For the past 18 months, a lingering La Niña has unleashed a tizzy of hurricanes, floods and other severe weather around the world. Meteorologists have been waiting anxiously for signs that the mischievous weather event would go away. But the latest satellite data reveal that a series of La Niñas could pester the world for some time to come.
If the interpretation is correct, the Atlantic basin could be in for 20 years or more of active hurricane seasons. A series of La Niñas would also continue to drench Asia, Central and South America with above average rainfall — areas already ravaged by flooding in recent years.
The new satellite data reveal a new horseshoe-shaped pattern of warmer than average surface temperatures on the Pacific Ocean. The pattern has formed over the last 18 months in the North, West and South Pacific Ocean. As these areas of the ocean warm, a persistent wedge of cooler than average water (a La Niña) often forms around the equator.
The temperature shifts may be part of the "Pacific decadal oscillation," according to Dr. William Patzert, an oceanographer with the NASA Jet Propulsion Laboratory. The oscillation is marked by "positive" and "negative" phases which alternate every 20 to 30 years.
Since the 1970s, the ocean has been in what is called a positive phase of the oscillation cycle. This phase is marked by warm surface water in the tropics and cooler water in the North Pacific. The positive phase generally favors El Niños. But every 20 years or so, the warm and cool water switch places for reasons that are still unclear to scientists.
Some scientists believe that the switch from a El Niño-favored climate to a La Niña-favored climate has already begun. Since 1995, there have been four La Niñas and only one El Niño, according to Ants Leetmaa, director of the National Weather Service's climate prediction center.
La Niña, which means "the little girl," is characterized by unusually cold ocean temperatures in the eastern tropical Pacific, as compared to El Niño, which is characterized by unusually warm temperatures in the same waters. The climatic impact of one system is the opposite of the other.
In the United States during El Niño years, temperatures in the winter are warmer than normal in the North Central states, and cooler than normal in the Southeast and the Southwest. During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest. The changes in ocean temperatures also cause a shift in the jet stream patterns, which affect storm activity.
The cooling of the tropical Pacific can cause drastic changes to the climate and to jet stream patterns, wreaking havoc on weather patterns around the globe. Recently, La Niña has resulted in an increase in hurricane activity in the Atlantic basin and more rain in the tropics and subtropics, said Leetmaa, adding that the recent flooding in Venezuela could be linked to the changing ocean temperature pattern.
But many experts argue that it's too early to draw a conclusion about weather patterns that will occur over the next few decades. "It is not at all clear that the pattern that is seen this particular winter is part of a decadal change," David Battisti, atmospheric sciences professor at the University of Washington, told the Associated Press.
And even if scientists are right in their predictions, they can't forecast future weather events with absolute certainty. Climatologists, for instance, think that a decades-long shift in the Pacific would bring drier weather to the southwest United States and wetter weather to the Northwest. But they can't predict whether the effects would be disastrous — throwing some areas into drought and deluging others with floods. "These are questions that are just being raised and it might be five or 10 years before we answer those questions," Leetmaa said.