November 29, 2006

Hurricane Update, Again

Filed under: Climate Extremes, Hurricanes

Who could ever forget the hurricane season of 2005 with Katrina and no end of massive hurricanes all in one tropical cyclone season? In a recent article in Geophysical Research Letters, two Louisiana State University scientists examine one record-breaking characteristic of that fateful year. Keim and Robbins note “The 2005 Atlantic hurricane season set many records including the greatest number of named tropical storms and hurricanes at 27, and the greatest total hurricanes at 15. Another record from 2005 includes 7 named tropical storms and hurricanes before 1 August. The 2005 season began early and remained active from June to January (2006).”

They gathered data on start dates of hurricanes from 1851-2005, and found that “Other seasons with storms with early dates of occurrence include 1887, 1933, 1936, and 1995.” There is no trend here with three of the five years with early storms occurring between 1887 and 1936. They made no link to global warming, but they did conclude “All of these seasons are associated with a positive phase of the Atlantic Multidecadal Oscillation” which is the warm phase of the oscillation.

The next recent interesting article on hurricanes comes from the Journal of the Meteorological Society of Japan in which a team of scientists investigated the effects of greenhouse warming on tropical cyclone frequency. Yoshimura et al. used a very high resolution (110 km grid spacing) atmospheric general circulation model to examine the effect of sea surface temperature conditions, altered by global warming, on tropical cyclone (TC) frequency.

To their credit, they found that the geographical distribution of tropical storm formation was simulated realistically in the control experiments using their numerical model. Furthermore, they found that the impact of El Niño and La Niña on tropical storm frequency “in the model also seems realistic.” They admitted that “The maximum wind speed and the minimum central pressures of intense TCs, are not realistically simulated in the model.” What the heck – we’re talking modeling here.

Now for the bottom line on tropical storms (TS) and tropical depressions (TD) from the recent research out of Japan. In the global warming experiments, the team found the “frequency of TS formation decreases by 9.0–18.4% globally, and some of these changes are statistically significant. Total frequency of TSs and TDs decreases significantly in all of the warm-climate experiments. For relatively intense TCs (e.g., maximum surface wind > 25 m s-1), there are no coherent changes in global frequency.”

Is this big news that should surprise anyone interested in the subject? No — Yoshimura et al. present the table below showing the results from other research in which scientists used high resolution general circulation models to investigate hurricane frequency in response to global warming. As we see in their table, Bengtsson et al. (in 1996) gave it a go and found that global warming would decrease tropical storm frequency by 36.6% (Bengtsson et al. find similar results in a paper published too recently to be included in Yoshimura’s Table, see here for our coverage of the recent Bengstsson findings of decreased TC activity in a CO2-warmed world). Sugi et al. took a stab at the “problem” and found a 33.6% decrease in storm frequency (we are noticing a trend here) thanks to global warming. Yoshimura et al. used their various deep convection algorithms and found anywhere from a 9.0% to a 18.4% decrease in the tropical storms.


Table is from Yoshimura et al. (2006)

There is no doubt that 2005 was an incredible year for hurricane activity in the United States. The storms started early, ended late, and were more frequent and severe than anyone could ever have imagined. Climatologists will spend years examining the geophysical circumstances that produced the devastating events of that hurricane season. Global warming advocates will continue to waste no time and blame 2005 on the anthropogenic greenhouse effect. The Gore film (and many others) quickly associated the greenhouse gas buildup with warmer sea surface temperatures and with the development of extreme tropical cyclones. Never mind that the Intergovernmental Panel on Climate Change has yet to predict more storms or more intense storms due to higher global temperatures.

As these latest articles in the peer-reviewed professional scientific literature illustrate, the story about global warming and hurricane activity is more complex than the greenhouse crusade would have you believe. The highest resolution numerical models of climate clearly predict fewer hurricanes globally in the future and empirical studies show no trend in the start date of the hurricane season. We have covered this story over and over, but new articles keep popping up all over the world, and we continue to bring you the news from the scientific literature.

Obviously, you come to World Climate Report for the latest on the global warming debate, but imagine if Keim and Robbins had found a trend toward earlier start dates for the hurricane season – they would need press agents to handle the coverage. Had Yoshimura et al. found an increase in tropical cyclone frequency; they too would have become stars in the greenhouse crusade. Instead, the valuable research papers of Keim, Robbins, Yoshimura, Sugi, and Noda will be trumpeted only here at World Climate Report.

References:

Keim, B. D., and K. D. Robbins, 2006. Occurrence dates of North Atlantic tropical storms and hurricanes: 2005 in perspective, Geophysical Research Letters, 33, L21706, doi:10.1029/2006GL027671.

Yoshimura, J., M. Sugi, and A. Noda, 2006. Influence of greenhouse warming on tropical cyclone frequency, Journal of the Meteorological Society of Japan, 84, 405-428.




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