Climate Change and Tropical Cyclones (Yet Again)

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.

Validation

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

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