U.S. government meteorological experts say the global climate condition known as 'La Nina' has returned, heralding a realignment of weather patterns around the world for the next several months.
The U.S. agency that monitors the oceans and atmosphere, NOAA, says sea surface temperatures of the equatorial Pacific have cooled slightly, confirming that La Nina is back.
La Ninas are part of the continual but irregular cycle of changes in the ocean and atmosphere that affect global weather. The other part is called El Nino, which warms the equatorial Pacific waters.
NOAA climate prediction expert Edward O'Lenic says the cooler La Nina waters affect the atmosphere so that parts of the world accustomed to certain types of weather - rain or drought, for instance - will experience them more intensely until May or June, or possibly longer.
"One simple way to get a handle on the impacts of El Nino and La Nina is to think of them as the opposite sides of the same coin," he said. "Where La Nina tends to make places that are normally wet, wetter, and places that are normally dry, drier. El Nino does the opposite of that in those locations."
La Nina usually causes increased rainfall across Indonesia and northern Australia, the Amazon Basin, southeastern Africa, Scandinavia, and the U.S. Great Lakes region and Pacific northwest. In contrast, below average rainfall normally occurs across the eastern half of the Pacific at the equator, eastern equatorial Africa, Japan and Korea, the southern United States and northern Mexico. La Ninas also tend to bring increased Atlantic hurricane activity.
They occur every three to seven years at this time of the year. Ed O'Lenic says the average temperature drop on the equatorial Pacific this time has been half of one degree Celsius. This is brought about by breezes called tradewinds, which flow from east to west on both sides of the equator.
"The tradewinds put stress on the water of the ocean surface and they tend to force it to move toward the west," he said. "When that happens, the water that gets removed has to be replaced - and it comes from down deep, where the waters are colder."
O'Lenic says predicting such events is important to let farmers and other people dependent on weather make plans to deal with them.
The last La Nina occurred five years ago and was weak. The NOAA official says it is too soon to gauge the impact of this one.
"This is a weak one so far and we're not sure what its ultimate strength will be," O'Lenic said.
Although La Ninas and El Ninos have certain tendencies, he said predictions of intensity are not easy to make.