Tropical Storms in the North Atlantic: Projections and Observations

Two recent papers by Villarini et al. (2011a,b) examine tropical storms in the North Atlantic and their future projection in response to anthropogenic forcing. The first paper (Villarini et al., 2011a) examines the possibility of a substantial increase (up to 200%) in tropical storm frequency in the 21st century due to anthropogenic forcing, as was also suggested in earlier studies (Mann and Emmanuel, 2006; Vecchi et al., 2008). The projected increase is linked to a substantial increase of Atlantic SSTs (Sea Surface temperature) projected in climate model scenarios, which translates into an increase in tropical storm count through a causal relationship. The authors also used a statistical model based on the well-known 'Poisson Distribution,' where the rate of occurrence of tropical storms is modeled as a linear function of tropical Atlantic and tropical mean SSTs. These results do not support the notion of a large increase in tropical storms for the North Atlantic in the 21st century in response to increasing greenhouse gases. Further, they reveal several uncertainties, primarily due to uncertainties in climate model projections of future SST increase in the Atlantic and tropical oceans. The study concludes that large uncertainties on century-scale trends that are due to internal climate variability are likely to remain irreducible for the foreseeable future.

In the second paper, Villarini et al. (2011b) examine the recorded increase in tropical storms from the middle of the 20th century, finding that the increase is mostly due to "short-duration" storms lasting two days or less. The study used a statistical model combined with current understanding of the physical processes (of tropical storm formation) to analyze all short duration storms over the period 1878-2008. The study concludes that the large increase in tropical storms from the 1940s is likely due to changes in observing system of these storms and does not suggest any detectable climate change signal. The rapid increase in short-duration storm count since 1944 is attributed to the beginning of the aircraft reconnaissance period to track and locate North Atlantic storms. The improved observing technology for detecting tropical storms has substantially inflated the storm count since the middle of the 20th century.

Additional References
Mann, M.E. and Emanuel, K.A. 2006. Atlantic hurricane trends linked to climate change. EOS, transaction of the American Geophysical Union 87: 233, 238, 241.

Vechhi, G.A., Swanson, K.L. and Sodden, B.J. 2008. Whither hurricane activity? Science 322: 687-689.