The parade of storms that pummels the western fringe of the North Atlantic every year just got a bit more predictable. Scientists say they have developed a way to forecast how many Atlantic hurricanes there will be — not just for the upcoming year, as some groups already do each spring, but for several years out.
“This is the first time anyone has reported skill in predicting the number of hurricanes beyond the seasonal time scale,” says Doug Smith, a climate modeler at the Met Office Hadley Centre in Exeter, England. A paper by Smith and his colleagues appeared online Nov. 7 in Nature Geoscience.
Knowing how hurricane trends could change in the future, he says, will help society prepare for the damage of the kind that Hurricane Tomas recently dealt the Caribbean.
Atlantic hurricane activity waxes and wanes over a cycle of several decades, and since 1995 has been in an active part of that cycle. Researchers have been working to tease apart the causes of this cycle and to predict how future changes, like rising sea surface temperatures, might affect storms.
Smith’s team uses one of the hottest areas of climate modeling: decadal climate prediction, which aims to understand both how the climate system varies internally along with external factors like greenhouse gases and volcanic eruptions.
The researchers used nine versions of its decadal prediction model to “hindcast” Atlantic hurricanes each year from 1960 to 2007. The model was set to May 1 for each of those years and then was asked how many storms would come that season. Averaging across the nine versions, the model results closely matched the changing number of hurricanes that occurred over those decades. Smith says: “We’ve found that there is some skill there.”
Next the team tackled long-term predictions, by starting on November 1 of each year between 1960 and 2005 and forecasting the number of hurricanes for 10 years out. Again, Smith says, the model tracked the observations well, particularly within the first couple of years.
But the study can’t yet show exactly what makes hurricanes more frequent. Climate researchers disagree over whether past increases in hurricane activity were due to internal variability, external factors like greenhouses gases, or both.
By watching how well the model matches reality while changing these factors, Smith’s team suggests that at least some of the recent increase in Atlantic hurricane frequency comes from external factors. The next step, Smith says, is to run more comparisons and see which of those might be most important.
Many researchers think that global warming will indeed affect how many storms form. If atmospheric carbon dioxide levels double over preindustrial levels by the year 2100 as many expect, “we should expect an increase in the frequency of the strongest hurricanes in the Atlantic, roughly by a factor of two by the end of the century,” says a group led by modeler Morris Bender of NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. Their analysis appeared in Science in January.
Within the next few months, Smith’s team plans to forecast how many Atlantic hurricanes there will be over the next few years.
So far this season the Atlantic has had 20 named storms, 12 of them hurricanes. That’s in line with NOAA’s May forecast, which called for 14 to 23 named storms, of which 8 to 14 would be hurricanes.
The Atlantic hurricane season ends November 30.