STORMS VARY WITH CYCLES, EXPERTS SAY

The New York Times, 30 August 2005
http://www.nytimes.com/2005/08/30/national/30cycle.html?hp&ex=1125374400&en=881f1863a6416e02&ei=5094&partner=homepage

By KENNETH CHANG

Because hurricanes form over warm ocean water, it is easy to assume that the recent rise in their number and ferocity is because of global warming.

But that is not the case, scientists say. Instead, the severity of hurricane seasons changes with cycles of temperatures of several decades in the Atlantic Ocean. The recent onslaught "is very much natural," said William M. Gray, a professor of atmospheric science at Colorado State University who issues forecasts for the hurricane season.

From 1970 to 1994, the Atlantic was relatively quiet, with no more than three major hurricanes in any year and none at all in three of those years. Cooler water in the North Atlantic strengthened wind shear, which tends to tear storms apart before they turn into hurricanes.

In 1995, hurricane patterns reverted to the active mode of the 1950's and 60's. From 1995 to 2003, 32 major hurricanes, with sustained winds of 111 miles per hour or greater, stormed across the Atlantic. It was chance, Dr. Gray said, that only three of them struck the United States at full strength.

Historically, the rate has been 1 in 3.

Then last year, three major hurricanes, half of the six that formed during the season, hit the United States. A fourth, Frances, weakened before striking Florida.

"We were very lucky in that eight-year period, and the luck just ran out," Dr. Gray said.

Global warming may eventually intensify hurricanes somewhat, though different climate models disagree.

In an article this month in the journal Nature, Kerry A. Emanuel, a hurricane expert at the Massachusetts Institute of Technology, wrote that global warming might have already had some effect. The total power dissipated by tropical cyclones in the North Atlantic and North Pacific increased 70 to 80 percent in the last 30 years, he wrote.

But even that seemingly large jump is not what has been pushing the hurricanes of the last two years, Dr. Emanuel said, adding, "What we see in the Atlantic is mostly the natural swing."

Copyright 2005, NYT