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.

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