El Nino may have a split personality.
The warming of the tropical Pacific Ocean has long been known to affect weather around the world, but researchers now say it may come in two forms with different impacts.
The traditional El Nino tends to reduce the number of Atlantic hurricanes. But a form Georgia Tech scientists call El Nino Modoki can lead to more hurricanes than usual in the Atlantic Ocean. Modoki, from Japanese, refers to something that is "similar but different."
The traditional El Nino involves a periodic warming of the water in the eastern part of the tropical Pacific. Indeed, it was first noticed by Peruvian fishermen, who named it after the baby Jesus because it tended to first appear around Christmastime.
In El Nino Modoki, on the other hand, the warming occurs farther to the west, in the central Pacific.
It's not clear why this new form is occurring, said Peter Webster, a professor at Georgia Tech's School of Earth and Atmospheric Sciences and a co-author of a report on the finding in Friday's edition of the journal Science.
"It may be responding to some (climate) oscillation or it may be in response to global warming," Webster said in a telephone interview.
Regardless of the cause, improved understanding of El Nino and it's cold-water counterpart La Nina could help atmospheric scientists better forecast weather, including the number of Atlantic hurricanes.
Traditionally, La Nina has been associated with more Atlantic hurricanes than normal and El Nino with fewer.
Just last month government forecasters reported signs that an El Nino may be developing in the Pacific.
So which type might it be?
"We spent all last week trying to figure that out," Webster said. 'It looks like it might be a hybrid," with warming starting in the east and them moving west, possibly meaning more hurricanes late in the season.
Co-author Judith Curry said she feels that "there is about a 50 percent chance that we could have one of the Modoki years emerging by late summer."
"We'll have to see how it plays out, but we could be seeing increased (hurricane) activity," Curry, chair of atmospheric sciences at Georgia Tech, said in a telephone interview.
Greg Holland of the National Center for Atmospheric Research welcomed the study, saying it "has important consequences for the predictability of global weather patterns."
Predicting the number of Atlantic hurricanes may be improved by breaking El Nino into two modes, eastern Pacific warming and central Pacific warming, Holland, who was not part of the research team, said in a commentary on the paper.
Curry explained that the Modoki version of El Nino had been looked at in general by Japanese and Korean researchers, but not in relation to hurricanes. Joining in the project was research scientist Hye-Mi Kim who came to Georgia Text from Korea, she said.
The discovery of a different mode of El Nino that led to increased storms "was quite startling," she said.
In 2004 an El Nino was developing, causing forecasters to expect fewer Atlantic hurricanes.
But instead it turned out to be one of the Modoki years. There were 15 named storms, including six major hurricanes. Overall activity was nearly two and a half times the long-term average, resulting in more than 3,100 deaths in the region including 60 in the United States. There was record property damage in the United States.
The American Meteorological Society's Monthly Weather Review reported that in 2004 "Florida, the 'Sunshine State,' became known as the Plywood State after being battered by Charley, Frances, Ivan and Jeanne."
The National Weather Service is predicting nine to 14 named tropical storms this year, of which four to seven are likely to be hurricanes.
Commerce Secretary Gary Locke has pointed out that "more than 35 million Americans live in regions most threatened by Atlantic hurricanes."