The September 16,2005 article by Webster et al. in Science concludes that there has been a large increase in the number and proportion of hurricanes reaching Saffir-Simpson category 4 and 5 hurricanes over the past decade. They report that these increases have taken place while the number of tropical cyclones and tropical cyclone days has decreased in all basins except the North Atlantic.
This is a clearly written article by very well-respected scientists. There are, however, several substantive issues with the study. First, an informative figure illustrating the maximum potential for hurricanes as a function of SST was described by a 1988 paper by Robert Merrill entitled “Environmental Influences on Hurricane Intensification” (see Figure 2 in that paper). This research was completed for the Atlantic hurricane region, but the SST thresholds should be the same for the other basins. As presented in that paper, Category 4 and 5 hurricanes require sea surface temperatures (SST) of over 27°Celsius. Thus the criteria that should be examined are anomalies in SST that result in increases of temperatures above the 27°C criteria. Category 5 hurricanes require temperatures 28°C. Has the area of SST above these thresholds increased, for example?
The Webster et al. Science article actually present a range of SST values during the respective hurricane seasons for the different hurricane basins in Figure 1 of their paper. These range from around 29.5°C for the north Indian Ocean to around 27.5°C for the north Atlantic and eastern Pacific Ocean basins. Such an analysis suggests that regardless of SST temperature trends, the north Indian Ocean should have a greater porportion of Category 4 and 5 hurricanes. Clearly, there are other factors besides SST that determine the ability of the tropical cyclones to attain Category 4 and 5 intensities as we discussed in Pielke, R.A., Jr. and R.A. Pielke, Sr., 1997: Hurricanes: Their nature and impacts on society. John Wiley and Sons, England, 279 pp, and Pielke, R.A., 1990: The hurricane. Routledge Press, London, England, 228 pp. Indeed, it is rare for the hurricane to attain its maximum intensity due to other limitiations. The Science article is silent on the relation between the different SSTs in the different hurricane regions with respect to the proportion that reach category 4 and 5 intensities.
The major limitations that prevent hurricanes from reaching their full potential includes vertical wind shear, dry air intrusion, and less than optimal outflow aloft in the upper portion of the hurricane circulation. In idealized hurricane modeling it is relatively easy to create hurricanes that attain their maximum intensity, since these limitations are not prescribed in the model initialization or boundary conditions. In the real world, however, one or more of these limitations almost always exists (fortunately!). Hurricane Katrina is an example where a particularly effective outflow aloft, moist tropical air, and a lack of vertical wind shear, along with the elevated SSTs, pemitted the cyclone to attain a category 5 intensity.
In Nicholls, M.E., and R.A. Pielke, 1995: A numerical investigation of the effect of vertical wind shear on tropical cyclone intensification. 21st Conference on Hurricanes and Tropical Meteorology, AMS, Boston, April 24-28, Miami, Florida, 339-341, we investigated the role of shear on hurricane intensification. In Eastman, J.L., M.E. Nicholls, and R.A. Pielke, 1996: A numerical simulation of Hurricane Andrew. Second International Symposium on Computational Wind Engineering, 4-8 August 1996, Fort Collins, CO , we investigated the skill in the simulation of a Category 4 and 5 hurricane. A clip of our model simulation of Hurricane Georges is available from Video Clip of Hurricane Georges (8 Megabytes). We suggest that the use of high resolution models of hurricane intensification as influenced by SST anomolies should be a high priority in addressing the issue of their role, relative to other influences, on hurricane intensification. It is only with fine-scale hurricane model simulations of real world systems, that are able to resolve the eyewall region of the hurricane, that we can adequately address the issue of the relative role of the spatial pattern and magnitude of SSTs on the intensity that they attain.
As another issue, why use 5-year running averages? Tropical cyclones respond to the SST that exists when they occur. The analysis should have correlated tropical cyclone intensity with the specific SST values for each event. The conclusions of the authors would be more robust if they evaluated the Category 4 and 5 hurricanes on a case by case basis with respect to the ocean SST temperatures and SST anomolies over which the hurricanes moved.
Finally, the same analysis, as shown by Pat Michaels (Global Warming and Hurricanes: Still No Connection), when applied to an earlier time period (starting in 1945) than in the Webster et al. Science study, indicates that a high proportion of Category 4 and 5 hurricanes also occurred then. Webster et al. is clear as to why they chose to use the more recent era with the better data coverage. However, coverage for the Atlantic basin, for instance, is quite good since 1945 and should have been assessed against the more recent time period. The Michaels communication ideally should have been submitted to Science as a comment, so that Webster et al. would need to respond. Nonetheless, it highlights an important issue that needs to be resolved as to whether Webster et al. are analyzing the upward portion of a cyclic behavior of hurricane intensities or a real much longer-term trend.
Webster et al. do appear to recognize this issue. The Science article concludes with the statement (referring to the trend towards more frequent and intense hurricanes),
“This trend is not inconsistent with recent climate model simulations that a doubling of CO2 may increase the frequency of the most intense tropical cyclones, although the attribution of the 30-year trends to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state”.
This qualification of their work was lost when the news media highlighted in their reports (e.g., see “Experts say global warming is causing stronger hurricanes“).
The National Oceanographic and Atmospheric Administration (NOAA) provides a very valuable current assessment of SST anomolies , which can be directly related to the SST temperature anomolies presented in the Webster et al. paper. For example, for the September 17th data, above average SST temperatures in the Atlantic Ocean hurricane region is evident, as is the cooling to below average where the recent hurricanes have traveled. The analysis also shows a complex spatial pattern of SSTs which further supports the need for the Webster et al conclusions to be assessed with respect to the actual SSTs traversed by the hurricanes. The NOAA data also show that the hurricane region exceeds the threshold for Category 4 and 5 hurricanes, even without additional warming.