New study shows number of trade wind days decreasing

Published: Oct. 12, 2012 at 1:17 AM HST|Updated: Oct. 12, 2012 at 3:28 AM HST
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Pao-Shin Chu
Pao-Shin Chu
Professor Chu points out empirical evidence of decreasing trade wind activity in Hawai'i.
Professor Chu points out empirical evidence of decreasing trade wind activity in Hawai'i.

HONOLULU (HawaiiNewsNow) - The number of days of northeast trade winds is decreasing, according to new research from the University of Hawaii. And while it means more days with muggy weather and volcanic haze, it also is resulting in longer-term effects for the state.

The study began on a hunch from Pao-Shin Chu, who moved to Hawaii in 1985. "Over the last 27 years, I just feel that the trade winds do not blow as much as they used to," he said.

Chu also is a professor of meteorology at UH and is the head of the Hawaii State Climate Office. He was one of the scientists who looked at and analyzed 37 years of wind speed and direction data recorded at the four major airports in Hawaii: Lihue, Honolulu, Kahului and Hilo. They also looked at data from the four major weather buoys in waters around the state.

The analysis showed a decrease in the number of northeast trade wind days per year overall from 1973 to 2009, with the biggest decrease in Honolulu. "We used to have 291 days of northeast trades at Honolulu Airport, but now it dropped to only 210 days," Chu said.

The trade winds are responsible for much of the rainfall around the state, especially in windward areas. That's where moisture is pushed up against the mountains by the trade winds, causing condensation and producing rain.

Besides showing the decrease in northeast trade wind days, the research also showed that the trade winds are becoming more easterly, which has long-term effects for rain. As northeast trade wind days decreased, drought conditions increased over the islands. "Since the late 1970s, you do see there's a large drop in rainfall over the last 30 years," said Chu, pointing to a graph of statewide rainfall.

Chu said climate change may be one reason for the decrease, but scientists are still trying to determine exactly why it is happening.

The study was recently published in the Journal of Geophysical Research.

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