"Anthropogenic factors are likely responsible for long-term trends in tropical Atlantic warmth and tropical cyclone activity," Penn State and Massachusetts Institute of Technology (MIT) researchers report in an upcoming issue of the American Geophysical Societys EOS.

Researchers Michael Mann, associate professor of meteorology and geosciences at Penn State, and Kerry Emanuel, professor of atmospheric sciences at MIT, evaluated the record of global sea surface temperatures, hurricane frequency, and aerosol impacts.

They also studied the Atlantic Multidecadal Oscillation (AMO), an ocean cycle similar, but weaker and less frequent, than the El Nino-La Nina cycle in the eastern Pacific ocean.

"Though other scientists have suggested that the AMO, a cycle of from 50-70 years, is the significant contributing factor to the increase in number and strength of hurricanes, their statistical analysis and modeling indicate that it is only the tropical Atlantic sea surface temperature that is responsible, tempered by the cooling effects of some lower atmospheric pollutants," a Penn State communique said.

"We only have a good record of hurricanes and sea surface temperature for a little more than the last 100 years," noted Mann, also director of Penn States Earth System Science Center.

"This means we have only observed about one and a half to two cycles of the AMO. Peer-reviewed research does suggest that the signal exists, but it is difficult to estimate the period and magnitude of the oscillation directly from observations," he explained.

On the other hand, Mann and Emanuel said, "There appears to be a strong historical relationship between variations in tropical Atlantic sea surface temperature and tropical cyclone activity extending back through the 19th century."

"The cause of increased tropical Atlantic sea surface temperatures is the real question," according to Penn State's statement. "One contributor must be overall global sea surface temperature trends."

"If the AMO, a regional effect, is not contributing significantly to the increase, then the increase must come from general global warming, which most researchers attribute to human actions," the statement said.

The researchers looked at the tropical Atlantic sea surface temperature record and compared it to global sea surface temperatures, finding that the two closely tracked together.

But global fluctuation did not account for everything, they said.

The Atlantic season officially starts Thursday and US experts say as many as 10 hurricanes could form in the Atlantic and four could slam ashore in the southern United States.

That could easily spell disaster for residents of coastal areas, thousands of whom have not yet finished repairing homes damaged by last year's Katrina, Rita and other massive storms.

Katrina ranked as category three on a five-point scale when it slammed ashore near New Orleans, causing the deaths of more than 1,500 people.

In all, 2005 saw a record 15 hurricanes, among an unprecedented 28 named storms that formed in the Atlantic. For the first time on record, seven of the hurricanes were considered major, meaning they hit category three or higher.

It was also the costliest hurricane season, with damage estimated at more than 100 billion dollars.

Related Links