluded people to talk about cattle farts. I just don’t think it’s worth talking to these people; their minds are made up and they won’t listen to reason.

    Tonya, if you want to counteract the effect, start your own blog and talk to those who will listen with an open mind.

  40. 40
    Edward Barkley says:

    Joe Fran’s comment #22 should have been jumped on and dissected by unbiased scientists by this time. The GW hysteria and its directly proportional funding inclinations must be weighing heavily on many of you. Much of the public’s misunderstanding of human-induced global warming centers on storm surges and sea levels, and yet this has hardly been the central focus of climate science research over the past 20 years. It is only the focus of the fear. Admit to yourselves and the lay people who follow you that the fear-mongering has been overdone and this site does nothing to “stem the tide”.

  41. 41
    tamino says:

    Re: #38

    Deech56, although I thought it not a profitable venue, if you think you can stem the tide … go for it! More power to ya.

  42. 42
    Nigel Williams says:

    Thanks Edward.

    IPCC commentary notes that even a metre of sea level rise (plus population growth over the intervening period) are likely to affect between 200 and 450 million people in the Asia Pacific region alone. Thats not fear mongering, its fact.

    The processes of land subdivision and development set in place urban features that last for millenia (the layout of streets in Rome, London, Florida etc), and so it is sensible today to start building all new infrastructure (roading layouts, utilities etc) to facilitate a logical response to the progressive sea level rise and withdrawl from the coastal areas.

    How old is the oldest building in your town? In sensitive areas building codes should consider increased hardening of structures to withstand the increased wind speeds expected over the life of that old building, at least.

    We must accept that as this starts to bite there will be very little aid money to go around to help areas in trouble because are all in the same boat, and its sinking. We must fend for ourselves or we (our communities and our future families) go down.

    We are having to think 100, 200, 500 and even 1000 years in advance of today, because we KNOW that the sea levels are going to continue to rise – it wont stop in 2090, and TCs are going to become stronger over the lifespan of the cities we are building today.

    There is a difference between being alarmist and prudent. Joe might be pushing it a bit, but thats better place to be than lost in apathy or denial.

  43. 43
    Paul M says:

    Lynn has a good observation with post 36. We should be scared s[edited]less that wind shear is not letting the hurricanes get the energy out. The energy will come back and haunt us later in some nefarious storm of some sort. Could this be the creation of the superhurricane?

  44. 44
    Timothy Chase says:

    Re: Edward

    The rise in sea level is one thing. I think Nigel pretty well covered it. However, with me at least, a bit part of the deal is the increased acidity reducing fish harvests, water shortages, droughts severely reducing crops (sure – more rain, but more over the ocean, less on land – and with greater evaporation before the water trickles to a dry stream bed), increased heat reducing rice production and other heat sensative crops, the heat waves, etc.. This isn’t exactly hysteria, either. In fact, I would consider it a good recipe for war – especially since we haven’t seen the end of population growth – and the good majority of it will be taking place in less developed countries.

    In any case, #22 was a little uninformed. No doubt I will look that way at one point or another – and appearances won’t be deceiving. But you can’t expect the experts who are here to correct anyone who comes in to vent for a couple of lines before moving on, can you?

    Besides – if you are paying any attention, you can see what side the genuine and deliberate distortions are coming from. You know the types. They speak about “GW hysteria” and “fear-mongering” at the drop of a hat – and are highly selective about what details they are willing to acknowledge.

  45. 45
    Ken Winters says:

    Tonya:
    Never try to teach a pig to sing; it wastes your time and it annoys the pig.

    If the people going to that site were actually interested in learning anything about climate science, they’d certainly stumble across RC sooner or later (a few well placed links might speed up that process). I wouldn’t waste a bunch of time trying to argue with them at that site. I spent 2 years (15 – 25 hours/week) back in the late 90′s trying to convince a bunch of on-line dogmatists that Y2K would NOT be The End Of The World As We Know It (TEOTWAWKI). Other than a handful of people who appreciated my efforts, I think all my time was spent in vain as 90% of the people there held a level of dogmatic certitude that was unassailable by logic.

  46. 46
    Nigel Williams says:

    Very true Timothy.

    I know this may be spinning (or is it shearing?) off the post a bit, but an interesting couple of observations from last week.

    1 Australia announces that for the Darling-Murray Basin:

    *Unless there are very substantial early, inflows there will be insufficient water available to allow any allocation at the commencement of the 2007-08 water year for irrigation, the environment or any purpose other than critical domestic supplies.

    And

    *In the Murray Valley inflows have been less than 60% of the previous minimum.

    http://www.smh.com.au/pdf/waterreport.pdf (a good read)

    This affects about 30% of Australian horticulture, dairying etc and probably closer to 50% of its wine industry.

    2 Last week, just quietly, market gardeners in New Zealand were being offered $4.00 a cabbage at the gate by Australian buyers. [Pers coms]

    So we in NZ are thus expecting to pay between $8 and $10 (around $5USD) for a cabbage sometime soon. I suspect that this is just the tip of a belt-tightening iceberg (if you’ll pardon the mixed metaphor!).

  47. 47
    Craig Allen says:

    Edward, there is a big difference between hysteria and prudent concern with appeals to act in order to avoid a clearly apparent problem with huge implications. Do you see people running around in the streets wailing and doing crazy things? What happened during Cyclone Katrina was hysteria. But that was because no-one took prudent action beforehand to prepare for and mitigate what was a clearly possible but by no means certain catastrophe. (OK, I suspect that it also had a lot to do with the fact that it happened in the US. By comparison, when Darwin in Australia was wiped out by Cyclone Tracy in 1974, there was an orderly disaster response run by a competent public service, and a pragmatic and supportive public.)

    Speaking of Australia; our response to impending potential disaster borders on lethargy.

    * We have been warned by scientists and environmentalists for decades that the nation’s water resources are dangerously over stretched, that this is leading to the severe degradation to our aquatic ecosystems and is subjecting our agricultural economy to unacceptable risk (see here for example).
    * Our leaders have consistently ignored the risks and have continued to allocate water to a burgeoning irrigation industry and continued to encourage population growth in areas that have limited water resources.
    * Over the last decade or two, rainfall has been markedly below average in our temperate south-west, south and east, and this along with increased average temperatures has caused a major decrease in runoff from our catchments. The record low inflow of March 2007 was the tenth consecutive month of record low inflow (see here and here).
    * Now the Prime minister has announced that the Murry Darling Basin has run out of water and there will be zero water allocation to irrigation in the coming season. The MDB accounts for 70% of the nations irrigated agriculture and normally produces 40% of our food produce.
    * Most of our major cities are under increasingly severe water restrictions and the prediction is that many towns will run our of water or have to pay significant pumping and carting costs in the coming year unless we get significantly higher than average rains (e.g see here).
    * The Prime Minister’s solution is that we should pray for rain. No kidding that was the best he could do when announcing the bad news last week. (Other than then pausing to scoff at the idea that we should get serious about reducing carbon emissions.)
    * 90% of the public wants us to take global warming seriously (see here for example). Not because they’ve been duped by loopy left wing scientists, but because, well, when rainfall decreases year after year and it keeps getting hotter and hotter, sooner or later you start to get a little suspicious about all the assertions by the politicians and high emission industry spokes-people that everything is peaches and cream and we should just go shopping and not think about it.
    * The government insists that dealing with global warming is way down the priority list and since our economy is booming because of mineral exports (including heaps of coal don’t you know) we should concentrate on that and not get too worried about the water situation.
    * Water from the Murray Darling is being reserved for towns and cities. As part of the response, most of the remaining large wetlands will be drained (some of these are the last refuge for critically endangered fish species and for hard-pressed bird life. But the city of Adelaide at the bottom of the Murray is still on stage three water restrictions out of a maximum of 5, because its unlikely that next year will be as dry as this I guess (heck what are the odds of it being as dry or drier than a record dry year after all!).

    You’ve got to wonder. I mean, we could deal with this. But first we have to decide to deal with it!

    On a lighter note, this is what we hope the government’s treasurer will say in his up-and-coming budget speech.
    Plus you can make a bet on how many times he will mention climate change.

  48. 48
    pat n says:

    Re: 4

    Actually, the leash on NOAA scientists has been tightened. See ‘Broken policy on hurricanes and climate change (Apr 19, 2007) at:

    http://npat1.newsvine.com/

  49. 49
    emlak says:

    I looked at the site, and it seemed to be a place for deluded people to talk about cattle farts. I just don’t think it’s worth talking to these people; their minds are made up and they won’t listen to reason.

    Tonya, if you want to counteract the effect, start your own blog and talk to those who will listen with an open mind.

  50. 50

    Tonya –

    I’ll try and drop by the site some time. I’m learning not to let snarky opposition comments make me respond in kind. The temptation is always there, but I think I can control it. One key is not to demonize the other side. These folks are not deliberately trying to do the wrong thing; they’ve only read the propaganda from one side and don’t even realize there’s another side to the issue. I don’t agree with the other comments here that say it’s useless to talk to people like that. I may not reach all of them, but maybe I can help at least one or two to start wondering if they might be wrong. Light one candle and all that.


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