Something about Tropical Cyclones

There’s an interesting Realclimate post discussing whether or not global warming makes tropical cyclones stronger. The basic answers is that we might expect the number of tropical cyclones (TCs) to decrease overall, but see an increase in the frequency and intensity of the strongest ones.

Credit: Kerry Emanuel, MIT.

As indicated by the figure on the right, there are also suggestions that such increase is actually being observed. For more details, it probably reading the Realclimate post. You could also read Elsner et al. (2008) and Kossins et al. (2013), both of which find an increase in the intensity of the strongest TCs (although the homogenisation method used in the latter does reduce the trend).

However, it seems that there is not complete agreement that we are indeed observing an increase in the frequency and intensity of the strongest TCs. It’s a difficult analysis and the results can vary as the datasets improve. However, ,any who do dispute the existence of any trend, tend to highlight that there are no trends in the accumulated cyclone energy (ACE). This is actually what I was wanting to discuss. I’ve been looking through a paper by Kang and Elsner called Climate Mechanism for Stronger Typhoons in a Warmer World.

Credit: Kang & Elsner (2016)

I’m not entirely sure that I quite get it, but I think a key point is demonstrated by the figure on the left. It shows a correlation screen of global ocean warmth (global mean sea surface temperatures) and climate variability of the top 10% of TCs in the Western North Pacific. INT is essentially intensity, FRQ is frequency, ACT is essentially ACE, and EINT is efficiency of intensity (fewer TCS but stronger).

What (I think) it shows is that increasing sea surface tempertures correlates with a reduction in TCs but an increase in the intensity of the strongest ones. What it does not show is any correlation with ACT, or ACE. As the paper says

Increasing EINT in a warmer year shows that this environment further inhibits the TC occurrences over the region, but TCs that form tend to discharge stored energy to the upper troposphere with stronger intensities. As the increasing intensities compensate for the loss of ACT by decreasing number of TCs, the ACT remains largely unchanged.

So, this paper seems to be again consistent with the general expectation that we would expect the strongest TCs to get stronger as we warm, but that we might expect a reduction in TCs overall. These two factors then tend to compensate in such a way that there is then little change in the accumulated cycle energy (ACE, or ACT). Hence, it would seem that one should be careful of using ACE to infer things about trend in TCs.

Links:
The increasing intensity of the strongest tropical cyclones (Elsner et al. 2008).
Trend Analysis with a New Global Record of Tropical Cyclone Intensity (Kossin et al. 2013).
Climate Mechanism for Stronger Typhoons in a Warmer World (Kang & Elsner 2016).
Does global warming make tropical cyclones stronger? (Realclimate)

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14 Responses to Something about Tropical Cyclones

  1. I should acknowledge that I partly got this from a tweet from James Elsner.

  2. Something I should add is that even if there isn’t a clear, statistically significant, trend in TC activity, I think there is a statistically significant correlation between sea surface temperature and intensity. In other words, when the sea surface temperatures are higher, the maximum wind speeds are higher. Even if this cannot yet be attributed to human influences, this is consistent with expectations, and also suggests that as the sea surface temperature increases (as it will if we continue to dump CO2 into the atmosphere) we would expect the strongest TCs to get stronger.

  3. To confuse and complicate things further, AIR Worldwide, advisors to the insurance industry and staffed by several people with serious technical chops, does relate destructive potential to ACE, keeps on top of and reports on hurricane forecasting science — including on machine learning approaches — and has a regular blog regarding the influence of climate change on hurricanes and their landfalls. But they argue that the case for hurricanes being stronger due to elevated Atlantic temperatures (SSTs) isn’t there, at least by their evidence. I don’t understand that, but I haven’t invested a lot of time in it yet.

    Still, another good source.

  4. Part of what motivated this was that someone (RPJ, if you really must know) tweeted this paper. It ends with

    Evidence that these warmer temperatures, along with warmer oceans, have increased the
    frequency and intensity of tropical cyclones remains to be seen.

    As far as I can see, it only uses ACE. Also, the expectation is not that there will be an increase in the frequency and intensity of TCs, but that there will be an increase in the frequency and intensity of the strongest TCs (and, possibly, a reduction in TCs overall).

  5. TTauriStellarBody says:

    While only a minor quibble, dropping anthropogenic sulphates are likely to impact (i.e. increase) tropical storm activity in the Atlantic basin.

    https://www.nature.com/articles/ngeo1854

    The period they studied (post 1979), this effect is likely a factor.

    But then again as the old joke goes, Atlantic tropical storms, 11% of the global activity 90% of the press coverage.

  6. TTauriStellarBody says:

    “So, this paper seems to be again consistent with the general expectation that we would expect the strongest TCs to get stronger as we warm, but that we might expect a reduction in TCs overall. These two factors then tend to compensate in such a way that there is then little change in the accumulated cycle energy”.
    ACE squares the velocity so I am not sure the two trends will cancel each other out.
    But that said it is not a great metric for the potential destructiveness of tropical strom winds, wind the damage is cubic of velocity (IIRC) as its a fluid so you not only increase the momentum of the air mass but the mass itself as more volume passes through a given theoretical 2D vertical area.
    (Bit of a rush so apologies if this is not quite what is being talked about here)

  7. TTauri,
    There’s also this paper, that says

    Although greenhouse gas–driven warming increases potential intensity, climate model simulations suggest that aerosol cooling has largely canceled that effect over the historical record.

    I’m also a little surprised that the increase in strong TCs, and the reduction in TCs overall, leads to little change in ACE. My guess is that it may not exactly cancel, and that it may not continue to cancel, but that the net effect is that there is little change in ACE even if there is a change in TCs (more stronger ones, but fewer overall).

  8. Ragnaar says:

    Here’s the message: Less but more intense. So I’ll pay you twice a month instead of weekly. So question is not why am I such a jerk but are you getting paid the same as before? But they’ll be more intense doesn’t tell me.

    We could survey payment arrangements and changes in those arrangements over time. And then conclude something about corporations. We could also predict changes in those arrangements based upon our understanding of corporations and their response to a variable.

    At this point I conclude, I am glad I am self employed. We try to control corporations. We come up with reasons why we should be doing this.

  9. Skaffy says:

    Has anyone looked at the effect of rising SST on the ocean area warm enough to support TC development? “All other things being equal” wouldn’t one would expect an increase in ACE and TC count simply because they can maintain their warm cores further away from the tropics or develop in previously too-cold areas (Vince/2005, and Ophelia/2017 come to mind)? Or does this effect just get swamped by the other factors discussed here?

  10. Dave_Geologist says:

    Here’s the message: Less but more intense. So I’ll pay you twice a month instead of weekly. So question is not why am I such a jerk but are you getting paid the same as before? But they’ll be more intense doesn’t tell me.

    Here’s a better analogy Ragnaar. You get paid weekly in arrears. Your landlord collects the rent weekly in advance. You had to find a bit of up-front money to move in, but now incomings equal outgoings and you’re fine. Your lease is up for renewal, and your landlord says OK, but you have to pay monthly in advance. You can’t make the first payment and BANG!, you’re homeless. How the hell did that happen? Your monthly incomings and monthly outgoings are unchanged.

  11. Skaffy,
    There are studies that show a correlation between sea surface temperature and maximum wind speed. So, I think there is now reasonable agreement that warmer oceans will lead to strong TCs. However, I think there are also other factors that can inhibit TC formation (wind shear, for example). So, the general picture is that we will see fewer TCs as we warm, but will see an increase in the frequency and intensity of the strongest ones.

  12. I posted this chart recently with average wind speed of v=12.4 m/s

    If warming is 1C, then the average wind speed will increase as absolute T proportional to v^2, so ~(300+1)/300 = v^2/12.4^2, or v=12.42 m/s.

    All other values, including extreme values, will increase proportionally to sqrt(301/300) which isn’t much. But that is the Maximum Entropy model of minimal information using the mean only.

    Interesting topic but I don’t understand how a huge increase can come about based on thermal excitation alone. I guess the recent argument is related to equatorial vs polar differential warming having an impact. Another one of those scalar predictions that doesn’t have a lot of resolving power, except at the margins in the regions already sensitive to changes, such as in low-lying areas.

  13. @WHUT,

    I have no understanding of the underlying science to speak on, but I have listened in to meteorological discussions of the matter and there are, apparently, some threshold effects in the mechanics of how these arise. As far as I know, these were not recorded, but were presented at meetings of the Southern New England Meteorological Conference which I attended. But there are some reviews:
    * https://journals.ametsoc.org/doi/full/10.1175/JAS-D-17-0008.1
    * https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-15-0585.1 (mechanism for increasing EINT)
    * https://sci-hub.tw/10.1038/nclimate2646 (this described the key tradeoff and is cited by the paper just above)

    * The Kang and Elsner paper also has a supplement: https://media.nature.com/original/nature-assets/nclimate/journal/v5/n7/extref/nclimate2646-s1.pdf

  14. Magma says:

    A serious weakness of ACE is that it does not take storm diameter (and therefore the mass of air involved) into account. A relatively small hurricane with very high winds could therefore have a higher ACE than a far more massive storm containing much more energy (e.g. Patricia vs. Sandy).

    I’m not sure why this metric became popular when it could have been relatively easy to include a height (possibly taken as constant), radius squared, and a wind speed vs. radius weighting factor (could be measured or standardized).

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