OREGON STATE UNIVERSITY

Hurricane Floyd signal of a new weather era?

09/15/1999

CORVALLIS, Ore. - The same long term climatic forces that Wednesday will cause Hurricane Floyd to ravage the Carolinas is also the culprit behind other weather events literally around the world - ranging from a wet, cool winter projected this year for Portland, Ore., to better salmon conditions in the Pacific Northwest and normal rain for the drought-stricken Sahel region of Africa.

But those forces also suggest a 15-20 year period of more regular and destructive hurricanes in the Atlantic Ocean and resulting decades of destruction for coastal cities of the United States.

This process, which scientists just began to understand in the early 1990s, is sometimes called a thermohaline circulation pattern or "conveyor belt" of ocean waters that has implications for El Ninos, La Ninas and countless other global impacts, said George Taylor, the state climatologist at Oregon State University and president of the American Association of State Climatologists.

"This hurricane season started late, but it's already had about an average amount of hurricane activity and it's only halfway over," Taylor said. "It's going to be a very busy hurricane season, just as four of the last five years have been. And the Pacific Northwest is going to have its sixth year in a row of wetter than average weather. This is not a coincidence. If you want to understand what is happening, you have to look at some of the climate forces that are driving it."

What's happening, Taylor believes, is that a conveyor belt of ocean circulation which operates over vast distances has kicked into a higher level of activity - a pattern that began about 1995 and generally lasts around 20 to 25years before it quiets down again. It's a pattern separate from, and not to be confused with possible global warming caused by the greenhouse effect.

When this conveyor belt of ocean waters is most active, Taylor said, it tends to produce warmer water in the Atlantic Ocean and cooler waters in the Pacific. Unusually warm water, as any meteorologist can tell you, is the fuel of more frequent and violent hurricanes. And cold water in the Pacific Ocean tends to produce the type of "La Nina" years that, among other things, makes Oregon even colder and rainier than usual in the winter. The fish love it, but no one else does - except some diehards like Taylor.

"When the conveyor belt is in high gear, the Atlantic tends to be warmer than usual and the Pacific is cooler," Taylor said. "Then the cycle seems to die down for about 20 years."

The correlation is particularly strong for Atlantic Ocean hurricanes, Taylor said. The quietest four year period of the last 50 years was from 1991-94, but then 1995 had the largest number of hurricanes since storms were first named in the early 1950s. The latest four year period is the busiest on record.

These theories, Taylor said, were first developed with some pioneering work by oceanographers and climate experts Wallace Broeker and William Gray in the early 1990s. They identified a global scale current that operates on a time scale of several decades and affects world weather patterns.

When it's active, this "conveyor belt" transports warm ocean water from the Pacific through the Indian Ocean and into the Atlantic Ocean. In the North Atlantic, this warm water, which is now very salty due to evaporation during the journey, cools and sinks, setting up a subsurface countercurrent that transports cooler water back to the Pacific and Indian Ocean.

The end result, Taylor says, is more El Nino events when the conveyor belt is inactive, and more La Nina events - or cool Pacific Ocean phases - when the conveyor belt is really moving.

"Since the theories were developed, we've identified several things that correlate most closely to high levels of conveyor belt activity," Taylor said. "Certainly one of the most prominent is Atlantic Ocean hurricanes. Also very clear is more La Nina years, and wetter, cooler conditions in the Pacific Northwest. In fact, we have a graph showing the tight correlation between severe hurricane seasons like the one we're now having, and wet winter conditions in Portland, Ore.

"Following the very dry 1975-94 period, which saw two significant statewide droughts in Oregon and 10 consecutive dry years, we are now completing the fifth consecutive above average year for precipitation in the Pacific Northwest," he said.

Taylor said it's still not certain what causes the "conveyor belt" to kick into gear. One likely candidate is fluctuations in the sun's magnetic field, which affects the amount of solar energy reaching the Earth and tends to change on a 20-27 year cycle.

What is far more certain, he said, is the implication of the trend once it's clearly underway.

Among other things, it means that East Coast and Gulf Coast cities are in for at least 15 more years of unusually frequent and destructive hurricanes. And that residents of traditionally soggy Oregon and Washington will have very wet, snowy winters with some regularity.