U.S. forecasters now say it’s a little less likely that this year will see a weather-roiling La Nina that could dry up crops in Brazil and trigger more Atlantic hurricanes.
Forecasters say there is a 55 to 60 percent chance La Nina will form by the northern hemisphere’s winter, down from 75 percent last month, the U.S. Climate Prediction Center said in a report Thursday. A La Nina watch is still in place.
In the past, La Nina conditions have led to droughts across southern Brazil, a major soybean producer, and heavy rains in Malaysia that can make harvesting palm oil crops difficult. In the U.S., the phenomenon has brought cooler winters boosting natural gas demand.
“We still favor a La Nina,” said Michelle L’Heureux, a forecaster with the Climate Prediction Center in College Park, Maryland. “There is still momentum in that direction.”
La Ninas can also decrease wind shear across the tropical Atlantic allowing more hurricanes to form. Shear, when winds blow at opposite directions or varying speeds at different altitudes, will often rip apart tropical systems.
Before forecasters confirm an El Nino or La Nina, corresponding changes in air and ocean temperatures must last for at least a month, with an expectation they will persist even longer.
Last year’s El Nino was one of the three strongest on record, generating the hottest global temperatures in more than 130 years, according to the U.S. National Centers for Environmental Information in Asheville, North Carolina.
The track record for La Ninas emerging after strong El Ninos is mixed, L’Heureux said. After the powerful El Nino of 1982-83 the Pacific cooled, however a La Nina didn’t form until the fall of 1984.
In contrast, after the El Nino of 1997-98 a La Nina formed almost immediately.
The odds for a La Nina have dropped because forecasters haven’t seen a reaction by the atmosphere above the ocean, L’Heureux said. Prior to last year’s El Nino, there were times when the ocean temperature met the criteria without the atmosphere reacting to it.
“It is always the atmosphere,” L’Heureux said. “The atmosphere is much less constrained. The ocean is a big mass of water, so it can’t move that quickly. Once things get going with the ocean it is almost unstoppable.”
It’s possible the atmosphere hasn’t reacted because of the pooling of warm and cool water in the Pacific, L’Heureux said. A strip of colder water extending west from South America along the equator is accompanied by warm seas to the north and south.
“So it is still very warm surrounding that strip,” L’Heureux said by telephone. “There is already speculation in the scientific community that that is prohibiting coupling at this point.”
This reaction in the atmosphere also could be a key in determining gas prices going into the winter. Colder-than-normal temperatures in large U.S. cities during the November-March heating season would cause more of the fuel to be burned to heat homes and businesses.
“If the atmosphere doesn’t click in here, we wouldn’t get effects over the United States in the upcoming winter,” L’Heureux said.