Although La Nina occurs in the Pacific, it can affect weather in the Atlantic region by altering wind patterns in the upper and lower atmosphere, which make it easier for hurricanes to form in the Atlantic and harder for them to form in the eastern Pacific. In the Atlantic, La Nina often lessens the power of winds that normally would tear a hurricane's circular motion apart, but increases them in the eastern Pacific.

Scientists at the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs, Md., report sea-surface temperatures have been warming back to normal. During April, sea-surface temperatures were slightly cooler than normal in the extreme eastern equatorial Pacific, but conditions are returning to near average in that region.

"The current temperature signal at the end of April is near normal and the ocean surface temperature has not yet caused the atmosphere to respond in a La Nina-like way," said David Adamec, an oceanographer at NASA's Goddard Space Flight Center.

Adamec has been tracking La Nina data using a NASA atmosphere-ocean-land computer model. The model, developed at Goddard, can forecast ocean and atmosphere patterns for periods three to 12 months into the future. The model uses data from two NASA satellites: Jason and QuikSCAT. Jason provided sea-surface height information, and QuikSCAT provided surface wind data.

Adamec said in order for La Nina to affect the Atlantic Ocean hurricane season, it must persist for a much longer time than it is doing this year - it really must last well into the peak hurricane season of August and September.

Furthermore, he explained, another factor associated with La Nina is the Southern Oscillation Index, which currently is normal. The index indicates atmospheric pressure of large-scale surface winds. "La Nina is already a memory," Adamec said.

According to 12 major ocean-atmosphere computer models, the equatorial Pacific will be neutral to warm in August, when it really matters for hurricanes. August and September constitute the peak season for hurricane formation in the Atlantic Ocean, and the atmosphere takes about two weeks to react to changes in ocean-surface temperature, he explained.

Forecasters and other scientists still expect a greater-than-average number of Atlantic Ocean hurricanes this year, but they agree La Nina probably will not be a factor. The active season is expected because of other environmental conditions favorable to hurricanes: the location of the Bermuda high - which removes much of the wind shear in the western Atlantic that thwarts hurricanes - and warm sea-surface temperatures in the Gulf of Mexico.

La Nina also influences where Atlantic hurricanes form. During La Nina, more hurricanes form in the deep tropics from African easterly waves - long waves in the atmosphere that occur between 5 and 15 degrees north latitude. They originate in Africa and move across the Atlantic, causing about 60 percent of Atlantic tropical storms and minor hurricanes.

Hurricanes spawned from easterly waves have a much greater likelihood of becoming major storms and threatening the United States and the Caribbean islands.

"The recent increased frequency of the hurricanes is thought to be part of a larger decades-long cycle of alternating increases and decreases of hurricane activity," said Bill Patzert, an oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"The current busy hurricane cycle began in 1995 and could continue for another 10 to 25 years. For the U.S. East and Gulf coasts, the fading La Nina is a real good thing, but Atlantic sea surface temperatures are still very toasty. It's the summer conditions that will dictate the fall hurricane activity, and I suspect those forecasts will be modified."

Related Links
El Nino/La Nina Info
NOAA Climate Prediction Center
Pacific Watch at TOPEX/JPL