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A clearer picture how climate change affects El Niño?

Filed under: — rasmus @ 28 November 2014

I still remember the first time I was asked about how climate change affects El Niño. It was given as a group exercise during a winter school in Les Houghes (in France) back in February 1996. Since then, I have kept thinking about this question, and I have not been the only one wondering about this. Now I had my hopes up as a new study was just published on the evolution and forcing mechanisms of El Niño over the past 21,000 years (Liu et al., 2014).

The main results published by Liu et al., (2014) rely on simulations with one climate model (CCSM3), but we know that different climate models indicate that a global warming will have different effects on El Niño. So why would I trust new computer simulations with a model that has a coarser representation of the atmosphere and ocean?

The spatial resolution of the model they used was described by the cryptic description “T31x’3″. The “T31” part of this refers to the atmosphere and is a technical term describing how the calculations are made.

Since our planet is spherical, it is possible to represent the differential equation describing the motion of air as a set of spherical harmonics rather than calculating the results for each grid box seperately. Basically, this is a clever mathematical “trick” also used in weather forecasting. “T” is short for triangular truncation of spherical harmonics, and “T31” means that the model represents all total wave numbers from 0 to 31.

In other words, “T31” means that the description of the atmosphere is rather coarse (resolution of 3.75×3.75 degrees) in terms of its spatial details. This is understandable because it would require too much computer resources to compute the climate with more details if you want to account for 21,000 years.

You can use a similar model with very high degree of detail for a weather forecast because you only want to compute the atmospheric state for a few days.

A climate model also includes an ocean component, and the description of the ocean usually requires finer details (higher spatial resolution) than the atmospheric model (due to different Rossby radius of deformation).

Although the atmosphere model is low resolution in this case, the ocean is relatively high. But some crucial aspects are not clear from the paper: if the ocean resolution is 0.3 degrees or 0.9 degrees near the equator.

Nevertheless, the ability to represent the El Niño phenomenon is limited in the model experiment, and indeed, its time scale tends to be closer to 2 years than the observed 2-7-year span.

I think the most interesting aspect of the model experiment was running the model several times with different inputs: CO2 forcing, changes in solar forcing due to orbital changes, the effect of changes in the ice sheets, the effect of meltwater from shrinking ice sheets, and one with all combined.

Also, I liked the way the paper tried to draw the lines to different causes, both through the experimental set up and the choice of diagnostics.

The results indicate that El Niño is fairly sensitive to the different aspects, but the picture is quite messy. Nevertheless, (Liu et al., 2014) reported a dominant role of precessional (orbital changes) forcing, but they also found that the glacial melt had an effect on El Niño.

The figure reproduced below also suggests that the ice (green curve) had a strong effect at around 14,000 years ago (a big jump). Yet, the orbital (light blue) and meltwater (dark blue) gave an evolution that was closest to the effect of the combined forcing (black).

However, all this is very qualitative. I don’t see a clear picture on the connection between the forcers and El Niño, and I’m no wiser. The quote from the press release “El Niño is driven by an intricate tango between the ocean and the Earth’s atmosphere.” [Kelly April Tyrrell] is still quite descriptive in my mind.

Liu et al. Figure 3: ENSO in single forcing experiments.

Fig.3 from Liu et al., (2014): a, Amplitude of ENSO. b, Amplitude of the annual cycle.

References

  1. Z. Liu, Z. Lu, X. Wen, B.L. Otto-Bliesner, A. Timmermann, and K.M. Cobb, "Evolution and forcing mechanisms of El Niño over the past 21,000 years", Nature, vol. 515, pp. 550-553, 2014. http://dx.doi.org/10.1038/nature13963

16 Responses to “A clearer picture how climate change affects El Niño?”

  1. 1
    Aaron Lewis says:

    AGW puts more energy in the system. El Nino can use that energy, but cannot collect energy to force the system. A large melt water pulse is the sudden release of potential energy – to El Nino, who can then do what he wants with that energy. In this light, it seems reasonable that a melt water pulse would intensify El Nino.

    Compared to the orbital mechanics forcing energy signals, AGW is the much larger signal. El Nino is one way the Earth System adjusts to a new level of signal. This paper correlates behaviours but does not attempt to elucidate the energy flows of El Nino.

  2. 2

    According to Clarke at FSU [1], the physics behind ENSO and therefore El Nino is a shallow water wave equation (2nd order differential equation) with a characteristic frequency of about 4¼ years. Together with the periodic forcing characterized by the QBO winds this leads to an erratic yet largely predictable time series. A group of us are pursuing this rather straightforward analysis at the Azimuth Project forum.

    [1] Clarke, Allan J, Stephen Van Gorder, and Giuseppe Colantuono. “Wind Stress Curl and ENSO Discharge/recharge in the Equatorial Pacific.” Journal of Physical Oceanography 37, no. 4 (2007): 1077–91.

  3. 3
    Salamano says:

    Are there ways to disentangle potential “chicken/egg” issues where climate affects El Nino affects climate affects El Nino, etc. etc.? Or is there no such possibility, where instead one exclusively manipulates the other. Or, even if they are all ‘tango-ing’ together, the component emanating outward from El Nino is not sufficient to wind up feed-backing onto itself at a later stage of the dance..?

  4. 4
    Patrick Fish says:

    The link to the Summary appears to be broken: The requested URL /pdf/assessment-report/ar5/syr/SYR_AR5_SPM.pdf was not found on this server.

  5. 5
    Henry says:

    I wonder if a more apt title would have been;
    “A clearer picture how El Nino affects Climate” ?

  6. 6
    wili says:

    SOI now below -17.

  7. 7
    R. Gates says:

    Interesting model study. There is some interesting research done a few years back that found an interesting potential for a La Niña like state during the warmth of the Pliocene. The GH Gas concentration were about the same as now (methane and N2O were lower), but the essential dynamics appears to be a reduction in latent and sensble heat flux from the ocean, perhaps caused by a warmer and wetter atmosphere. Given recent studies showing the SW absorption in the atmosphere being stronger than LW in terms of anthro GH induced warming, it seems reduction in latent and sensible heat flux, and thus warming oceans (increasing OHC) and an actual La Niña like state is worthy of further study as a potential feedback to rising GH gas climate. Certainly the Pliocene is also a worthy analogue in general. The study I’m referring to is:

    https://www.sciencemag.org/content/307/5717/1948?related-urls=yes&legid=sci;307/5717/1948

  8. 8
    Susan Anderson says:

    As an example of earth’s venting system with knock-on consequences, El Nino and other events are fascinating. As symptoms of a sick planet, they are depressing. Too much focus on the symptoms without noting the underlying cause is at this point distracting from the main point, which is acting soon and acting fast is beyond important.

    We have much the same chicken-egg imperception about the new prevalence of polar incursions in the most populated and politicized areas buying the obfuscatory efforts of people wishing to kill the messenger.*

    Aaron Lewis at #1 puts it well, thanks.

    *For a startlingly clear example of this, this shameless distortion gives you a snapshot of how more knowledge will not communicate with a broad segment of those whom it is necessary to bring up to speed on what is real and what is not. I don’t much care for the style (substance is largely true) of Russell Brand’s responses, but she provides a stark example of effective counterfactual PR propaganda.

    https://www.youtube.com/watch?v=K3BlotUe4G8

  9. 9

    #6–Where did you see that, Wili? BOM wasn’t that low when I looked, and NCDC has a different numerical scheme, so I’m a tad confused.

  10. 10
    wili says:

    It looks like the polar vortex has basically split in two. Is that common? http://earth.nullschool.net/#current/wind/isobaric/70hPa/orthographic=-15.56,74.65,282

    (please delete if this is a duplicate…what’s going on with recaptcha?)

  11. 11
    R. Gates says:

    @10:

    The disruption of the upper level winds this time of year is not exceptionally unusual, but usually this kind of disruption occurs late December into January during SSW (Sudden Stratospheric Warming) events. This year, the Super Typhoon Nuri in the western Pacific in November which turned into an extratropical cyclone really did a number on the Rossby Wave pattern over the N. Hemisphere. Not only did it bring the cold outbreak to the US back in November, but it really led to this continued rather messy “split” vortex pattern. Not a classical SSW split of the vortex, but the result has been very similar. I think most interestingly of all is how the exceptional warmth of the N. Pacific ocean this year led to all of this, as it takes warm water down to 300m that gets churned up by a normal typhoon to feed the growth of the storm into a Super Typhoon. Thus, we can trace the vortex disruption, the cold outbreak in the US in Nov. all back to record warm water in the Pacific.

  12. 12
    wili says:

    That was the value for just that day (Nov. 29), and it was preliminary. I realize that it’s the longer-term average that are most important. But it struck me as being surprisingly low. But looking at the whole month now, I see that there were a few days that were even lower. It was from BOM, but I haven’t checked back since. Maybe it’s been corrected or adjusted by now?

  13. 13
    Hank Roberts says:

    > it looks like the polar vortex has split in two. Is that common?

    You might think so if you trust Google
    https://www.google.com/search?q=“polar+vortex”+”split+in+two”

    But if you look with Scholar, you get a clearer idea of what’s known:
    http://scholar.google.com/scholar?as_ylo=2014&q=%22polar+vortex%22+%22split%22&hl=en&as_sdt=0,5

  14. 14
    patrick says:

    Thanks for this clear look at current research.

    As you say, the question was put to you. It’s a question that is bound to arise, per the first link in this post and otherwise.

  15. 15
    Hank Roberts says:

    For Wili — BOM tells you about daily values for the SOI:

    http://www.bom.gov.au/climate/glossary/soi.shtml

    The SOI is usually computed on a monthly basis, with values over longer periods such a year being sometimes used. Daily or weekly values of the SOI do not convey much in the way of useful information about the current state of the climate, and accordingly the Bureau of Meteorology does not issue them. Daily values in particular can fluctuate markedly because of daily weather patterns, and should not be used for climate purposes.

  16. 16
    wili says:

    Thanks, Hank (and sorry, for the nth time, for being such a lazy, laggardly cur.) I didn’t realize that this pattern is often associated with a sudden stratospheric warming event. That makes sense.


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