CNN - Cool Pacific, warm Atlantic and La Nina leave U.S. ripe for hurricanes

This hurricane season is far from over; oceanographers are placing blame on two oceans and a weather phenomenon VIDEO CNN's Ann Kellan looks at the weather conditions contributing to this powerful hurricane season. (September 15) Windows Media 28K 80K

September 15, 1999
Web posted at: 8:19 p.m. EDT (0019 GMT)

From Correspondent Ann Kellan

(CNN) -- The hurricane season of 1999, just like its 1998 counterpart, was relatively quiet in the beginning, only to spawn the most powerful storms in its second half.

Oceanographers and forecasters blame these storms on the effects of two oceans and one weather phenomenon, La Nina.

The force and direction of Hurricane Floyd, for instance, are being guided by a cool Pacific Ocean, which steers storms, and a warm Atlantic Ocean, which fuels them.

"Really, the ocean is simply the fuel tank for these great hurricanes, and right now, I would characterize the South Atlantic as high octane," says Bill Patzert, an oceanographer with NASA's Jet Propulsion Laboratory.

"The magic number is 80 degrees Fahrenheit. What we are seeing in the Atlantic right now is temperatures that are 82 to 86 degrees Fahrenheit, almost two to four degrees above their normal temperatures," says Patzert.

Activity in the Pacific Ocean dramatically influences Atlantic storms, via the jet stream, upper level winds that steer most major U.S. weather patterns.

During an El Nino, when Pacific waters are warmer than normal, the tropical jet stream tends to block and weaken tropical storms headed toward the eastern United States.

Activity in the Pacific Ocean dramatically influences Atlantic storms via the jet streams, seen in red and blue

But this year, the opposite phenomenon, La Nina, means cool Pacific waters. And they tend to steer the jet stream north, leaving the door open for oncoming storms such as Floyd.

"The hurricanes naturally tend to move from east to west because of the rotation of the Earth," says Patzert. "The northern part of the hurricane moves faster than the southern part of the hurricane, so they tend to move across the Atlantic toward the Caribbean and the United States."

"Conditions last year are very similar to this year -- warm water in the Atlantic, cool water in the Pacific," says NASA oceanographer Tony Busalacchi.

"Even though we're on the historical average right now -- five to six tropical storms being generated so far -- last year at point this time, nine more tropical storms were generated (including) big ones such as Mitch during the second half of the hurricane season," he says.

Busalacchi predicts that La Nina will stick around another one to three months. With Atlantic waters still warmer than normal, conditions are ripe for more storms to threaten U.S. shores.