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Severe Tropical Cyclone Pam and Climate Change

Filed under: — stefan @ 18 March 2015

Guest post by Kerry Emanuel

In the past 16 months, two exceptionally intense tropical cyclones, Haiyan and Pam, have struck the western Pacific with devastating effect. Haiyan may have had the highest wind speeds of any tropical cyclone on record, but we will never know for sure because we do a poor job estimating the intensity of storms that are not surveyed by aircraft. (Currently, only North Atlantic tropical cyclones are routinely reconnoitered by aircraft, and only if they threaten populated regions within a few days.) Pam’s analyzed intensity puts it within 10 knots of the most intense storms on record in the South Pacific, but here again this is within the error bars of satellite-derived intensity estimates.

Pam’s high intensity and terrible impact on Vanuatu have invariably raised the question of the possible effect of global warming on its characteristics. For example, Vanuatu’s President Baldwin Lonsdale blamed the disaster partly on climate change. Just as predictable is the backlash to the effect that no single event can be attributed to climate variations of any kind. What can we say about the effects of climate change on South Pacific tropical cyclones?

We can begin by looking at the record of tropical cyclones in that region. Unfortunately, for the reasons discussed above, these records are poor and those that exist only go back to about 1980, though there are longer records of storms making landfall in Australia. Perhaps the best existing analysis of South Pacific tropical cyclones is that of Kossin et al. (2013), who homogenized the satellite data record from 1982 to 2009 to create a temporally consistent record, and compared that to the problematic historical data base of storms over the world. While the historical data in the South Pacific region show a highly significant upward trend in the incidence of high intensity events, the satellite-based record shows a less prominent and significant trend of 2.5 m/s per decade with a p value of 0.09. Thus there is some evidence of a trend toward higher intensity of high category tropical cyclones in the South Pacific over the period 1982-2009, but it is not conclusive and in any event spans a limited time interval.

We can also look at trends in important environmental factors that are known to influence tropical cyclones. The usual suspect among these is sea surface temperature (SST) and there has been much talk about the elevated SST’s in the region where Pam developed. But SST by itself is not the main factor in the existing theory for the upper bound on tropical cyclone intensity, known as the potential intensity; instead, the potential intensity depends more nearly in the difference between SST and a measure of the bulk temperature of the troposphere as well as the temperature of the tropopause. An expression of the potential intensity, measured in maximum possible wind speed, is


where Vp is the potential maximum wind speed, Ts is the surface temperature, Tt is the tropopause temperature, hs* is the saturation moist static energy of the sea surface, and h* is the saturation moist static energy of the free troposphere, which is nearly uniform with height if the lapse rate is moist adiabatic. In the deep tropics, temperature is nearly uniform on pressure surfaces because there is not enough Coriolis acceleration to balance strong pressure gradients, thus h*, which is just a function of pressure and temperature, is horizontally as well as vertically uniform in the free troposphere. Therefore, the potential intensity depends mostly on variations of SST (which controls hs*) for climate variations that do not affect the mean temperature of the troposphere. But global warming very definitely does affect the temperature of the tropical free troposphere, so it is not possible to conclude, as alas many have, that increasing SST per se means increasing tropical cyclone intensity (though it usually does signify more TC-related rain).

It is not difficult to calculate the actual potential intensity from SST and atmospheric soundings, and this can be done as well for reanalysis and global model data sets. The map below shows the potential intensity at 12 GMT on 7 March, 2015, calculated from the NCEP operational analysis at that time. The track of PAM is superimposed in blue.


Pam traversed a region of potential intensities around 75 m/s, consistent with the storm’s estimated peak intensity. These values are not unusual in this region, but for the past few decades they have been increasing if the reanalyses are to be believed. I calculated potential intensity trends over the period 1980-2012 using three different reanalysis products: NASA’s MERRA, the European Center’s ERA Interim, and the NCAR/NCEP reanalysis. The MERRA and ERA Interim reanalyses show upward trends of around 2 m/s per decade in the region where Pam formed, while NCEP’s trend is closer to 3 m/s per decade; all these trends have p values less than 0.1. These trends are broadly consistent with the Kossin et al. (2013) trend of 2.5 m/s per decade in the observed intensity of high category tropical cyclones in this region.

Thus the weight of evidence points to increasing potential intensity in the region where Pam developed, and consistent with this, increasing intensity of the highest category storms based on satellite-derived measurements. But to what do we attribute such increases? The roughly thirty year period over which we have reliable reanalyses and satellite measurements is too short to rule out the influence of natural climate variability, such as the Pacific Decadal Oscillation. We can at least check to see what kinds of trends climate models produce. I looked at eight CMIP 5 models whose output I had ready access to and calculated linear trends of potential intensity over the period 2006-2100 under the RCP 8.5 emissions pathway. The eight models were the NCAR CCSM4, a super-parameterized version of the same, referred to as SP-CAM, The GFDL CM3, the UKMO HADGEM-ES, the IPSL CM5A-LR, the Max Planck MPI-ESM-MR, the CCSR/NIES/JAMEST MIROC5, and the Meteorological Research Institute’s MRI-CGCM3. Of these, two models showed insignificant trends in the region in which Pam developed, and the rest showed positive trends averaging around 0.5 m/s per decade, considerably less than the observed trend over the last 30 years. The largest trend was produced by the GFDL model, whose global trend distribution is shown below. (White areas represent p values less than 0.1., and the units are m/s per decade.)


It is interesting that the largest increases are at the polar peripheries of the tropics, indicating a general expansion of the regions that are thermodynamically favorable for tropical cyclone development; this general feature is present in most of the model potential intensity trends as well as the reanalysis trends over the past 30 years, and may be behind the poleward migration of the latitudes at which observed tropical cyclones reach their peak intensity, as documented by Kossin et al. (2014).

The disparity between the reanalysis potential intensity trends over the past 30 years and the projected trends over this century suggests either that most of the observed increase in potential intensity (and actual intensity of high category storms) is due to natural variability, that decreasing anthropogenic aerosol loading over that period may have played a role, or that the model projections are too conservative. Yet the projected increase is not insignificant, amounting to about 5 m/s over 100 years. Note from the figure above that there are somewhat larger increases elsewhere, particularly in the northern hemisphere.

All of this is consistent with the strengthening consensus that the frequency of high category tropical cyclones should increase as the planet warms (Knutson et al., 2010). Basic theory and a variety of numerical simulations support this, as well as the projection that tropical cyclones should produce substantially more rain, owing to the increased moisture content of the tropical atmosphere. This is important because most destruction and loss of life are caused by high category storms and their attendant storm surges, and by freshwater flooding from torrential rains. Most of the disagreement in the literature on tropical cyclone projections concerns the incidence of weak storms, but these are usually far less consequential in spite of being more numerous.

Besides the oft-discussed issues of TC frequency and intensity, changes in genesis locations and tracks are potentially very important, as are the diameters of TCs, which affect the area covered by strong winds and which greatly affect the magnitudes of storm surges. In the present climate, the diameter of storms, as measured by the radius at which their circular wind component becomes indistinguishable from environmental winds, appears to be log normally distributed, with a mean around 420 km (Chavas and Emanuel, 2010). There is some indication from modeling studies that the size of the dangerous inner core scales with the potential intensity divided by the Coriolis parameter (Khairoutdinov and Emanuel, 2014). If this turns out to be true in nature, then storm inner core dimensions should increase over time.

While Pam and Haiyan, as well as other recent tropical cyclone disasters, cannot be uniquely pinned on global warming, they have no doubt been influenced by natural and anthropogenic climate change and they do remind us of our continuing vulnerability to such storms. Destructive TCs in any one place tend to be generational…enough time for people to forget them and go back to risky behavior, including over-development of coastal regions. We adapt more successfully to the more frequent events which are always in the back of our minds (and often in the front). But this human adaptation time scale may be longer than the time over which climate change affects storms, so that comparatively small changes in the frequency of generational events can have large social consequences. When a 100-year event becomes a 50-year event, it may take a few destructive hits before we adapt to the new reality. This is of particular concern with tropical cyclones, where the application of existing damage models to projected changes in tropical cyclone activity predict large increases in damage, as documented, for example, in the recent Risky Business report commissioned by Michael Bloomberg, Hank Paulson, and Thomas Steyer*.

Now if only we could better measure tropical cyclones to record how they may change in coming years.

* [Update, 1pm ET] It has been pointed out that in my reference to the Risky Business report, I might have mentioned that I contributed synthetic hurricane event sets that were used by Risk Management Solutions, Inc., to estimate damages from tropical cyclones.


Chavas, D. R., and K. A. Emanuel, 2010: A QuickSCAT climatology of tropical cyclone size. Geophys. Res. Lett., 37. 10.1029/2010GL044558.

Khairoutdinov, M. F., and K. Emanuel, 2014: Rotating radiative-convective equilibrium simulated by a cloud-resolving model. J. Adv. Model. Earth Sys., 5, In press.

Knutson, T. R., and Coauthors, 2010: Tropical cyclones and climate change. Nature Geosci., 3, 157-163.

Kossin, J. P., T. L. Olander, and K. R. Knapp, 2013: trend analysis with a new global record of tropical cyclone intensity. J. Climate, 26, 9960-9976.

Kossin, J. P., K. A. Emanuel, and G. A. Vecchi, 2014: The poleward migration of the location of tropical cyclone maximum intensity. Nature, 509, 349-352.


Kerry Emanuel is professor of atmospheric science at MIT and one of the world’s leading tropical storm experts.

62 Responses to “Severe Tropical Cyclone Pam and Climate Change”

  1. 51
    Racetrack Playa says:

    There’s been a fair amount of discussion of the role of increased winds driving coastal upwelling due to increased land-ocean temperature gradients in a warming world (continental interiors). Does this imply that higher wind shear will tend to disrupt hurricane formation along coastal boundaries? On the other hand, if wind shear dropped to low levels from time to time, that would provide a window of opportunity for monster hurricanes to hit the coast, driven by high atmospheric water moisture, high sea surface temperatures, and a deep pool of warm water to draw energy from. So: less frequent but more destructive?

    @Thomas O’Reilly: there’s really no need to overstate or exaggerate any aspects of climate science in order to influence the press or politicians or industry. Also, there are many uncertainties – not in the notion that burning fossil fuels is driving global warming, but in the details of how that will play out from region to region, and the complex interactions involved – here’s something very interesting related to this topic:

    These two systems, rotating on either side of the equator, drove a trade wind reversal across the region that appears to have amplified a weak El Nino. Why is this so fascinating? Because normally we think of ENSO phases as modulating global hurricane activity, but this seems to be the reverse. ENSO may be a bit more sensitive to external perturbations then people thought (volcanoes, etc.) – but that’s not certain by any means:

    “What’s hard to determine is whether the cyclones created the wind burst or the other way around, as westerlies are also conducive to cyclone formation in that area. Both could also be linked to a climate pattern called the Madden-Julian Oscillation (MJO), which features alternating areas of enhanced and suppressed rainfall that move from west to east across the globe. When enhanced rainfall is over the western Pacific, it tends to whip up the westerlies that can feed cyclones and the El Niño. And this round of MJO could be record-breaking. But it’s hard to predict how big an impact this push of winds will have on the El Niño.”


    However, this very fascinating science doesn’t change the fact that any exposed country not investing heavily in cyclone/hurricane/coastal surge preparedness infrastructure right now is making a big mistake that will have nasty economic repercussions down the line.

  2. 52
    Thomas O'Reilly says:

    #51 Racetrack Playa
    “@Thomas O’Reilly: there’s really no need to overstate or exaggerate any aspects of climate science in order to influence the press or politicians or industry.”

    Do tell what in your view would “influence the press or politicians or industry?”

    As yet it has all been counter-productive and ineffective relative to the known risks involved. Nothing has changed – CO2e emissions continue to rise daily, will continue to rise from now to 2040 at least – new coal mines continue to be opened including the largest ever in the world is fully approved and in train now – land clearing continues unabated – global warming continues unabated – the impact of severe weather events – climate change is clear and self-evident globally now and can only get worse if Physics and Maths play any role whatsoever in measuring that accurately (which it does).

    The IPCC Reports do not use excessive CAPS – and yet no one who should read those reads them either (nor understands them.) But I do and have since the late 1990s.

    Therefore the use of CAPS is not in fact part of the problem here. I use them for a specific purpose, and even if others do not understand that, it does not change the logic nor the validity of their use. I have my reasons. I write for a very limited audience, maybe one in a thousand here, who knows. Kudos, fame, agreement, and compliments is of no interest to me.

    People adopt all kinds of unconscious beliefs that are not true. One of these is someone once said long ago that using CAPS meant that the writer was SHOUTING and/or Angry. Billions on the planet now accept that as true. It was never true, and it will never be true.

    Just as many people believe that Christopher Monckton or Watts on WUWT really are clever people, very wise, well informed and logical – doesn’t make that true either.

    My suggestion to all is to make a conscious focused effort to “parse the content”, and see what it is that I am really saying above. Wisdom requires effort – it isn’t left in a wrapped box under the Xmas tree as a free giveaway. 8^)

  3. 53


    What are you trying to accomplish? In my view, it’s already too bloody late. The science continues to be interesting, but I fully expect the society I live in to collapse, and that fairly soon (2028 is my current best estimate, with big error bars). I don’t think there’s any combination of advice, exhortation, calm, anger, or any other damn thing that could move our ruling classes to act helpfully in time. There are nice trends, like rising use of solar and wind, but the problem is simply out of control at this point without coordinated international effort–and let’s be honest, there’s no such thing as coordinated international effort. World War II came close, but even beating the Axis you had Stalin pressing the US and UK to invade France NOW, while we pressed Stalin to invade Japan NOW, and while both eventually listened, it took years–and in the latter case, happened after the issue was already determined.

    We’re screwed. Fatally screwed. If you can find an isolated place, high up, with plenty of rainfall and good land, hole up there with people you can trust, agricultural tools and seeds, and arm yourselves to the teeth. I’m not even going to bother with that, because without high-tech medicine, I’m going to die.

    But you know, I was going to die sometime whatever I did. So are you. So is everybody else. The death rate is the same for everybody in the end. Just deal with one day at a time. What other choice have we got?

  4. 54
    Ray Ladbury says:

    Thomas O’Reilly: “My suggestion to all is to make a conscious focused effort to “parse the content”, and see what it is that I am really saying above.”

    Might I humbly suggest that you parse the content of what you meant and try again to state it clearly. The problem is not that the scientists have not done their job to call society’s attention to a very credible threat. The evidence establishes beyond all reasonable doubt that we are warming the planet and that it will have adverse consequences. That many choose to live in denial is not the fault of the scientists.

  5. 55
    Thomas O'Reilly says:

    #53 BPL “What are you trying to accomplish?”
    I was curious if anything substantial has changed yet, in thinking or strategy, that’s all. Been out of the loop for long time, after deciding it was best to ignore the issue and live my life. With Paris coming up I had a few questions to see what the go was now in the scientific approach. I’m a bit of a realist with a pragmatic approach to most things, so I tend to avoid pollyanna wherever possible. Like positive thinking is all well and good, but it’s counter-productive when disconnected from the reality. So I am not trying to accomplish anything. More curious about what the climate scientists were trying to accomplish these days. Because I couldn’t envision the UNFCCC COP21 in Paris accomplishing much at all.
    By the look of it, as you said, “it’s already too bloody late.” thanks

  6. 56
    Thomas O'Reilly says:

    #54 Ray Ladbury – if I had to spend months on months parsing the content of the thousands of pages in IPCC reports, then anyone who is genuine can spend a few mintutes parsing my content.

    They do not have to, and I do not demand that they do either. But God helps those who help themselves .. and I know what I know because I have done the work. And made the effort to UNDERSTAND WTF the IPCC reports and the scientists were saying without any help from THEM and without any help in the way THEY communicated what I and the other 9 billion on this planet are supposed to know in our own best interests. Pointing out the bleeding obvious, to me, is not complaining nor critizing and it sure isn’t BLAMING!

    RE “That many choose to live in denial is not the fault of the scientists.”
    Do tell em something I don’t know! And knock that chip off your shoulder while you are at it. Make up your mind as to what is really important and what isn’t. 8^)

  7. 57
    Thomas O'Reilly says:

    #54 Ray Ladbury “That many choose to live in denial is not the fault of the scientists.”

    It’s foolish beyond reason to keep blaming and finding fault in “deniers” for the current state of affairs regarding AGW/CC causes and inaction.

    Remove every single “denier” from the planet post-1992 and there would still be a AGW/CC Catastrophic Problem needing to be solved today.

    Until people (and especially climate scientists) realize this fact, nothing will change. Which means, nothing will change until it’s too late, and it already is. 8^)

  8. 58
    Steve Fish says:

    Re- Comment by Thomas O’Reilly — 26 Mar 2015 @ 6:54 PM, ~#57

    Thomas, I am curious. Don’t you think that societal attitudes about the fake tobacco/cancer controversy would have changed more quickly if there hadn’t been a corporate disinformation campaign?


  9. 59
    15120092 says:

    Could you elaborate on the following sentence in your piece, hopefully with an example.
    “There is some indication from modeling studies that the size of the dangerous inner core scales with the potential intensity divided by the Coriolis parameter”. (15120092)

  10. 60

    Remove every single “denier” from the planet post-1992 and there would still be a AGW/CC Catastrophic Problem needing to be solved today.


    But there’d probably also be an international, comprehensive, agreed solution, well along in the process of implementation.

    As it is, we have a patchwork of semi-coherent, partially-coordinated actions at multiple jurisdiction levels, and the best-case possibility of achieving a rather namby-pamby international agreement this year, which would only come into full force in 2020. It’s amazing that global emissions dropped slightly last year–even if ‘unforced variation’ was part of the reason.

    Denialism has a lot to answer for–and will soon enough have a lot more, I’m afraid.

  11. 61
    Hank Roberts says:

    > remove every single “denier” from the planet post-1992

    Perhaps the worst two hundred million would suffice?

  12. 62
    Mark says:

    The news about the increase in number of cyclones as well as increase in intensities due to increasing temperature was always the message to the people of Vanuatu during climate change awareness throughout the country over the past few years.

    Cyclones form when the temperature increases and reduction in pressure. During the PAM development the population in Vanuatu was experiencing a very hot and humid weather and now that it has passed we are experiencing cooler evenings.

    most of the coastal areas in Vanuatu is currently being affected by sea level rise. We are also observing fruit trees flowering and fruiting outside their normal seasons. Very different from past experiences.

    If this changes are not related to climate change then, what is causing all these changes? Is the Ozone layer really is depleting as learned in school by all Ni-Vanuatu or we have been mislead to belief that these chances is caused by climate change.