In a weekly update, scientists at the NOAA Climate Prediction Center said that as the 2006-2007 El Niño has faded, surface and subsurface ocean temperatures have rapidly decreased.

Recently, cooler than normal water temperatures have developed at the surface in the east-central equatorial Pacific, indicating a possible transition to La Niña conditions.

La Niña conditions occur when ocean surface temperatures in the central and east-central equatorial Pacific become cooler than normal.

These changes affect tropical rainfall patterns and atmospheric winds over the Pacific Ocean, which influence the patterns of rainfall and temperatures in many areas worldwide.

Typically, across the United States during the spring and summer months, La Niña conditions do not significantly impact overall inland temperature and precipitation patterns,but La Niña episodes often do have an effect on Atlantic and Pacific hurricane activity.

"Although other scientific factors affect the frequency of hurricanes, there tends to be a greater than normal number of Atlantic hurricanes and fewer than normal number of eastern Pacific hurricanes during La Niña events," said NOAA Administrator Conrad Lautenbacher.

"During the winter, usual La Niña impacts include drier and warmer than average conditions over the southern United States," he said.

"NOAA's ability to detect and monitor the formation, duration and strength of El Niño and La Niña events is enhanced by continuous improvements in satellite and buoy observations in the equatorial Pacific," Lautenbacher explained.

The observing systems currently at work include the TAO/TRITON moored and Argo drift buoys, as well as NOAA's polar orbiting satellites.

"La Niña events sometimes follow on the heels of El Niño conditions," said Vernon Kousky, research meteorologist at the NOAA Climate Prediction Center. "It is a naturally occurring phenomenon that can last up to three years."

"While the status of El Niño/La Niña is of vital importance to our seasonal forecasts, it is but one measure we use when making actual temperature and precipitation forecasts," said Kousky.

La Niña episodes tend to develop during the four months from March through June, reach peak intensity during the December to February period, and then weaken during the following March to May period.

"The last lengthy La Niña event was 1998-2001, which contributed to serious drought conditions in many sections of the western United States," said Douglas Lecomte, drought specialist at the NOAA Climate Prediction Center.

Photo: Radar image of Hurricane Wilma making landfall in South Florida, October 2005. Wilma was the most intense hurricane recorded in the Atlantic basin and only the third Category 5 ever to develop in the month of October. With the formation of Hurricane Wilma, the 2005 season became the most active on record. (Image courtesy NOAA)

In addition, atmospheric scientists have uncovered fresh evidence to support the controversial theory that global warming has contributed to the emergence of stronger hurricanes in the Atlantic Ocean.

Scientists at the University of Wisconsin-Madison and NOAA's National Climatic Data Center report that all the physical variables that converge to form hurricanes - wind speeds, wind directions and temperatures - feed off each other in ways that only make conditions more ripe for a storm.

The unsettling trend is confined to the Atlantic and does not hold up in any of the world's other oceans, researchers have also found.

James Kossin, a research scientist at UW-Madison's Cooperative Institute for Meteorological Satellite Studies, says the pattern emerged from a new dataset that correlates a variety of different satellite data over 22 years from 1983 to 2005.

"While we can see a correlation between global warming and hurricane strength, we still need to understand exactly why the Atlantic is reacting to warmer temperatures in this way, and that is much more difficult to do," says Kossin. "We need to be creating models and simulations to understand what is really happening here."

NOAA will issue the U.S. Spring Outlook on March 15, and its Atlantic Hurricane Season Outlook in May. Both outlooks will reflect the most current La Niña forecast.

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