August 21, 2007

Tellus More About Hurricanes

Filed under: Climate Extremes, Hurricanes

Tellus is a professional scientific journal published on behalf of the International Meteorological Institute in Stockholm and is highly respected by atmospheric scientists throughout the world. A recent issue is devoted to hurricanes (a.k.a., tropical cyclones), and three articles in the issue are of significant interest to us at World Climate Report.

The first is by a scientist named Johnny Chan of the University of Hong Kong (didn’t he win the World Series of Poker a few years ago?). Anyway, Chan’s interest was on the interannual variations in intense hurricane activity in the western Pacific where tropical cyclones are often called typhoons. Chan began the article noting that two recent papers by Emanuel and Webster et al. (covered extensively at World Climate Report) “have claimed that the recent increase in sea-surface temperatures (SSTs) as a result of global warming is likely to be responsible to the concomitant increase in either the power dissipation or the number of intense hurricanes and typhoons through a direct forcing of enhanced thermodynamic energy supply. However, many papers in the past have failed to identify such relationships.” Chan developed a time series of accumulated cyclone energy (ACE) from 1960 to near-present and noted “a maximum in 1960s, a minimum in 1970s and early 1980s, another maximum in the early 1990s and a general decrease from 1997 to the present.” In attempting to explain variance in the time series, Chan found that the ACE could not be explained by sea surface temperatures (SST) in the western Pacific. He concludes “In other words, the frequency of occurrence of intense typhoons in this region is not likely determined by the average SST over the region.” The intense storms were associated with variations in vertical wind shear and moisture in the atmosphere, not by the thermal state of the ocean’s surface.

Next up in Tellus is an article by Vitart and Doblas-Reyes regarding the impact of elevated concentrations of greenhouse gases on tropical storm activity. The pair used a sophisticated global climate model to examine interannual variations in tropical cyclone activity from 1958 to 2001. At the end of the day, the scientists found that “Results suggest that an increase in greenhouse gas concentration is conducive to a decrease of tropical storm activity over all basins except the western North Atlantic and the eastern Pacific. This result seems consistent with the impact of an increase of greenhouse gas concentration on the thermodynamical and dynamical variables that have an impact on tropical cyclone activity.” Furthermore, they found that their numerical experiments with different concentrations of greenhouse gases “do not display significant differences in the intensity of tropical storms.” They note that the finding “seems to contradict” the famed Webster et al. and Emanuel recent studies but they warn “although the model has some skill in predicting the interannual variability of tropical storm frequency, it does not display strong skill in predicting the interannual variability of tropical storm intensity.”

The third piece in Tellus is by climate heavyweight Lennart Bengtsson and others from institutions in Europe and Japan and has the million dollar title “How may tropical cyclones change in a warmer climate?” Similar to some of his earlier work, Bengtsson used climate models from the Max Planck Institute to simulate tropical cyclone (TC) activity from 1860 to 2100. In the abstract, the Bengtsson team writes “While there is no significant change between the 19th and the 20th century, there is a considerable reduction in the number of the TC by some 20% in the 21st century, but no change in the number of the more intense storms. Reduction in the number of storms occurs in all regions.” However, those results are for a model with a resolution of 1.875° latitude and longitude. Bengtsson et al. then ran the same experiments with a resolution of 0.5625° latitude and longitude and found “there is a reduction by some 10% in the number of simulated TC in the 21st century compared to the 20th century but a marked increase in the number of intense storms.” However, the low-resolution and high-resolution experiments (referred to as the T63 and T213 experiments, respectively) have substantial differences in the way the atmosphere is coupled to the ocean, and Bengtsson et al. warn that the increase in intense storms in the high-resolution (T213) simulation “could be an effect of the lack of atmosphere–ocean interactions in the T213 runs as the lack of air–sea interaction may contribute to more intense cyclones.” They conducted further experiments at an even higher resolution (0.375° latitude and longitude) and found that “The results are consistent with the T213 study.”

This Tellus trio of articles tells us that tropical cyclones could become less frequent in the future and the jury is still very much out on the question of whether more intense storms will develop in the decades to come. All told, there is still a lot of work to be done, and the debate is anything but over!

References:

Bengtsson, L., K.I. Hodges, M. Esch, N. Keenlyside, L. Kornblueh, J.-J. Luo, and T. Yamagata. 2007. How may tropical cyclones change in a warmer climate? Tellus, 59A, 539-561.

Chan, J.C.L. 2007. Interannual variations of intense typhoon activity. Tellus, 59A, 455–460.

Vitart, F. and F. Doblas-Reyes. 2007. Impact of greenhouse gas concentrations on tropical storms in coupled seasonal forecasts. Tellus, 59A, 417-427.




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