Climate Change and Tropical Cyclones (Yet Again)

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.

The differences between the conclusions of the two studies are significant. Using essentially the same IPCC model projections, the two studies come to very different conclusions with regard to key projected quantities, such as the seasonally-integrated powerfulness of TCs or ‘power dissipation index’ (PDI). While the Emanuel et al study predicts a clear increase in PDI, the Knutson et al study does not. So which is right?

Well, Knutson et al fully acknowledge that their RCM still has too low resolution to produce realistic TCs (the model resolution is about 20 km, while theoretical estimates indicate that a resolution of about 1 km is likely required to simulate the inner core of intense TCs). TCs are very likely being artificially prevented from intensifying in a warmer climate in the Knutson et al study because of this. By contrast, Emanuel et al‘s approach does not suffer from such resolution limitations.

To their credit, Knutson et al openly acknowledge this weakness in their treatment of TC intensity and PDI. What about their conclusions about a projected decrease in Atlantic TC frequency, which are, after all, the central point of the paper? Here we have reservations as well (and if we were the betting kind might even put forward a wager with regard to future trends). In part, these reservations are a result of the very same issues. The limitations of the RCM for example, as the authors note, also lead to incorrect seasonal and geographic distributions of TC genesis.

Small-scale processes

In the real world, small-scale phenomena such as convection, clouds, gravity waves, and various sorts of eddies may influence mesoscale systems such as TCs, and these unresolved small-scale phenomena are represented through often somewhat simplistic statistical ‘parameterizations’. This limitation is of course common to essentially all atmospheric models, and in and of itself is no reason to dismiss the conclusions of the study.

But TCs do also play a role in terms of the larger scales, as they facilitate transport and a redistribution of heat, moisture and momentum (‘upscaling’). This action is simulated more explicitly in RCMs and more implicitly in GCMs by their parameterization schemes, but it is still not really known if these two levels of modelling provide a physically consistent picture of the scale interactions. The RCM solutions, however, are constrained by the results generated by the GCMs and thus depend on how well the parameterization schemes capture this upscaling effect. Yoshimura et al., 2006 have shown that the solutions may be sensitive to the choice parameterization schemes: they found an increase in TC number over the Indian Ocean if the model used the Kuo cumulus parameterization but a decrease if the Arkawa-Schubert cumulus parameterization scheme was used.

Seasonality

Also significant, perhaps, is the seasonality issue touched on above. The Knutson et al study involves an assumption of a fixed August-October TC season. Yet one important impact of large-scale climate factors which influence Atlantic TC frequency such as ENSO and the NAO (see e.g. one of our own papers on this topic) is their influence on activity during the latter part of the Atlantic hurricane season (one might argue for example that the primary reason for the low TC count during the 2006 season was the early ‘shut-down’ of the season due to increasingly strong El Nino-related wind shear in the autumn as the El Nino set in).

The role of ENSO

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