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NOAA satellite image of Hurricane Andrew on August 23, 1992.July 19, 2001 — NOAA Scientists 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. (Click NOAA satellite image for larger view of Hurricane Andrew taken August 23, 1992.)

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 will be published in the journal Science on July 20.

Click image for larger view.
Effects of vertical wind shear.

"Overall, from 1995-2000 we saw the highest level of North Atlantic hurricane activity ever measured," Goldenberg said. "Compared with the relatively inactive time of 1971 to 1994 in the Atlantic, there was twice as much overall activity in the Atlantic, two and a half times more major hurricanes—those reaching Category 3 strength (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 co-authors Christopher Landsea, a research meteorologist at NOAA's Hurricane Research Division; Alberto M. Mestas-Nuñez, a physical oceanographer at NOAA's Cooperative Institute for Marine and Atmospheric Studies at the University of Miami; and William M. Gray, professor of atmospheric science at Colorado State University, also tried to determine if the recent increase 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 Mestas-Nuñez. "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."

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. Nonetheless, rapidly increased population and development means that hurricane damage will be far more than ever experienced by coastal residents. "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 our 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. Whereas, weak wind shear encourages them.

Goldenberg and Landsea are part of NOAA's Hurricane Research Division which studies hurricanes and tropical storms. The scientists are among a cadre of researchers who fly into the storms gathering data that are relayed to NOAA's National Hurricane Center, which issues the warnings. Goldenberg has flown into the eye of a hurricane more than 100 times.

The Hurricane Research Division is located in NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami.

Relevant Web Sites
NOAA's Hurricane Research Division

NOAA's Atlantic Oceanographic and Meteorological Laboratory

NOAA's Hurricanes Page

Highlights of Hurricane Season 2000

Hurricanes 2000 — Climate summaries and satellite images

Historic Hurricanes

Hurricane Basics

Archived NOAA satellite imagery of historical events

Atlantic Tropical Events 2001 — NOAA satellite imagery

NOAA’s “Hurricane Hunter” Aircraft


Media Contact:
Jana Goldman, NOAA Research, (301) 713-2483 ext. 181