The intricate hurricane cycle
By now, you’ve heard about what Hurricane Katrina has done to the Gulf Coast, specifically, to New Orleans. Many people have been wondering why there’s been so much hurricane activity in the last few years, myself included. There have been more hurricanes and tropical storms, and on average, they’ve been fiercer and more destructive. So what’s going on? NASA’s Earth Observatory has got an article up about the hurricane cycle.
Using mathematical models, Doctors Xie and Pietrafesa developed a way to analyze patterns relating to tropical storms dating from 1887 to 1999. By analyzing patterns, they predict that the current hurricane resurgence will continue for the next 10 to 40 years. Not a very exact (or encouraging) prediction, but it does indicate that if New Orleans is rebuilt the same way the old city was, it could very well be wiped out again by another powerful hurricane like Katrina. In the the prophetic words of the Times-Picayune, “It’s only a matter of time.”
Their model is based on four cycles, shorter cycles within larger ones. A long-term cycle contributes one or two tropical storms per year to the total number, and a short-term cycle contributes up to five per year for the North Atlantic. Since 1995, there has been a dramatic increase in hurricane activity, with storms like Fran and Floyd causing widespread destruction.
An earlier, July 20 study published in Science suggests that the recent increase in hurricanes and tropical storms is the beginning of a 20 to 50 year trend in hurricane activity. But Xie and Pietrafesa believe that their analysis technique, called Empirical Mode Decomposition (EMD) more accurate describes the patterns of tropical storm occurences.
The most energetic cycle - the one that shows the greatest variation in the number of landfalling storms between its peak and low-point - is one that lasts three to five years. That cycle, Xie says, essentially adds or subtracts one or two landfalling tropical storm events every year on the East Coast. The eight-to-12-year cycle can add or subtract an average of one and a half hurricanes per year; the 20-40-year cycle can add or subtract an average of about one-half hurricane per year; and the longest, 40-60 year cycle (similar to the cycle described by Goldenberg and his colleagues) can add or subtract about one hurricane per year.
The causes of these trends are not immediately clear, but global warming has been implicated, along with the oscillations of ocean currents in the Atlantic and Pacific Oceans. Among these oscillations are El Nino (the warming) and La Nina (the cooling) of these two oceans.
During the 112-year period for which the NC State researchers have data, an average of 3.23 tropical cyclones pounded the East Coast each year. During El Nino years, that number dropped to an average of 2.47 storms. North Carolina saw an overall average of 0.81 tropical cyclones annually, and 0.56 during El Nino years.
Jonathan has great coverage of more of the science behind how and why Katrina got as powerful as she did in this week’s Science.Ars. I will investigate the global warming angle on the hurricane season in the next few days. The scientific understanding of the ferocity of a given hurricane season is not fully understood, but headway is being made. Observing patterns is the first step towards explaining them, and right now it seems we’re on the upswing of a few of the observed hurricane cycles — possibly all four.
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[...] As I’ve written before, the hurricane cycle is an intricate dance of four cycles each within one another. Influences like El Nino and La Nina also play a small role in how severe a hurricane season can be. Remember that El Nino is the natural warming of the Atlantic and Pacific oceans, and that during this regular warming period, the hurricane season tends to be more mild. Although Katrina began as a relatively small hurricane that glanced off south Florida, it was supercharged with extraordinary intensity by the relatively blistering sea surface temperatures in the Gulf of Mexico. [...]
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