Tony Barnston predicts above-normal 2010 hurricane season

The Atlantic hurricane season of 2010 is predicted to be more active than normal. That’s according to forecasts released in early June by the National Oceanic and Atmospheric Administration and the International Research Institute for Climate and Society, or IRI. EarthSky spoke with Tony Barnston, lead forecaster for IRI.

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Tony Barnston: There will be more hurricane activity. The main three months that our forecast applied to are August, September, and October, when most of the hurricanes occur.

That’s peak season for hurricanes, Barnston said, but the official season is from June to November. He said in a normal year, almost ten hurricanes and tropical storms are expected. This year, forecasters expect fourteen or fifteen. Barnston said there are two reasons they believe there will be so many storms:

Tony Barnston: The water temperature being above normal in the main development region. And number two, the likely development of at least a weak La Nina by hurricane season.

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Barnston said that the surface temperatures of the seas where Atlantic hurricanes develop are one to two degrees Celsius above normal this year. Warmer waters fuel hurricanes. And La Nina – a climate event connected to ocean and air circulation in both the Atlantic and Pacific Oceans – can create atmospheric conditions that help hurricanes stay intact as they develop. Barnston explained that both the warmer waters and the possibility of La Nina are related to last year’s El Nino. El Nino is associated with higher tropical Pacific sea surface temperatures in winter, and higher north tropical temperatures the following spring and early summer, and it is not unusual for El Nino to be followed by a La Nina.

Tony Barnston: The main reason we’re expecting more active than normal, because sea surface temps, the temperature of the surface water of the ocean, in an area we call the main development region which extends from the west coast of Africa to the Caribbean, that water temperature is above normal by one or two degrees. That doesn’t sound like huge deviation from normal, but it makes a big difference to the likelihood of getting hurricanes. The warmer the water, more hurricane activity there is.

Barnston said that global warming may play a small part in the warmer temperatures, but the main cause is the El Nino that just died in May.

Tony Barnston: We had an El Nino this last year. Often during the spring season following an El Nino, the north tropical Atlantic surface temperatures of the water become above normal. That’s one main reason that we have warm temperatures now. That period comes to an end by the middle of hurricane season, which is why we think temperatures won’t be as far above normal by September – the main month of hurricane season – as they are now. But they most likely still will be above normal. He explained that there is a 50 – 60% chance that La Nina will follow on the heels of the El Nino this year. That increases the likelihood of an above-normal hurricane season, he said.

Tony Barnston: El Ninos and La Ninas change tropical air circulation. It changes it in such a way that when we have a La Nina, circulation features in the Atlantic are more favorable for hurricane development. It has to do with the winds, in the lower part of the atmosphere over the tropical Atlantic versus the upper part of the atmosphere.

Barnston said that El Ninos suppress hurricanes, while La Ninas supports their development.

Tony Barnston: To have a hurricane stay intact, the prevailing winds in the lower part of the troposphere cannot be greatly different from the upper part of the troposphere. When there is an El Nino, the winds differ more than average, which we call a high wind shear. What happens, literally, is that the hurricane clouds are torn apart because as the hurricanes grow vertically, the differing winds whip them apart. It stretches and breaks the hurricane structure. When there’s a La Nina, the winds in the lower versus the upper part of the atmosphere are more consistent with each other, so that the clouds of the hurricane can remain intact as the clouds grow. They won’t be ripped apart.

Barnston said that there is a loose correlation between the number of hurricanes, the number of landfalls, and the amount of destruction during hurricane season. He said that the coast of Florida and the Caribbean islands, including Haiti, are at greater risk from hurricanes this year.

June 28, 2010

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