WASHINGTON — For decades, Americans have flocked increasingly to East and Gulf coast beach communities, prepared to ride out the occasional tropical storm in exchange for ocean vistas.
Now the winds of change are blowing in a new era of more storms, a change that could last for 20 to 30 years, scientists say.
After a quarter-century of fewer than normal hurricanes in the North Atlantic, the 1995 to 2000 hurricane seasons were the busiest on record, according to a report in last Friday's issue of the journal Science from a team of weather researchers.
The ocean has been warming in recent years, which provided increased energy to fuel the development of the massive storms, according to the group led by Stanley Goldenberg of the National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami.
At the same time, the scientists found a decline in wind shear, which can inhibit the development of the storms. Wind shear occurs when wind speeds differ sharply at different altitudes, and powerful winds high in the sky can prevent the vertical development of storms below them.
"When we see this combination, we better be prepared for a very busy period for hurricane activity," Goldenberg said.
The group also noted a cycle of periods with more hurricanes alternating with quieter times.
"Looking at the changes in oceanic and atmospheric conditions, we think this shift is due to a natural ocean cycle called the Atlantic Multidecadal Mode, a North Atlantic and Caribbean sea surface temperature shift between warm and cool phases that lasts 25 to 40 years each," said Alberto Mestas-Nunez of NOAA's Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.
"The data suggest that we are in the beginning of a warm Atlantic phase, and thus an active Atlantic hurricane era may be under way similar to that last seen from the late 1920s to the late 1960s," he said.
That reinforces a report this year by James Elsner of Florida State University, who found busy and quiet hurricane periods in the Atlantic and Gulf appeared to alternate and said a period of more than normal storms is beginning.
Hurricane season began June 1, though the strongest storms tend to form in late summer and early fall, when the sea that provides energy has warmed.
The National Weather Service has forecast 11 tropical storms this year, of which seven will be hurricanes, and disaster experts worry that coastal residents have been lulled into a false sense of security by recent quiet years.
Also increasing the danger is that the number of people living in coastal areas has skyrocketed in recent years, and evacuating masses in advance of a storm can be chaotic, sometimes dangerous.
"From 1995-2000 we saw the highest level of North Atlantic hurricane activity ever measured," Goldenberg said. The data showed that compared with the previous 24 years, twice as many hurricanes occurred in the Atlantic, including 2 1/2 times more major hurricanes, with winds reaching more than 110 mph. More than five times as many hurricanes hit the Caribbean islands than in the previous two dozen years.
Lennart Bengtsson of the Max Planck Institute for Meteorology in Hamburg, Germany, ventured, however, that the period of accurate storm record keeping may be too short to draw conclusions about alternating cycles.
The "records are too short and incomplete to claim that the coastal United States may be in for a longer period of hurricane activity," Bengtsson said in a commentary on the paper. He was not part of the study team.
Besides Goldenberg and Mestas-Nunez the research group included Christopher Landsea of NOAA's Hurricane Research Division and William Gray of Colorado State University.
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