The El Nino weather phenomenon, which caused damage worldwide in 1997-98, has returned, government climate experts said Thursday.
This El Nino will be milder than the last one, but could begin affecting weather in the United States in the fall, according to climatologist Vernon Kousky of the National Oceanic and Atmospheric Administration's Climate Prediction Center.
"This time around, El Nino will not be as powerful as the 1997-98 event, but we'll track it closely for any change in its projected strength," said Kousky. Once it matures, he said, El Nino should maintain a weak-to-moderate strength.
El Nino is first characterized by a warming of the tropical Pacific Ocean, and scientists at the Climate Center in Camp Springs, Md., have been tracking its progress for months as conditions began to move in the direction of warming.
They said Wednesday that with consecutive months of warmer-than-normal sea-surface temperatures and abnormally heavy rainfall in areas of South America, conditions meet the threshold to be classified as an El Nino.
The warmer-than-normal seas result in rising wet air that can affect the winds that steer the movement of weather. This can trigger a chain reaction of weather changes around the globe, including warmer, rainy weather in the southern United States during winter and drier weather in much of Indonesia throughout El Nino's life cycle.
Kousky said El Nino normally does not affect summers in the United States, although historically it tends to suppress Atlantic hurricane activity.
However, he said El Nino may not be strong enough to be a factor in this year's hurricane season.
In late April and during the last half of May, the low-level equatorial easterly winds substantially weakened throughout the Pacific, followed by an increase in sea-surface temperature. Buoy data indicated that the oceanic thermocline — the boundary between warm surface water and cold water below — had deepened.
A strong El Nino can cause winter storms to be more vigorous along the Pacific coasts of Canada, Alaska and the U.S. Northwest. Winter storms also would tend to be more vigorous in the Gulf of Mexico and along the southeast coast of the United States, resulting in wetter-than-normal conditions in that region.
Other effects can include abnormally dry conditions over northern Australia, Indonesia and the Philippines, drier-than-normal conditions over southeastern Africa and northern Brazil and reduced Indian monsoon rainfall. It tends to produce wetter-than-normal conditions along the west coast of tropical South America and from southern Brazil to central Argentina.
El Ninos tend to occur every four or five years and can last as long as 12 to 18 months. In between, ocean conditions in the Pacific usually return to normal levels, but sometimes can cool abnormally, a condition known as La Nina, which also affects the weather in other areas. The causes of El Ninos and La Ninas are not yet fully understood.
Traditionally, the arrival of El Nino was first noted by South American fishermen at around Christmas time. They named it El Nino, Spanish for "little boy," after the Baby Jesus.