Stronger SST-intense hurricane link?

A link between global warming and increased intense hurricane activity is a very hot topic in hurricane research right now, and many new papers on the subject will be published this year. The latest paper, published March 15 in the on-line version of Science, Science Express, finds stronger evidence that the increasing number of Category 4 and 5 hurricanes globally since 1970 is directly linked to increases in Sea Surface Temperature (SST). The paper by Hoyos et al. was called, "Deconvolution of the Factors Contributing to the Increase in Global Hurricane Intensity". Two of the co-authors--Peter Webster and Judith Curry of Georgia Tech--were also authors of a paper published in Science magazine in 2005 that reported a worldwide increase in the number of Category 4 and 5 hurricanes of 80% in the past 30 years. The paper, (Webster et al., 2005), titled "Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment", linked the rise in storms to increasing sea surface temperatures and concluded that "global data indicate a 30-year trend toward more frequent and intense hurricanes." As I reported in my blog on the subject, their findings should be considered as preliminary evidence that the global incidence of Category 4 and 5 hurricanes may be increasing. There are some severe problems with the quality of the data set used to, and there are good reasons to believe that the actual increase in Category 4 and 5 hurricanes is far lower than the 80% increase found by Webster et al.

The new paper by Hoyos et al. uses a mathematical technique called information theory to study the relative effects of SST, wind shear, humidity, and wind patterns on global incidence of Category 4 and 5 hurricanes. The study found that only SST can explain the observed increase in these storms. One thing I like about the new study is that it directly addesses the issue of data quality in the record of Category 4 and 5 hurricanes, something the authors neglected to do in their previous paper. The authors write, "Recently, the quality of the hurricane data has been questioned and even a reanalysis of the tropical cyclone databses has been suggested in order to ratify that the results of recent studies are not due to problems in the data." The authors go on to say that they performed their analysis without using suspect data from the North Indian Ocean, and found no difference in their results. Well, that's not too surprising, since the number of Category 4 and 5 hurricanes in that ocean basin represents only about 2% of the global total. What I would have liked to have seen was the analysis re-done using the latest reanalyzed results for typhoons from the Western Pacific, which accounts for 48% of global Category 4 and 5 hurricanes. In a paper accepted for publication but not yet finalized, Knaff and Zehr (2006) make convincing arguments that typhoon intensities during the 1973-1986 period were too low due to measurement error, and the number of Category 4 and 5 storms in the region have been roughly constant for the past 50 years. Dr. Knaff and Charles Sampson have performed a preliminary re-analysis of maximum typhoon intensities for the period 1966-1987 based on the Knaff and Zehr (2006) results. In a paper to be presented at the upcoming 27th Conference on Hurricanes and Tropical Meteorology (April 24-28, 2006), they show that after correcting for the measurement errors, the number of Category 4 and 5 typhoons during the 1966-1987 period increased by 1.5 per year, leaving only a slight upward trend in Category 4 and 5 typhoons during the period 1970 - 2004. The 16% increase in Category 4 and 5 typhoons found by Webster et al. during the past 15-year period is reduced to just 3%. I suspect that if the information theory techniques of Hoyos et al. were applied to this modified data set, the connection between SST and an increase in global Category 4 and 5 hurricanes would be much weaker.

The realclimate.org blog has more information on the paper, along with links to quotes in the media from many of the scientists involved in the hurricanes/global warming debate.

My next blog will be on Friday. Apparently, NHC has "found" a new Atlantic subtropical storm that formed in 2005, bringing the total for the season to 28 named storms. If the final report on this new storm has been issued, I'll discuss that.

Jeff Masters

references
Hoyos, C.D., P.A. Agudelo, P.J. Webster, and J.A. Curry, "Deconvolution of the Factors Contributing to the Increase in Global Hurricane Intensity", www.scienceexpress.org, 16 March 2006, 10.1126/science.1123560.

Knaff, J.A., and R.M. Zehr, "Reexamination of Tropical Cyclone Wind-Pressure Relationships", accepted to Weather and Forecasting, 2006.

Webster, P.J., G.J. Holland, J.A. Curry, and H.-R. Chang, "Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment", Science, 309, 1844,1846, 16 September 2005.