BOSTON — It may be of little comfort to people recovering from Hurricanes Fran or Hortense, but relatively few high-intensity hurricanes have made landfall in the US and Caribbean during the past 25 years.
That may be about to change. One of Earth's most powerful heat exchangers, the North Atlantic Ocean, appears poised to send more hurricanes with sustained winds exceeding 111 miles per hour (Category 3 or above) spinning onto the East Coast, Florida, and into the Caribbean during the next few decades, according to a team of researchers.
Concerns about increased hurricane activity have been sounded in recent years by scientists focused on global warming. They warn that the number and intensity of such storms are likely to grow as higher average air temperatures heat the sea's surface in regions that spawn Atlantic hurricanes.
Yet a team led by William Gray of Colorado State University cites evidence that Atlantic hurricane activity has gone through 20- to 30-year cycles at least for the past century and perhaps for the past 3,000 years. Such swings, his team argues, are likely to continue regardless of global warming. The challenge, the researchers say, is to develop techniques that will allow forecasters to determine when such shifts are likely to happen.
For now, scientists are trying to get a better handle on the forces driving the cycles.
In developing an increasingly accurate annual forecast, Gray's team has used a variety of atmospheric conditions as hurricane harbingers. They include rainfall in the Sahel region of western Africa, the state of El Nio activity in the Pacific, and wind conditions in the stratosphere over the equator.
Last year, the team added three new bellwethers, including two measures of Atlantic sea-surface temperatures.
"Our governing hypothesis is that water temperatures in the tropics alters circulation in the atmosphere," says Christopher Landsea, a meteorologist with the National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami and a member of Gray's team. The warmer waters in the subtropics not only feed moisture to the "waves" that can evolve into hurricanes, he says. They also tend to lower the air pressure at the surface and change upper and lower-level wind patterns in ways that nurture the tropical disturbances.
As they looked at long-term cycles in these conditions, Dr. Gray and his colleagues noticed that when the atmospheric factors favoring hurricane formation strengthened, so did the large-scale movement of water in the North Atlantic. Known as thermohaline circulation, the northward migration of warm surface water and the southward flow of cold deep water is the ocean's way of trying to smooth out temperature differences between the poles and the tropics. The team is concerned that if the rate of movement increases, warm water will be forced farther north, allowing hurricanes to retain more of their strength at higher latitudes.
ACCORDING to Michael McCartney, a senior scientist at the Woods Hole Oceanographic Institution, at Woods Hole, Mass., that flow is mostly driven by temperature differences between water in the far North Atlantic and water near the equator. The colder, denser water sinks and flows south along the sea floor. This forces warmer water on the surface to flow north. The more dramatic the temperature differences, the greater the flow rate. The rate is altered by the salt content of cold northern waters. Increased salinity raises the water's density, and thus the rate at which it sinks.
Observations over the past few years suggest that the North Atlantic's conveyor belt has been stuck in low gear since the early '70s - roughly equivalent to the "lull" in intense hurricanes making landfall. But shipborne measurements and satellite readings suggest the gears may be shifting: Salinity has been building in northern waters while water temperatures have been increasing near the tropics. Referring to attempts at correlating changes in the North Atlantic's conveyor belt with atmospheric changes, Dr. McCartney says "the trick is to decide what's causal."