In a highly publicized article in the journal Science this summer, a team of meteorologists predicted that the current resurgence in North Atlantic hurricane activity will continue for at least the next 10 to 40 years.

That may indeed be the case, say two hurricane researchers at North Carolina State University. But it's only a small part of a complex tropical storm picture facing the U.S. Eastern Seaboard.

Those NC State meteorologists and oceanographers, Dr. Lian Xie and Dr. Leonard Pietrafesa, have used a recently developed mathematical technique to analyze patterns relating to the tropical cyclones that made landfall along the East Coast between 1887 and 1999.

Their findings: The number of hurricanes making landfall in a given year is controlled not only by the long-term, multi-decade trend described in the Science article, but by three shorter-term cycles as well. These four distinct "temporal modes" – each probably the result of a different atmospheric and oceanic phenomenon – combine to determine the number of tropical storms that make landfall each year, Xie and Pietrafesa explain.

"It's the short-term modes that have the most effect on the number of Atlantic tropical cyclones that make landfall each year," Xie said. "There's a danger if you don't look at all the modes, not just the longest-term mode."

Pietrafesa adds that while the long-term cycle contributes about one or two tropical cyclones per year to the total, the shorter-term cycles typically contribute up to five in a given year for the entire North Atlantic, several of which hit the coast.

The North Atlantic has seen dramatic hurricane activity since 1995, with powerful storms like Hurricanes Fran and Floyd pounding the eastern United States and resulting in many lost lives and extensive property damage. The authors of the July 20 Science article – Drs. Stanley B. Goldenberg and Christopher W. Landsea of the National Oceanic and Atmospheric Administration, Dr. Alberto M. Mestas-Nunez of the University of Miami and Dr. William M. Gray of Colorado State University – suggest that the recent upswing in the number of hurricanes is the beginning of a 20-50 year trend characterized by an increased number of hurricanes.

Xie and Pietrafesa believe that their mathematical analysis technique, called Empirical Mode Decomposition (EMD), more accurately describes the temporal patterns of tropical storm occurrences along the East Coast than the procedure used by Goldenberg and his colleagues.

EMD mathematically analyzes the differences in the number of tropical storms occurring over different time scales, resulting in a series of wave-like graphs describing cycles in the number of landfalling tropical cyclones. Using EMD, Xie and Pietrafesa found four different cycles, or modes.

The number of tropical storms that make landfall in a given year depends on whether each of the four cycles is at its peak, its low point, or somewhere in between. The highest number of hurricanes is likely to occur in years during which most or all of the cycles are "in phase" at their peaks.

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 longer-term cycles indicate that in the early 21st century, higher-than-normal landfalling tropical cyclone activity is likely to occur along the eastern United States coast, and in North Carolina in particular," Xie said. "The causes of these decadal trends are not yet clear, but may be related to multi-decadal oscillations of ocean circulations in the Atlantic and Pacific Oceans." Those oscillations, he said, include the long-term weather events known as El Nino, which is linked to the periodic warming of the Pacific Ocean off the coast of South America, and La Nina, which is the cooling of those same waters.

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.

The EMD technique was developed by former NC State oceanic engineering professor Dr. Norden E. Huang, who is now senior fellow and chief scientist for oceanography at the NASA Goddard Flight Center in Maryland.

Xie and Pietrafesa note that scientists still have a great deal to learn about what causes fluctuations in the number of hurricanes, especially over the short-term. "The ability to predict the number of hurricanes for a given year is improving, but we're not there yet," Pietrafesa said.

Their research was funded by the Office of Naval Research and the National Oceanic and Atmospheric Administration.

- potter -