Model Investigations of the Effects of Climate Variability and Change on Future Gulf of Mexico Tropical Cyclone Activity

Abstract According to three recent assessments, global warming will likely cause increased hurricane activity1 in the future. If true, this raises the possibility that new coastal and offshore facilities are being under-designed, and that older facilities may need hardening in order to maintain presently accepted risk levels. As these three assessments readily admit, many uncertainties remain concerning the accuracy of their forecast. The study summarized in this paper has sought to narrow the uncertainties by using several methods. A set of relatively high resolution regional climate simulations are being made with the NCAR Nested Regional Climate Model (NRCM) embedded in the global Community Climate System Model (CCSM). These are being combined with statistical and statistical-dynamical downscaling techniques to provide an assessment of changes in North Atlantic hurricane activity out to 2050. Comparisons with the historical record show that the model can reasonably duplicate both the observed frequency and severity of hurricanes in the North Atlantic as well as in the Gulf of Mexico. This initial analysis suggests that under the business as usual IPCC A2 scenario North Atlantic tropical cyclones will experience an accelerated increase in numbers from 3.4% per decade near the present to 10% per decade leading up to 2050; the region of maximum storm frequency and formation will move equatorward over the same time period; and there will be a modest increase of mean intensity of ~2 ms-1, but a more marked increase in the frequency and intensity (~3.5 ms-1) of the most intense hurricanes that can be resolved by the model (Cat 3). All of these changes are statistically significant at the 95% level or greater. For the Gulf of Mexico, the results are more ambiguous in part because of the limited number of storms in each decadal time slice which is exacerbated by a strong multi-decadal natural climate variability. These are preliminary results from a more comprehensive prediction and analysis program that is in progress. We invite community involvement in helping to analyze the several hundred terabytes of model output from existing and in-process model simulations that are being archived. Introduction Recent synthesis reports (IPCC 2007, CCSP 2008) concluded that global warming will likely result in increased hurricane activity1 in the North Atlantic. Their conclusion is based onModeling and observational evidence that shows near-surface ocean temperatures have increased due to global warming, and are very likely to increase in the future (Meehl et al. 2007, Gillett et al. 2008). It is also well established from theoretical and observational considerations that hurricane activity tends to increase with warming ocean temperatures (Henderson-Sellers et al.. 1998, Emanuel 2007, Elsner et al. 2008);An increase in the observed hurricane power associated with the near-surface sea temperature in the hurricane generation region of the North Atlantic (Emanuel 2005);Numerical model results which have almost universally found an increase in intensity and rainfall, but there are substantial differences on whether the numbers of storms will increase or decrease (Knutson and Tuleya 2004, Knutson et al. 2007, Bengtsson et al. 2007).