September 15, 2006

More Stormy Weather Ahead for Hurricane Doomsayers

Filed under: Climate Extremes, Hurricanes

“Storm Tracks and Climate Change” is the title of an article that appeared in the August issue of the Journal of Climate by Lennart Bengtsson, Kevin Hodges, and Erich Roeckner. Sounds interesting…

Bengtsson and colleagues ran the European Centre/Hamburg Model Version 5 (ECHAM5) climate model under conditions of increasing greenhouse gases as defined by the IPCC’s A1B scenario—a mid-range scenario that produces about 3ºC of global warming between 1990 and 2100. They then compared future storm tracks and intensities (in the years 2071-2100) against model-generated tracks using observed changes in greenhouse gases and particulates.

In their own words:

Compared to previous studies on the change of storm tracks in a future climate as summarized by Geng and Sugi (2003), the following aspects of this study are highlighted: 1) This study has used data from a coupled model at relatively high resolution (compared with many previous studies) with observed anthropogenic forcing, including tropospheric and stratospheric ozone for the past century and a selected SRES scenario for the future. The analysis has made use of an ensemble of three 30-yr integrations. 2) The reduction of storms in the extratropics at both hemispheres is minor and only includes the weaker storms. 3) There is no indication of an increase in the intensity of extratropical storms except to a smaller degree in the SH, which we suggest may be related to a poleward translation of the storm track. 4) There are no changes in the extremes of tropical storms in spite of increased tropical SST by 2°–3°C. 5) The regional changes in the storm tracks are suggested to be associated with regional SST changes. The mechanisms for the tropical SST are consistent with Rossby wave propagation and the extratropical SST for the locking in of the storm tracks as suggested by Inatsu et al. (2002).

In case point number 4 got lost in the jargon, here is how Bengtsson et al. described it earlier in their paper (note that italics are in the original):

The Atlantic [tropical] storms are reduced in number, in particular the stronger ones, while the storms in the eastern Pacific are virtually unchanged though there is some indication of fewer extreme storms. In the western Pacific there is little change. It is interesting to note that the change in SST by between 2° and 3°C has not had any influence on the numbers and intensities of the more powerful tropical storms. The geographical changes in the storm tracks have some similarity to what occurs during El Niño events. This may also be expected as the coupled models predict a larger warming in the eastern tropical Pacific, creating a pattern that bears some resemblance to a warm ENSO event.

In other words, despite a 2 to 3˚ C warming of SSTs, the ECHAM5 climate model produced no increase in the number of strong tropical cyclones. And in fact, it produced an overall decrease in the frequency of all Atlantic tropical systems, probably because the model pattern of future warming is much like an El Nino—which is bad for hurricanes.

The authors do go on to caution about the generalization of the results, since they are based only upon a single climate model. But they follow that caution with a bit of bragging that the atmospheric component of their model “is capable or reproducing essential features of the life cycle, distribution, and intensity of transient storms in the extratropics and to a reasonable degree also in the Tropics” and that “for the present climate the coupled model generates very similar storm tracks as the atmospheric model, providing confidence in the overall results.”

The work of Bengtsson et al. provide yet another example that the present and future of tropical storms, and the potential human impact on their frequency and intensity, is far, far from being settled in the pages of scientific journals (see our two recent posts, here and here, for other current examples), despite what you may be reading in the pages of your favorite newspaper.

Reference:

Bengtsson, L., et al., 2006. Storm Tracks and Climate Change. Journal of Climate, 19, 3518-3543.




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