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Global Warming?
from Environment News Service July 24, 2001
Hurricane Activity Accelerates in U.S. Long Term Forecast
MIAMI, Florida, July 23, 2001 (ENS) - The United States is in for 20 or 30 years of above normal hurricane activity, say researchers with the National Oceanic and Atmospheric Administration.
In an average three year period, roughly five hurricanes strike the United States coast, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically what scientists term "major" or "intense" hurricanes with winds greater than 110 miles per hour.
A new study shows warmer sea surface temperatures in the North Atlantic combined with a decrease in vertical wind shear contribute to conditions for more hurricanes in the weather forecast. But the new pattern will not produce more hurricanes this year, federal government scientists say.
Scientists at the National Oceanic and Atmospheric Administration (NOAA) say many of the hurricane seasons in the next two or three decades may be much more active than they were in the 1970s through early 1990s. And they think they know why.
Stanley Goldenberg, a research meteorologist at NOAA's Hurricane Research Division, part of the Atlantic Oceanographic and Meteorological Laboratory in Miami, conducted research that shows warmer sea surface temperatures in the North Atlantic combined with a decrease in vertical wind shear contribute to conditions for more hurricanes over a several year period.
"When we see this combination, we better be prepared for a very busy period for hurricane activity," Goldenberg said. Goldenberg's findings appear in Friday's issue of the journal "Science," a publication of the American Association for the Advancement of Science.
"From 1995 to 2000 we saw the highest level of North Atlantic hurricane activity ever measured," 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. We started looking at the records to find out why."
Goldenberg and his coauthors also tried to determine if the recent increase in hurricane activity reflects a long term climate shift.
"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-Nuņez, a physical oceanographer at 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.
The study finds that the record amount of hurricane activity could possibly be caused by a combination of the multidecadal scale of ocean temperature changes plus a small contribution from the long term warming trend. However, deficiencies in the data record make this a difficult issue to resolve.
NOAA has predicted normal levels of hurricane activity for the 2001 season. Goldenberg noted that while anticipating generally high activity during the hurricane seasons for the next few decades, not every year is expected to be hyperactive.
But rapidly increased population and development means that hurricane damage will be far more than ever experienced by coastal residents.
Hurricane hazards come in many forms: storm surge, high winds, tornadoes, and flooding.
"Even weak storms can cause devastating flooding," Goldenberg said.
The scientists note that an active hurricane season does not necessarily mean more storms making landfall, but caution that even years with a low level of activity can produce disasters. In 1992, Hurricane Andrew became the costliest disaster in U.S. history and was the only hurricane to make landfall that year.
The North Atlantic hurricane season officially lasts from June 1 to November 30. The tropical storms that can turn into hurricanes and threaten the east and Gulf coasts of the United States form in the Gulf of Mexico, Caribbean Sea and Atlantic, many developing from easterly waves moving off the west coast of Africa.
Hurricanes are fueled by warm water as they travel across the ocean. An abundance of warm water provides more energy allowing the storm to increase in strength. However, the researchers found that the warm water alone was not enough.
The winds between the upper and lower troposphere (the first seven miles of the atmosphere starting from the ground or ocean and going upwards) also play a major role. Strong vertical shear in the wind inhibits the formation or intensification of tropical cyclones. Weak wind shear encourages them.
Each year, an average of 10 tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean. Six of these storms become hurricanes each year.
Meteorologists expect near normal levels of hurricane activity for the 2001 Atlantic hurricane season. Today, forecasters at the National Weather Service in Miami are keeping an eye on a tropical disturbance with thunderstorms over portions of the Florida Peninsula and the adjacent Gulf and Atlantic waters. It is associated with a broad low pressure system centered about 30 miles west of St. Petersburg, Florida.
Forecasters warn businesses and vessels to monitor the system closely. While it is not a hurricane yet, it could gradually acquire hurricane characteristics if showers and thunderstorms continue to develop and persist near the low-level circulation center.
To learn more about hurricane research, visit:http://www.aoml.noaa.gov/hrd
