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Climate Change and Tropical Cyclones (Yet Again)

By Rasmus Benestad & Michael Mann
Hurricane Katerina
Just as Typhoon Nargis has reminded us of the destructive power of tropical cyclones (with its horrible death toll in Burma–around 100,000 according to the UN), a new paper by Knutson et al in the latest issue of the journal Nature Geosciences purports to project a reduction in Atlantic hurricane activity (principally the ‘frequency’ but also integrated measures of powerfulness).

The close timing of the Knutson et al and Typhoon Nargis is of course coincidental. But the study has been accorded the unprecedented privilege (that is, for a climate change article published during the past 7 years) of a NOAA press conference. What’s the difference this time? Well, for one thing, the title of the paper: “Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions” (emphasis added).

The idea that climate change might actually decrease the frequency of tropical cyclones (TCs) is not an entirely new idea. Indeed, similar conclusions have been reached in earlier work using climate model projections (e.g. Yoshimura et al., 2006, J. Meteorol. Soc. Japan; Bengtsson et al., 2006 J. Clim.; Chauvin et al., 2006 Clim. Dyn.). So what are the key developments in this latest work?

Some background

Before we tackle that question, it is helpful provide a bit more background on the problem. First, it needs to be recognized that GCMs are too coarse to provide a realistic description of ‘small-scale’ (mesoscale) features such as TCs. The atmospheric components of climate models were never really designed for the study of TCs, but the fact that they can produce features with TC-like character when run at sufficiently high resolutions, gives us increased confidence in the possibility that climate models can be used to analyze climate change impacts on TCs. In order to get a more realistic description of the TCs in coarsely resolved climate models, one needs howeveer to ‘downscale‘ the model results.

Knutson et al project future changes in Atlantic TC behavior by using a regional climate model (RCM) which produces tropical cyclones (though ones that are too weak–see discussion below) to ‘downscale’ climate change impacts. This is accomplished by driving the RCM with boundary conditions provided from the various 21st century model projections described in the IPCC 4th Assessment report (IPCC AR4).

Contrasting two recent studies

In certain respects, this new paper is closely related to a paper published last month by Emanuel et al in the Bulletin of the American Meteorology Society (‘BAMS’ to those in the know) which received some press of its own (some of it quite distorted). Emanuel et al . also use a downscaling approach applied to more-or-less the very same climate model simulations. And both studies project a decrease in the frequency of Atlantic tropical cyclones (though see caveats below). But here is where the similarities end.

Emanuel et al use a very different downscaling approach. The use a ‘seeding’ method to randomly generate small vortices analogous to ‘short wave’ tropical disturbances in the real world (the tracks they take are defined in terms of the background atmospheric circulation of the model combined with the so-called ‘self advection’ of the TC itself). They define the probability of development of these vorticies into TCs through a ‘genesis’ model that conditions the favorability of development on various characteristics of the background climate state, and they use a theoretical model to predict TC intensities.

The differences between the conclusions of the two studies are significant. Using essentially the same IPCC model projections, the two studies come to very different conclusions with regard to key projected quantities, such as the seasonally-integrated powerfulness of TCs or ‘power dissipation index’ (PDI). While the Emanuel et al study predicts a clear increase in PDI, the Knutson et al study does not. So which is right?

Well, Knutson et al fully acknowledge that their RCM still has too low resolution to produce realistic TCs (the model resolution is about 20 km, while theoretical estimates indicate that a resolution of about 1 km is likely required to simulate the inner core of intense TCs). TCs are very likely being artificially prevented from intensifying in a warmer climate in the Knutson et al study because of this. By contrast, Emanuel et al‘s approach does not suffer from such resolution limitations.

To their credit, Knutson et al openly acknowledge this weakness in their treatment of TC intensity and PDI. What about their conclusions about a projected decrease in Atlantic TC frequency, which are, after all, the central point of the paper? Here we have reservations as well (and if we were the betting kind might even put forward a wager with regard to future trends). In part, these reservations are a result of the very same issues. The limitations of the RCM for example, as the authors note, also lead to incorrect seasonal and geographic distributions of TC genesis.

Small-scale processes

In the real world, small-scale phenomena such as convection, clouds, gravity waves, and various sorts of eddies may influence mesoscale systems such as TCs, and these unresolved small-scale phenomena are represented through often somewhat simplistic statistical ‘parameterizations’. This limitation is of course common to essentially all atmospheric models, and in and of itself is no reason to dismiss the conclusions of the study.

But TCs do also play a role in terms of the larger scales, as they facilitate transport and a redistribution of heat, moisture and momentum (‘upscaling’). This action is simulated more explicitly in RCMs and more implicitly in GCMs by their parameterization schemes, but it is still not really known if these two levels of modelling provide a physically consistent picture of the scale interactions. The RCM solutions, however, are constrained by the results generated by the GCMs and thus depend on how well the parameterization schemes capture this upscaling effect. Yoshimura et al., 2006 have shown that the solutions may be sensitive to the choice parameterization schemes: they found an increase in TC number over the Indian Ocean if the model used the Kuo cumulus parameterization but a decrease if the Arkawa-Schubert cumulus parameterization scheme was used.


Also significant, perhaps, is the seasonality issue touched on above. The Knutson et al study involves an assumption of a fixed August-October TC season. Yet one important impact of large-scale climate factors which influence Atlantic TC frequency such as ENSO and the NAO (see e.g. one of our own papers on this topic) is their influence on activity during the latter part of the Atlantic hurricane season (one might argue for example that the primary reason for the low TC count during the 2006 season was the early ‘shut-down’ of the season due to increasingly strong El Nino-related wind shear in the autumn as the El Nino set in).

The role of ENSO

ENSO itself, and how it’s influences are represented in the analysis, is potentially an even more fundamental issue. It is well known (and openly acknowledged in both the Emanuel et al and Knutson et al studies) that tropical Atlantic TC frequency is heavily influenced by ENSO variability. This is primarily through its influence on vertical wind shear in the Caribbean and tropical Atlantic, which in turn determines how favorable of an environment incipient TCs encounter as they form and intensify. We have discussed this here in detail before.

Given that ENSO is the dominant source of variability on interannual timescales, it is likely that future changes in ENSO (more specifically, the mean state of the climate and whether it is more “El Nino” or “La Nina” like, i.e. is there a strengthened or weakened ‘Walker Circulation’) could have a profound influence on Atlantic TC frequency. Although the IPCC models project overall a more El Nino like mean state with a weakened Walker circulation, there is far from a consensus among the models. Several credible state-of-the-art coupled models project precisely the opposite. And all of the models used in the IPCC assessment suffer to a varying extent from certain fundamental biases (the inability to produce a realistic ‘ITCZ’ over a large part of the equatorial Pacific ocean–the so called ‘split ITCZ problem’).

The CMIP3 model projections are essentially evenly split as to whether they project an increase or decrease in the magnitude of individual El Nino and La Nina events. Yet the frequency of large El Ninos and large La Ninas means everything in terms of the likelihood of very active Atlantic tropical storm seasons. If all of this sounds familiar to you, its because we made essentially the same point about a year ago in response to a paper that was more or less making the same argument as Knutson et al, though not quite as fleshed out.


The fact that the RCM-based downscaling approach can reproduce the observed changes when fed modern reanalysis data is used by Knutson et al as a ‘validation’ of the modeling approach (in a very rough sense of the word–there is in fact a non-trivial 40% discrepancy in the modeled and observed trends in TC frequency). But this does not indicate that the downscaled GCM projections will provide a realistic description of future TCs in combination with a multi-model GCM ensemble mean. It only tells us that the RCM can potentially provide a realistic description of TC behavior provided the correct input.

Indeed, other purely statistical approaches using large-scale climate predictors of Atlantic TC activity, and which seem to imply different relationships between projected climate change and future Atlantic TC activity (more on this in the future!), also pass similar validation tests with flying colors. So validation against the modern record alone (be it with a dynamical or statistical model) cannot demonstrate the reliability of the future projections. It can only indicate the self-consistency of the analysis.

Future research

Scientifically, where do we go from here? How do we achieve greater clarity on these issues? Obviously, there is need for significant increases in resolution of the RCMs, as past studies indicate a significant sensitivity of results to model resolution. The arguably required, aforementioned 1 km resolution may not be practically achievable in the near term, but the community must strive to move in that direction, particularly if projections of future changes in TC strength, intensity, and power dissipation are to be useful.

Better yet would be to run the coupled ocean-atmosphere models themselves at very high resolution (e.g. 10 km or even finer). This could in principle eliminate many of the thorny issues discussed above, including the potential artifacts of using embedded models with one-way only coupling. But this may be wishful thinking, at least for the foreseeable future.

Final Thoughts

Of most fundamental significance to assessing the reliability of these current projections, in our view, is the “junk in/junk out” factor. The detailed projections made using either the RCM approach of Knutson et al or the ‘random seeding’ approach of Emanuel et al, can only be as good as the large-scale scenarios used to drive them. And since key aspects of those large-scale scenarios as far as Atlantic TC activity is concerned (i.e. what really happens to the ENSO mean state and amplitude of variability) are currently not confidently known, neither can we be confident using the model projections to say what will happen to Atlantic TC activity in the future.

In this respect, we have to consider the entirety of currently available evidence that can inform our assessment of climate change impacts on Atlantic TCs. We know, for example, from the work of Santer et al. that the warming trend in the tropical Atlantic cannot be explained without anthropogenic impacts on the climate. Knutson et al. do not contest this. Furthermore, they do not dispute that the late 20th century increase in Atlantic TC frequency is tied to large-scale SST trends (though they argue that the influence may be non-local rather than local). So we know that (i) the warming is likely in large part anthropogenic, and (ii) that the recent increases in TC frequency are related to that warming. It hardly seems a leap of faith to put two-and-two together and conclude that there is likely a relationship between anthropogenic warming and increased Atlantic TC activity.

What Knutson et al are asking us to do in essence is to put all that aside (because, they argue–in short–that its not the warming but the pattern of warming that matters here) and instead take on faith the perhaps not-much-more-than 50/50 proposition that the mean changes in ENSO state and variability projected by the IPCC multimodel ensemble (which are a key determinant in the projected future Atlantic TC activity) should be trusted.

Given these considerations, we would argue that coastal homeowners, insurers, the re-insurance industry, and every other potential stakeholder in this debate would be wise not to take false comfort from the notion (which the headlines resulting from this paper will inevitably feed) that climate change poses no future Atlantic hurricane threat. In fairness to Knutson et al, they do explicitly point out that their projected decrease in frequency is mostly coming from the weak end of the TC intensity spectrum. In principle, therefore, we imagine that they might perhaps even agree with this message themselves. Indeed, we invite them to comment here!

Update: The authors of the paper have put out an FAQ about their research.

Correction (5/21/08): It has been brought to our attention by NOAA representatives that the NOAA press conference for the Knutson et al paper was not an unprecedented event in recent years. In fact, similar press conferences were held for two other papers (both also questioning the premise that climate change is likely to lead to an increase in tropical cyclone activity) by Wang and by Vecchi and Soden

62 Responses to “Climate Change and Tropical Cyclones (Yet Again)”

  1. 1

    Thank you for that! I had cut out all slides on hurricans from my presentation on climate change after reading the Nature Geoscience paper, because my sense of not trusting the conclusion of the paper was not good enough to stand ground in front of a skeptic. I will now immediately put those slides back in.
    I love your blog – it is absolutely essential for non-climatologists like me to stay informed and well equipped to help educate the public.

  2. 2
    Pepe Larios says:

    Thank you!
    very good job, I read all your post

  3. 3
    Ike Solem says:

    Thanks for the very informative post.

    One issue might be the boundary conditions.

    Knutson et al project future changes in Atlantic TC behavior by using a regional climate model (RCM) which produces tropical cyclones (though ones that are too weak–see discussion below) to ‘downscale’ climate change impacts. This is accomplished by driving the RCM with boundary conditions provided from the various 21st century model projections described in the IPCC 4th Assessment report (IPCC AR4).

    Section 10.3.4 ( Changes in Atlantic Circulation) of the 4th IPCC report states that:
    “In spite of a slowdown of the MOC in most models, there is still warming of surface temperatures around the North Atlantic Ocean and Europe due to the much larger radiative effects of the increase in greenhouse gases.”

    The problem here is that estimates of changes in sea surface temperature and the depth of the warm mixed layer might be very unreliable, since the general behavior of the Atlantic circulation is only now being directly observed – and the most recent findings are that flow rates vary over a whole order of magnitude:

    John Church, Science 2007, “A Change in Ocean Circulation”

    The RAPID/MOCHA (Rapid Climate Change/ Meridional Overturning Circulation and Heat Flux Array) array was deployed in March 2004 to continuously monitor the meridional overturning circulation at 26°N. These observations are a component of a larger set of activities of the World Climate Research Programme’s CLIVAR Project to monitor the North Atlantic meridional overturning circulation. . .

    Cunningham et al. report a year-long average meridional overturning circulation of 18.7 ± 5.6 Sv (3), but with large variability ranging from 4.4 to 35.3 Sv over the course of the year. This range includes all five meridional overturning circulation values estimated from the snapshots analyzed by Bryden et al.; thus, the apparent long-term decrease inferred by these authors may merely be a result of large intra-annual variability. . .;317/5840/935

    The problem is the limited sampling – imagine if the only avaliable measurements of the global jet streams were from airplane-borne monitoring systems. Trying to infer changes in atmospheric circulation from that data would be a very tricky business – but until know, that’s what oceanographers have been stuck with – and the system could use a lot of expansion.

    Some odd things are also going on with relation to large areas of spreading ocean anoxia – with the two most likely culprits being ocean warming and/or nutrient loading (global industrial fertilizer production).

    Not only that, there seems to be a new view that seasonal wind-driven upwelling may play an equally important role in maintaining the Atlantic MOC. For more on how changes in winds are affecting upwelling regimes, see

    That matters because the trickiest part of global climate models appears to be how they handle ocean-atmosphere interactions, and I really have no idea how well they link changes in local wind-driven upwelling to the net thermohaline circulation. In particular, estimates of any future changes in El Nino frequency and intensity seem pretty speculative – and the frequency component might be essentially unpredictable.

    Isn’t it possible to run these regional climate models with a range of boundary conditions? Rather than saying “The IPCC predictions are ironclad”, simply include them as one of the boundary condition sets (toward the low end). That might be a better way to compare the output of different RCMs.

    Soundbite version: “Global warming is expected to increase sea surface temperatures, create a thicker and warmer ocean surface layer, and increase the moisture in the atmosphere over the oceans – all conditions that should lead to a general increase in hurricane intensity and maybe frequency.”

  4. 4
    Jim Bullis says:

    Is it just folklore that hurricanes (I think these are TC’s) occur due to warm ocean water that causes air to rise over a region, drawing in air that then develops into circular winds?

    [Response: Well, that’s part of it, but not the full story. All other things being equal, warmer SSTs mean greater potential latent heat release, which ultimately is the energy source from which the storms derive their power. However, as Knutson et al and others have pointed out, warmer SSTs won’t necessarily be more conducive to more TCs. In a very simple sense, thermally-driven circulations are associated with horizontal gradients in heating. The rising motion tends to take place over the relatively warm surface regions, and the subsiding over the relatively cool surface regions. relatively is the operative word. For example, if SSTs are increasing in the main development region for Atlantic tropical storms, but increasing even more over other regions, then the main development region may not become more favorable for TC development. On the other hand, larger-scale atmospheric circulation changes impacting vertical wind shear (which can place a constraint on TC development) are also important, and this is really where we get to the heart of the current uncertainties, because of issues like how the Walker circulation might change, and what impacts that would have on all of these considerations. I hope that provides a bit of clarification. -mike]

  5. 5
    Mr Henderson says:

    Thanks for the response to no. 4, which clarifies things a lot. The material in the response could usefully have been in the main article for the benefit of us mere arts graduates.

  6. 6
  7. 7
    Spencer says:

    #1 Maiken: cutting out your hurricane slides may not be necessary, but you must be careful not to give an impression that we have a pretty good idea hurricane strength will increase–that will leave you open to just criticism, like Al Gore for one.

    My own approach is to show pictures of hospitals during the 2003 European killer heat wave and pictures of hurricane Katrina submergence and refugees, and to say that it is highly likely that there will be many more such scenes in the future (that is, scenes of mortality, flooding and environmental refugees), due to the probable increase in heat waves and flooding. Confidence in the latter is not related to TC intensity or frequency; it comes from the probable increase in heavy rain events and the inevitable rise in sea level that will make it easier for storm surges to go inland.

  8. 8

    That is why I had left out the hurricane slide in the first place. There is so much very clear evidence of the devastating effects anthropogenic climate change will have on the natural environment and on us humans, that subjects of debate could just distract from the clear catastrophe we are heading towards.

    But from this sentence…

    “So we know that (i) the warming is likely in large part anthropogenic, and (ii) that the recent increases in TC frequency are related to that warming. It hardly seems a leap of faith to put two-and-two together and conclude that there is likely a relationship between anthropogenic warming and increased Atlantic TC activity.”

    ..I was under the impression that TC activity is indeed, at least to a large part, related to anthropogenetic warming.

    Actually, what alternative explanation is there for the increase of TC activity but anthropogenic climate change? And shouldn’t the relative T-difference even increase in the future as shallower coastal waters heat up more quickly than deeper ocean water (except probably in upwelling areas)?

  9. 9
    Steve Bloom says:

    Mike, could you compare/contrast the results of the Wang et al paper (final draft version) that just appeared in G3? There appears to be a contradiction with Knutson et al:

    Atlantic Warm Pool Acting as a Link between Atlantic Multidecadal Oscillation and Atlantic Tropical Cyclone Activity

    “Multidecadal variability of Atlantic tropical cyclone activity is observed to relate to the Atlantic Multidecadal Oscillation (AMO) – a mode manifesting primarily in sea surface temperature (SST) in the high latitudes of the North Atlantic. In the low latitudes of the North Atlantic, a large body of warm water called the Atlantic Warm Pool (AWP) comprises the Gulf of Mexico, the Caribbean Sea, and the western tropical North Atlantic. AWP variability occurs on both interannual and multidecadal timescales as well as with a secular variation. The AWP multidecadal variability coincides with the signal of the AMO; that is, the warm (cool) phases of the AMO are characterized by repeated large (small) AWPs. Since the climate response to the North Atlantic SST anomalies is primarily forced at the low latitudes and the AWP is in the path of or a birthplace for Atlantic tropical cyclones, the influence of the AMO on Atlantic tropical cyclone activity may operate through the mechanism of the AWP-induced atmospheric changes. The AWP-induced changes related to tropical cyclones that we emphasize here include a dynamical parameter of tropospheric vertical wind shear and a thermodynamical parameter of convective instability. More specifically, an anomalously large (small) AWP reduces (enhances) the vertical wind shear in the hurricane main development region and increases (decreases) the moist static instability of the troposphere, both of which favor (disfavor) Atlantic tropical cyclone activity. This is the most plausible way in which the AMO relationship with Atlantic tropical cyclones can be understood.”

  10. 10
    Steve Reynolds says:

    >(the model resolution is about 20 km, while theoretical estimates indicate that a resolution of about 1 km is likely required to simulate the inner core of intense TCs). TCs are very likely being artificially prevented from intensifying in a warmer climate in the Knutson et al study because of this.

    Can you say what evidence justifies the second statement?

  11. 11
    Cheska says:

    …”The idea that climate change might actually decrease the frequency of tropical cyclones (TCs) is not an entirely new idea.’…

    I can see where they are coming from if I keep in mind that there are several
    schools of thought regarding quantum reality that physicists hold, and sub schools of thoughts.
    Some believe there is no deep reality (Copenhagen interpretation), some do not, many worlds interpretation, quantum logic, neorealism, to name a few. Quantum theory schools of thought proponets range from Heisenberg to Schrodinger to Dirac to Von Neumann to Bohm to Bell to Feynman… Crowd waves (dilution), non cloud Frequency canceling, repetitious oscillations recycling in time and space, waves meeting in and out of phase, phase relations, periodicity, sum of amplititudes add, out of phase waves do not, superposition principle, and so on. Another words, the ocean can indeed be very calm if the next incoming wave arrives out of phase ( half a cycle or so), even if the preceding wave was two feet in amplititude due to two waves arriving in synch (on time). Waves adding or subtracting (subject to wave differences) is called interference (constructive, destructive). Wave’s intensity, (energy), amplitude squared.

    Not that I could say whether I would agree or disagree with their findings, but I can understand where they are coming from.

  12. 12
    Timothy Chase says:

    The BBC, which quotes Held, emphasises the high resolution of the model and the high degree of confidence the authors have in the results:

    Because there was a high degree of confidence that the sea surface temperature trend was going to continue to rise, Dr Held explained, people had “tried to conclude that hurricane activity will increase rather dramatically in the future”.

    “We tried to simulate the fundamental fluid dynamics and thermodynamics that control hurricane genesis in the Atlantic in a numerical model to a very high resolution.”

    ‘Fewer hurricanes’ as world warms

    By Mark Kinver
    Science and nature reporter, BBC News
    Sunday, 18 May 2008

    In contrast, ABC News quotes Knutson, and gives us quite a different take:

    And Knutson said this study significantly underestimates the increase in wind strength. Some other scientists criticized his computer model.

    Study says global warming not worsening hurricanes
    May 18, 2008 (AP)

    The following would seem particularly relevant:

    Knutson acknowledges weaknesses in his computer model and said it primarily gives a coarse overview, not an accurate picture on individual storms and storm strength. He said the latest model doesn’t produce storms surpassing 112 mph.


    … since the mainstream view, to the extent that such exists on the topic of hurricanes in the Atlantic, would seem to be that the number of hurricanes will not increase, but the intensity of the strongest storms will. It is this very point which the study is entirely unable to address.

  13. 13
    Ike Solem says:

    A better paper on this topic is Holland & Webster 2007, and is worth looking at:

    Overall, there appears to have been a substantial 100-year trend leading to related increases of over 0.78C in SST and over 100% in tropical cyclone and hurricane numbers. It is concluded that the overall trend in SSTs, and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming.

    They also make an important point: historical records of hurricane intensity are of no value prior to 1945 or so:

    However, for the period prior to 1945, we consider that intensity estimates are too uncertain for definitive analysis, especially for differentiating between hurricane classes.

    Holland and Webster’s discussion breaks the 20th century down into three regimes, as follows:

    Between 1855 and 1900 cyclone numbers averaged 7–9 per year and the SST anomaly remained around -0.1 to -0.2C. This period was followed by a marked decrease of 30% to an average of six tropical cyclones per year from 1905 to 1930, which we designate climate regime TC1. Annual numbers then increased to a new stable regime averaging around 10 tropical cyclones per year from 1931 to 1994 (climate regime TC2). This increase was accompanied by a rise in SST of 0.2–0.3C and an upward shift of the annual variability to typically 6–14 named storms. Since 1995, the NATL has experienced a sustained upward trend in tropical cyclone numbers which may not as yet have settled into a stable regime. Accompanying this increase in tropical cyclone numbers is a marked SST warming to unprecedented anomaly levels exceeding +0.7C. We suggest that this trend may represent a transition to a third, and as yet undefined, climatic regime (TC3) in which cyclone frequency could stabilize at significantly higher numbers than characteristic of any previous period.

    I don’t see what there is in Knutson’s work that challenges that in any meaningful way, so if there is an anthropogenic influence on sea surface temperatures, there also must be an anthropogenic influence on hurricanes.

    In fact, the evidence for a good link between human activities, sea surface temperatures and hurricanes is a good deal more robust than the evidence for a periodic or quasiperiodic or chaotic cycle in the Atlantic Ocean circulation which is supposed to bring more warm water north – “a positive phase of the AMO”.

    Would someone care to propose or explain a mechanism for the AMO, maybe something along the lines of the explanation for El Nino / Southern Oscillation? I’ve never heard a single coherent explanation, other than that’s what you get when you squeeze the data through a time series analysis program… which can easily be nothing but applying Fourier analysis to noise and coming up with a quasi-periodic result. A “40-80 year time period” – based on one century of data? That’s pretty bad as an explanation for the observed increase in hurricanes. As it is, recent studies show that the annual yearly variation in thermohaline flow in the Atlantic can vary across an order of magnitude – how does that fit into the AMO hypothesis (which is still widely featured in news reports)?

  14. 14
    Alex J says:

    The AP/Seth Borenstein article has spread all over the place, including an abbreviated local news version (here). Interesting title: “Study says global warming not worsening hurricanes“, when the study only seems to suggest that the frequency of Atlantic hurricanes isn’t being heightened by warming, while intensity remains a different story.

  15. 15
    Alan K says:

    It seems there is no consensus on the effect of any warming on TCs. No one can agree on how they form, what forms them, whether there will be more or fewer of them or if they will be weaker or stronger. A TC is an important natural phenomenon, a product of climate manifested as weather. Clearly the science isn’t yet well understood and the significant gap in GCMs reflect this. Of course this begs the question of what else is not well understood by GCMs or is it just TCs?

  16. 16
    Pat N says:

    Unprecedented privilege by NOAA in doing a Press Release for the Knutson article is clearly at odds with what the NOAA director Conrad Lautenbacher said in urging creation of a National Climate Service in NOAA,- quotation from May 13, 2008 story:

    … Today everybody just cherry-picks the data that support their point of view, Lautenbacher said of the debate over climate change. “We need to deal with this in a scientific manner.” …

  17. 17
    Pat N says:

    Editing correction: Comments by NOAA’s Lautenbacher on May 13th were not “at odds” – but a case in point!

  18. 18

    If anyone is interested, I’ve revised my planetary temperatures page and the javascript calculator based on it. I eliminated the confusion over partial pressure, which resulted in a much better fit. I also added a page explaining what partial pressure is, and noting the paradox of the gravity-pressure equation giving a different number.

  19. 19
    Mauri Pelto says:

    #12 Excellent point Ike. I will take actual data over model generated any day. It is hard to explain the observed increase in the number of major hurricanes without invoking global warming. The concluding sentence on Knutsen’s webpage on hurrioanes and global warming is “An implication of the GFDL studies is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse-gas induced warming may lead to an increasing risk in the occurrence of highly destructive category-5 storms.”

  20. 20
    Mr Henderson says:

    Re. no 14: The slightest hint of a genuine scientific dispute, and someone always interprets it to mean that the whole of AGW theory is riddled with uncertainties. But it just isn’t like that: it’s clear that there must be broad agreement on how TCs form and what forms them, because if there wasn’t, the Knutson paper could not have been written; it uses that agreement as its starting point. The ‘significant gap in GCMs’ is not because ‘clearly the science isn’t yet well understood’; it just corresponds to the fact that Global Climate Models are not Regional Climate Models. And even if there was significant uncertainty about the probability of global warming, that would be no cause for complacency, since it could mean that things were going to turn out worse than predicted.

  21. 21
    Mr Henderson says:

    Whoops… I mean (no. 19, last sentence) ‘the probability of a particular level of global warming…’.

  22. 22
    Lawrence Coleman says:

    Australia’s most respected climate scientists and australian of the year last year has sounded this dire warning are some of his key points. 1/ there is now considerably more greenhouse gasses in the atmosphere to cause catastrophic and permanent climate change. 2/ cutting GG’s now, alone will NOT save the planet, we have to extract the CO2 out of the air en-masse, convert it to charcoal and plough it back into farmer’s feilds….but 3/ in the meantime we have to pump sulphur via aircraft exhausts into the upper atmosphere to cause global dimming (this is not the solution long term but it may buy us valuable time in the short term). This procedure is untested and unprecedented but may save most life on earth. This will change the color of the sky however. 4/ all wealthy farmers or individuals or corporations should pay poor farmers in the equatorial regions to plant trees, still having enough food crops to live by but plant O2 producing trees for income. He says that most if not all our natural feedback mechanisms are now failing and working against us and it is absolutely vital that we act decisively NOW. 5/ On a more national level he said that the Rudd gov should lift the means test for the $8000 solar panel rebate. He has based his speech today on the latest and most accurate climate data to date.
    I could only wish the CC skeptics out there could read this..and maybe just maybe the penny would finally drop.

  23. 23
    Thom says:

    I wonder how long it will take the Bush administration to start shoving Chris Landsea in front of every available camera like in the past.

  24. 24
    Alan K says:


    “cutting GG’s now, alone will NOT save the planet, we have to extract the CO2 out of the air”

    “in the meantime we have to pump sulphur via aircraft exhausts into the upper atmosphere to cause global dimming ”

    “This procedure is untested and unprecedented but may save most life on earth”

    beyond parody

  25. 25
    Ossi says:

    On a completely unrelated matter, are going to deal with that paper by Ferenc Miskolczi at some point? It’s being cited by climate change skeptics quite often and I would like to know how much merit it has.

    [Response: None. – gavin]

    [Response: The paper does have some value as a teaching tool in undergraduate physics or climate classes. It’s like those puzzles in kids’ books our Swedish friends call “Finn Fem Fel.” I ran the paper past a guest class I was teaching at Bowdoin and with a little encouragement the students were able to find and understand the two elementary mistakes Miskolczi made in the first 9 pages, which invalidate the rest of the paper. The students in that class are writing up their work, and it’s my intention that we’ll put it up on RealClimate as a guest post, once it passes muster with the rest of the RC crowd. I’m still waiting for the first draft. Meanwhile, you can try your own hand at finding the mistakes. The only hint I’m giving at this point, so as not to spoil the fun, is that you needn’t look beyond the first 9 pages. –raypierre]

  26. 26
    Aaron Lewis says:

    Does anyone want to address the issues North American tornadoes? Was the early start of the tornado season this year, in any way attributable to AGW? Can we expect greater numbers of more intense tornadoes over a larger geographical range in the course of longer tornado seasons?

  27. 27

    We appreciate your interest in our recent paper on Atlantic hurricane frequency and global warming. Our goal in this ongoing work is to move this discussion to firmer physical grounds by bringing new kinds of modeling, based more closely on the underlying fluid dynamics and thermodynamics, to bear on the question. Relying on statistical studies can take us only so far. Our regional modeling effort is far from perfect, as you and others have noted, but we feel that it is a significant advance to begin addressing the hurricane/global warming connection with these kinds of simulations. To get a feeling for the kind of model on which our paper is based, we encourage readers to look at the animations in the paper’s supplementary material (or see here .)

    Our work is a “downscaling” study, in which we first simulate past hurricane seasons, using as input observed sea surface temperatures (SSTs), the observed state of the atmosphere at the boundaries of our Atlantic domain, as well as the largest scales in the atmospheric flow over the Atlantic. We then perturb this input with the change in the seasonal mean SSTs and the seasonal mean state of the atmosphere as projected by an ensemble mean of global models for the end of the 21st century. Several of your reservations about our work are related to possible inadequacies in this input from the global model projections. Our paper does not address this issue. We are interested, as you are, in improving these large-scale projections, but we feel that the ensemble mean of the CMIP3 models is a logical place to start in this kind of downscaling study.

    We are in the process of downscaling individual global model projections, and preliminary results (as mentioned briefly in the paper) show substantial departures from the result using the ensemble mean. This is a slow process, as these simulations are at the limit of what we can do with the computer power available to us. But we see no indications, as yet, that any of these calculations will show an increase in Atlantic hurricane frequency remotely comparable to that obtained by extrapolating the recent trend. (As mentioned in the paper, when downscaling the GFDL CM2.1 model, rather than the ensemble mean, the number of hurricanes stays roughly unchanged by the end of the 21st century, and we see a substantial increase in the strongest model storms, those that exceed the surface pressure criterion for category 3. Of the models that we have examined to date, this is the one that shows the greatest tendency to enhance activity).

    You seem to criticize us for putting observed statistical relations aside and trusting in our dynamical model. But the question is whether or not these statistical relationships have any predictive value. If one examines our model’s control simulations for the 1982-2006 period, which show a trend towards increasing hurricane activity over this period, and correlates this activity with SST in the Main Development Region, and then tries to use this correlation to predict the 21st century behavior of the model, it clearly doesn’t work. Our results are supportive of a very reasonable physical picture in which relationships between hurricane activity and spatial structure in SST are more relevant than those connecting activity to local SST only. We believe that storm frequency in our model is controlled by shears and by the gravitational stability of the vertical temperature profile (we are currently trying to determine the relative importance of these factors). Both of these factors, in turn, are primarily controlled by the spatial structure in the SST field.

    Since our results represent, to our knowledge, the first calculation of this type, we do not know, and are anxious to learn, if different assumptions made in the downscaling result in substantially different results concerning changes in storm frequency, or if most of the uncertainty arises from the global model input.

    Turning to very important question of the frequency of the strongest storms, it is entirely possible that a large increase in category 4-5 storms will result from increasing greenhouse gases, despite an overall reduction in hurricane numbers. Our model adds little new information on this question because of its failure to simulate these very strong storms.

    [Response: Thanks for your comments. The way I look at it is whether the CMIP3 runs – which I too consider to be the best data set for future projections – are capable of providing regional details with sufficient accuracy and fidelity so that they can be used to make projections for TCs. There is a fundamental limitation to RCMs, as they are constrained by the coarse resolution boundary conditions provided by the GCMs (which tend to favour a more El Nino like state on average). These GCMs were – as far as I know – never designed to study TCs, but are suitable for studying the larger picture of our climate. RCMs provide more details, but don’t the ‘question’ the environment on which TCs depend. In Scandinavia, new efforts with RCMs have started, where RCMs are now coupled to regional ocean and sea-ice models, as there are air-sea coupled processes that become important for these small-scale processes and uncoupled models are not sufficiently realistic (otherwise, people wouldn’t bother). Likewise, cyclones play a role in the global heat, moisture and momentum budgets, and my concern is that the GCMs still have some problems capturing all these aspects in a realistic manner through simple parameterization schemes (they tend to have regional biases) even though they provide a realistic over-all description. Such details may or may not be important depending on your scientific question, but when the outcome depends on a fine balance of competing ‘forces’, then we really have to be careful. I would furthermore like to argue that one always should back up RCM studies with statistical downscaling which at least provides an independent assessment (although it may suffer from different caveats). Additionally, one should look at the 20C3M runs to see if they capture the trends of the past. -rasmus]

    [Response: Guys, thanks for your comment. The study is an important one and points us in the right direction in terms of approach. I just think that the confidence in the conclusions has in places been overstated. There will no doubt be other similar analyses in the months ahead using different approaches, and it will be interesting to compare the results and conclusions. -mike]

  28. 28
    Hank Roberts says:

    Aaron, Google: tornado shear helicity
    will help; lots of recent info, not enough data collected over enough years to tell if wind shear/helicity numbers are changing overall. My speculation — wind shear defusing hurricanes but pumping up tornados — didn’t seem apt, the size/scale is so different between the two kinds of storms that wind shear means something different for each type.

  29. 29
    Bocco says:

    Do any GCMs have variable grid sizes? I wonder because in some types of modelling one puts more grid points in areas where more interesting (or changable) things are happening. I could imagine (at a push) that say the modelling of the climate over a vast flat area of ocean might be achievable in a few less grid points than over say the Himalayas or Rockies. If one divides the earth into 50x50km blocks then I suppose that’s about 200k grid points (times however many layers you want in the atmosphere) and as for the oceans I have no idea. Did anyone try using different distributions of those 200k grid points, with more points in interesting places (I don’t know, erm, say places where hurricanes generally form)? Or do any of the models out there dynamically change the grid size, keeping the total number of points the same, but putting more grid points in at places where gradients in variables are largest? If you would expect better results from 1km grid, which is just not possible computationally at the moment, why not try other ways on tipping the balance in favour of even better results. I suspect however, that this has been tried, and the answer I’ll get will be something like: tried it, but the gains are marginal…

  30. 30
    doug rogers says:

    Back to #4’s response…. just me a non-science layperson trying to understand this; water vapour rising over warm water… condensing over cooler water…. but as the oceans are warming, there is less relative difference between the warm and cool, so less frequent release, meaning fewer storms, but of greater intensity. ?

  31. 31
    Harold Brooks says:

    Re: 26. It’s unlikely the early start to the tornado season was in any way attributable to AGW. It’s likely that the January-February tornado count will be the highest on record (adjusting for changes in reporting practices), but it’s not clear that it’s qualitatively different from the 1971 and 1999 starts. It’s also important to note that 2002-2004 are three of the six slowest starts to the season and 2001 and 2005 were also well below normal.

    There’s really no evidence to support an expectation of greater numbers of more intense tornadoes over a larger geographical range in the course of longer tornado seasons. If we restrict attention to F1 and stronger tornadoes, which are the strongest 1/3 of current reports and seem to have been relatively consistently reported, the biggest era in the US was the early 1970s, when the US average temperature was at its lowest since ~1920. The correlation of F1 tornadoes and annual temperature in the US since 1954 is -0.1, but that’s essentially zero. There’s also no particularly evidence to suggest that the temporal pattern of the tornado season has changed in any appreciable way. There is a lot of interannual variability. If we break the first two months of the year down into the most frequent and least frequent tornado years since 1950 (the official database goes back then, although it didn’t get really started until 1954), and count how many years in each decade are in the “big” and “little” starts to the year, we get

    Decade Big Lit
    1950s 4 2
    1960s 4 4
    1970s 6 4
    1980s 1 6
    1990s 4 1
    2000s 4 3

    (There are more in the big years because of a tie for 19th fastest start.) Other than the fact that the late 1970s through 1980s were slow starters (9 of the 12 years from 1977-1988 make the little list), there’s not much to say about the list.

    The climate model studies (three papers in the last year) have focused primarily on conditions associated with large hail, suggesting that environments supportive of the largest hail will become more frequent. There’s also some support for historical increases in large hail frequency in the US

  32. 32

    Re #29 where doug rogers Says:

    “Back to #4’s response… water vapour rising over warm water… condensing over cooler water…. but as the oceans are warming, there is less relative difference between the warm and cool, so less frequent release, meaning fewer storms, but of greater intensity?”

    Water vapour does not condense over cooler water. It cools when it has been raised to a colder altitude. This is obvious in the case of a hurricane where the clouds form above the warm water which is generating the hurricane, (not over the trail of cold water left behind the hurricane.

    But warm air can be lifted in two ways; natural convection and forced convection. Natural convection is due to the low density of the air caused by the lighter water vapour. One type of forced convection is when the cold air from the poles flows under the warmer subtropical air, which results in wind shear.

    This would imply, that in a warmer world where the Arctic ice had vanished, there would be less wind shear so allowing more hurricanes to form in the tropical North Atlantic.

    I can’t imagine that Knutson included the disappearing Artic ice in his model :-(

    Cheers, Alastair.

  33. 33

    No time to read all this, but here are a few of my ideas:

    1. That would be wonderful if at least Atlantic TC reduce or do not increase with GW, since GW is and will be doing so much greater harm thru droughts, floods, disappearing glaciers, disease spread, ocean anoxia (with HS outgassing likely to follow), species loss, heat deaths,…am I leaving anything out? TCs are only a small portion of the tremendous harm GW is and will be causing, perhaps for many millennia (brought to you future peoples by our generation). So that’d be great if Atlantic TCs decrease. We can put that along side less deaths due to a decrease in snow shoveling. The net harm from GW, however, is and will be vast.

    2. As pointed out in the post, increasing sea surface temps due to GW is a necessary, if not a sufficient cause for increased TC intensity and frequency. And I’d think predicting all those nitty-gritty sufficient causal variables would be harder to do than predicting global warming. Why they can’t even predict rain accurately a few days in advance. So if skeptics are attacking GW science, finding all sorts of (mostly bogus) problems with it, they ought to be able to rip TC causal variable prediction (minus the SST variable) to shreds.

    3. I’m still buying insurance for my house near the Gulf of Mexico; and I’m still reducing my GHGs.

    4. RE #31 & increasing hail. During Hurricane Emiliy in 2005 there was a serious hail storm in Brownsville, TX, breaking windows, etc. Our weatherman said he had never heard of hail during a hurricane. When I asked here at RC about it, someone who seemed to know what he was talking about said that indeed hail was not expected during hurricanes. So this might be some avenue to research — possible hail storms during hurricanes (even if perchance they do reduce with GW).

  34. 34
    Jeff says:

    Re: Ferenc Miskolczi

    What? You mean atmospheres don’t orbit?

  35. 35

    Dear Friendly Neighborhood Scientists,

    We need some help again over at Dot Earth. Andy published a post on these articles, and the comments are so discouraging. Thanks,

  36. 36
    doug rogers says:

    Okay Alistair, So what Knutson seems to be saying is that it’s kind of a zero-sum game. He is saying his modelling predicts fewer hurricanes, but of greater intensity. So, as to my understanding then, it’s not the cold water, but the cooler atmosphere, and as the atmosphere is warmer, then… there will be less relative difference and fewer occurances…. ?

  37. 37
    wmanny says:

    25 Ossi

    To those of you interested in seeing the actual 43-page, peer-reviewed article and its citations, over and above the curt dismissals of gavin, “raypierre”, and Nick Stokes, here is the link to the work itself:

    [Response: … caveat lector! – gavin]

  38. 38
    Ossi says:

    Re: 25
    “Meanwhile, you can try your own hand at finding the mistakes. The only hint I’m giving at this point, so as not to spoil the fun, is that you needn’t look beyond the first 9 pages.”

    Okay, I tried reading the paper. Are the mistakes related to the use of virial theorem and Kirchhoff’s law?

    [Response: Got it in one! There are probably other mistakes in there as well, but these will do. As a further exercise, you can ask whether the virial theorem, correctly applied, gives you any further information about an atmosphere in hydrostatic balance. –raypierre]

  39. 39

    Hi doug,

    As you climb up a mountain the temperature drops by 1F for every 300 feet of altitude you ascend. Eventually, if the mountain is high enough you will reach the snow line.

    It is the same with rising air. It cools, and if it get high enough any water vapour it contains that has not already turned into cloud, condenses into ice clouds. Global warming will not stop the water vapour condensing. It may make it condense at a higher altitude, but since the air at the surface will be warmer and hold more water vapour the condensation level may remain at the same height.

    Knutson et al. are saying that “Using projected boundary conditions for the end of the twenty-first century, the frequency of Atlantic tropical cyclones and hurricanes in a regional climate model of the Atlantic basin is reduced compared with observed boundary conditions at the end of the twentieth century. This is inconsistent with the idea that higher levels of atmospheric greenhouse gases will result in increased Atlantic hurricane activity.”

    In other words, global warming will lead to less North Atlantic hurricanes, not more as had been generally expected because of the rise in sea surface temperatures.

    What I am saying is that they used a regional climate model which did not include the changes in the Arctic region, and the boundary conditions they used were those predicted using GCMs that we know got the Arctic ice wrong! Their results are useless, but no scientist would dare to point that out. It is like saying the king is wearing no clothes :-)

    Cheers, Alastair.

  40. 40

    Re #38

    where Ossi wrote “Okay, I tried reading the paper. Are the mistakes related to the use of virial theorem and Kirchhoff’s law?”

    You seem to have found that problem pretty easy, although I have spotted another error. Perhaps you would like another challenge. Can you find where Miskolczi has got it right? No need to restrict yourself to the first 9 pages. But perhaps I should admit that I can’t answer that problem :-)

    Cheers, Alastair.

  41. 41
    Jerry says:

    Re: #32 “Natural[free]convection is due to the low density of the air caused by the lighter water vapour.”

    Nonsense! The “low density” of which you speak is a result of the release of latent heat that occurs when water vapor condenses to form clouds. (See any introductory meteorology textbook!)

  42. 42
    doug rogers says:

    I am trying to understand in lay terms, why Knutson et al feel there would be fewer low intensity hurricanes. Alastair points out that the model they use doesn’t deal with known variables, has initial presumptions that have been shown to be incorrect and that their sample is essentially too small.

    Okay, these criticisms I can understand. But it doesn’t explain plainly why they got the results they got.

    I appreciate that you guys are all quite the scientists and that kind of hard criticism needs to be leveled at these studies, no matter what. Part of the problem is with news media summing up stuff glibly, and uneducated critics trumpeting those ‘conclusions’ as some kind of proof.

    So how are they modelling these TCs? Same as everybody else? Why would they choose to use faulty sources?

  43. 43
    Aaron Lewis says:

    Re 28
    Hank, I guess the problem that I have is recent statement on Open Mind ( dismissing the recent increase in tornado sightings to improved detection and record keeping,

    I feel that there is a plausible expectation that more latent heat as a result of AGW would lead to more tornados of increasing intensity. Moreover, HB points out a shift in seasonality. In an example that he does not use, there were tornadoes in Wisconsin this last January, and January tornadoes had not been seen in Wisconsin since 1844. It is worth remembering that in the “old days” farms were smaller, so there were more farmsteads per section, and every farmstead had to keep an eye on the weather. My point is that we are seeing weather that has not been seen for a long time.

    We have an observed trend in the number of tornadoes supported by an expectation of more tornadoes as the result of more available latent heat to drive them. Why is everyone so reluctant to say, “Global Warming” when we have an exceptionally fine swarm of tornadoes?

    This was the heart of my question.

  44. 44
    tamino says:

    Re: #43 (Aaron Lewis)

    I don’t deny the possibility of an increase in tornado frequency or severity, but I haven’t yet seen hard evidence of it. Improvements in tornado detection are undeniable; the increase in population alone has an impact. The record of “wide” tornadoes (the measure I used to distinguish more powerful ones) shows a distinct change in 1995 but no other sign of any change. And since an active researcher in that field commented that the definition of, and measuring procedures for, width data were changed in 1994 (first affecting 1995 data), it’s also undeniable that the change in that year is at least in part (and possibly, entirely) due to the changed data procedures.

    I repeat, it’s by no means impossible that tornado frequency or severity has changed — but the data I’ve seen don’t establish it reliably.

  45. 45
    Harold Brooks says:

    Re: 43

    We also have an expectation of fewer tornadoes with decreasing vertical wind shear. Since shear is more important for tornadoes, given that you’ve got some CAPE, it’s not at all clear what impact global warming will be on tornado occurrence. It’s just as reasonable from physical principles to expect a decrease as to expect an increase.

    Actually, the last January Wisconsin tornado was an F2 or F3 in 1967 in Green and Rock counties.

    There’s no trend in the number of F1+ tornadoes, going back to 1954. The increase is entirely in F0 tornadoes, which we have good reason to believe are better observed now than in the past. Personally, my years of researching the climate and severe thunderstorm connection give me no evidence to predict a change in frequency or intensity of tornadoes. (I think the support for an increase in very large hail in the US is stronger, from theoretical and modelling perspectives, as well as a hint in the observational database. There’s also some support for the notion of a decrease in small hail associated with a raising of the freezing level height, based on observations from China.)

    I think one of the reasons why many people don’t jump on tornado outbreaks as associated with global warming is that it’s hard to say why this year is particularly different than 1999 or 1998 or 1971, which had big starts to the year and continued big through most of the year. You also have to explain the very slow starts to the tornado season from 2001-2005.

  46. 46
    Andy Gunther says:

    I thought the key issue with TC impacts is increasing storm power (power dissipation index), not increasing storm frequency. It seems to me that the popular press frequently does not make this distinction. At the end of their comment #27 the authors state “Turning to very important question of the frequency of the strongest storms, it is entirely possible that a large increase in category 4-5 storms will result from increasing greenhouse gases, despite an overall reduction in hurricane numbers. Our model adds little new information on this question because of its failure to simulate these very strong storms.”

    So isn’t not fair to state that we see increase in TC power in recent decades, and that this is expected giving rising SST, which are driven by greenhouse gas forcing. The new paper does not suggest a decline in storm power, and even if power does not continue to grow at the current rate we can still expect a future with more powerful and damaging TCs.

  47. 47

    Re #41 Where Jerry Says:

    Re: #32 “Natural[free]convection is due to the low density of the air caused by the lighter water vapour.”

    Nonsense! The “low density” of which you speak is a result of the release of latent heat that occurs when water vapor condenses to form clouds. (See any introductory meteorology textbook!)


    You are right that when the water vapour condenses the released latent heat causes further convection, especially in hurricanes.

    But in order for the air to reach the cloud base where condensation begins, the air has to be lifted from the sea surface (or an evapotranspiring land surface.) That happens because the density of the air is “diluted” by the addition of water vapour with a molecular weight of only 18 compared to air with a molecular weight of 29.

    Cheers, Alastair.

  48. 48
    Eric (skeptic) says:

    It seems a little odd to validate the models only at a seasonal macro
    scale after adjusting small scale parameters since that would
    aggregate the effect of parameterizations across large scale pattern
    changes within each season and make it impossible to determine cause
    and effect. It would seem better to validate the models using
    individual historical storms by replicating the actual historical
    conditions in the model.

    Can the small scale parameterizations also be validated (for example
    in smaller scale models)? For example, the predicted change in PDI
    would seem to be dependent on the rapid intensification process, which
    you imply is not modeled.

    One parameter for intensification is obviously SST which could dictate
    a percentage of storms of each intensity. Or more realistically, the
    time to rapid intensification could be based on SST. But SST is not
    the cause of intensification, only supportive of it. The other
    factors, storm dynamics, wind shear, vertical temperature profile,
    humidity, etc, can be added to the parameterization, but should be
    subject to small scale validation (i.e. weather forecasting).

    Once intensified, the SST would help sustain storm intensity, again
    with paramterizations that include the other factors subject to

    Seasonality questions: does negative NAO always go away in the summer
    in the models? What makes it go away and come back in the fall?

    [Response:The NAO is really just a measure of the dominant mode of variability of the Northern Hemisphere jet stream over the North Atlantic and neighboring regions. The jet stream is strongest (and most equatorward) during the boreal winter, and weakest (and most poleward) during the boreal summer. For these reasons, the NAO has the greatest opportunity to impact tropical cyclone behavior in the late fall, when its reach is beginning to extend into the northern tropics, its impact is beginning to become substantial, and the TC season is still in full gear. A similar argument holds for El Nino influences. – mike]

  49. 49
    Geoff Beacon says: (‘Fewer hurricanes’ as world warms) has been mentioned earlier. I can’t make sense of

    Hurricanes and tropical storms will become less frequent by the end of the century as a result of climate change, US researchers have suggested.

    But the scientists added their data also showed that there would be a “modest increase” in the intensity of these extreme weather events

    and what the authors say above

    Turning to very important question of the frequency of the strongest storms, it is entirely possible that a large increase in category 4-5 storms will result from increasing greenhouse gases, despite an overall reduction in hurricane numbers. Our model adds little new information on this question because of its failure to simulate these very strong storms.


  50. 50
    Sashka says:

    [W]e know that (i) the warming is likely in large part anthropogenic, and (ii) that the recent increases in TC [tropical cyclone] frequency are related to that warming.

    Can I ask you how exactly do you know about (ii)?

    [Response: This is demonstrated in the 20th century experiments by Knutson et al (as well as other studies using validated statistical models) so it is not being contested by the current study. -mike]