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Monday, November 24, 2014         

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Ocean warming saps tradewinds

The effect of weaker northeast breezes might be less rainfall in windward areas

By Gary T. Kubota

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A study showing a trend toward weaker tradewinds in the Atlantic and Pacific could mean less rain for the windward side of the Hawaiian Islands. Or not.

One of the researchers, Shang-Ping Xie, a University of Hawaii meteorology professor, said the reduction of northeast trades in Hawaii is due to stronger ocean warming over a portion of the North Pacific compared with the South Pacific.

What that means for rainfall is subject to debate.

A decrease in the strength of tradewinds could produce less rainfall on the windward side of the islands, Xie said.

Above-average warming of ocean surface temperatures around Hawaii projected by climate models tends to increase rain for the islands, said Xie, who works at the university's International Pacific Research Center.

"It's kind of a mix, so it's hard to say how Hawaii is going to fare as far as future rainfall trends," Xie said.

The study of regional climates by Xie and visiting researcher Hiroki Tokinaga was published online Feb. 6 in Nature Geoscience. The print edition is scheduled to be published in March.

The study, partially funded by the National Oceanic and Atmospheric Administration, included information from several decades of ship reports on the heights of wind-driven waves.

The study departed from the popular method of measuring wind speeds with anemometers placed on merchant ships. The heights of anemometers have risen with the heights of ships and resulted in an apparent artificial increase in wind speeds.

Weakened tradewinds in the Atlantic have resulted in less upwelling cold water and nutrients in the eastern tropical Atlantic that could affect marine life.

Rainfall has increased in the Amazon and Guinea Coast, where nearby ocean temperatures have risen above the tropical average.

But rainfall has decreased in an area of Africa known as the Sahel, a transition zone below the Sahara Desert.

Xie said places with ocean surface temperatures above the tropical mean show a trend toward increasing rainfall.

"It's not only about warming. ... It's the degree of warming," he said.

But he said the weakening of winds can play a factor in delivering the rain clouds to the Hawaiian Islands.

Xie said the next step for him and Tokinaga is to conduct a detailed regional study of tradewinds near the islands.






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