The winds of change are threatening millions of people living along America’s East and Gulf coasts.
The increase in the number of hurricanes in recent years is likely to continue, perhaps for decades, a team of weather researchers reports in today’s issue of the journal Science.
The waters of the North Atlantic Ocean have been warming in recent years, providing increased energy to fuel these massive storms, said the group, led by Stanley B. 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 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’d better be prepared for a very busy period for hurricane activity,” Mr. Goldenberg said. “We’re not talking about an increase in numbers, we’re talking about the magnitude and the number making landfall.”
The group also noted what appears to be a cycle of periods with more hurricanes alternating with quieter times.
The years from 1995 to 2000 were among the most active in history for hurricanes, following a quiet period that began in the 1960s.
“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 M. 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 earlier this year by James B. Elsner of Florida State University, who found that busy and quiet hurricane periods in the Atlantic and Gulf of Mexico appeared to alternate. He said a period of more than normal storms is getting under way.
Hurricane season began on June 1. The strongest storms tend to form later in the summer and early fall, when the sea, which provides them energy, has warmed.
The National Weather Service has forecast 11 tropical storms, of which seven will be hurricanes this year. Disaster specialists worry that coastal residents have been lulled into a false sense of security by recent quiet years.
Also increasing the danger is the fact that the number of people living in coastal areas has skyrocketed in recent years, and that evacuating masses of people in advance of a storm can be chaotic and sometimes dangerous.
“From 1995 to 2000, we saw the highest level of North Atlantic hurricane activity ever measured,” Mr. Goldenberg said.
“Compared with the previous 24 years, there were twice as many hurricanes in the Atlantic, including two-and-a-half times more major hurricanes those reaching Category 3 strength with winds reaching more than 110 mph — and more than five times as many hurricanes impacting the Caribbean islands.”
However, Lennart Bengtsson of the Max Planck Institute for Meteorology in Hamburg, Germany, cautioned in a commentary on the paper that the period of accurate storm record keeping may be too short to draw conclusions.
The research group also included Christopher Landsea of NOAA’s Hurricane Research Division and William M. Gray of Colorado State University.
m Matthew Cella contributed to this report.