Tropical Storms

Letters to the Editor—April 29, 2010

April 29, 2010 // 0 Comments

Home grown scientist comments on climate column With regards to changes in the frequency and intensity of tropical storms over time, the status of current research is, as you note, inconclusive. The problem of extracting definitive conclusions from the data is complicated by many factors, including the evolution (and variety) of measurement techniques, gaps in the historical storm record, diverse methods of quantifying storm intensity, and the complex and localized nature of tropical storms themselves. The most accurate and consistent long-term data sample comes from studies of the North Atlantic, so let’s avoid some of these complications by restricting our focus here. For example, the Pew Center on Global Climate Change claims that the average number of named tropical storms in the North Atlantic since 1996 has exceeded the previous historical maximum set in the 1950s by approximately 40%. You may argue that this is merely the result of spotty historical data being compared to complete modern data; you may reference other studies or measures of storm frequency that show no statistically significant trend; you may claim that the change is a normal multi-decadal variation; and you would be right to so! Nevertheless, it is just one of many studies that at least suggest a long-term trend of increased storm frequency. Clearly, further study is required. But tropical storm frequency, as it turns out, is less meaningful than intensity, and here the research becomes rather abstruse. You cite Ryan Maue, a Florida State University PhD candidate, whose studies of net global “Accumulated Cyclone Energy” (the integrated sum of all cyclones’ maximum wind speed squared) are very interesting. ACE is a convolution of global storm frequency, intensity, and duration, so it falls a bit outside the stated bounds of discussion. Still, as a comparison, let’s also consider MIT Professor of Meteorology Kerry Emanuel’s “Power Dissipation Index” (the integrated sum of cyclones’ maximum sustained wind speed cubed). Although similar in definition to Maue’s ACE, Emanuel’s PDI indicates a ~60% increase over the past thirty years in the North Atlantic. It is highly correlated with the surface sea temperature, which itself is correlated with global warming. There’s obviously a good deal of natural variability in the frequency and intensity of tropical storms, and, as discussed, making conclusions about […]