Monday, July 1, 2019

Hawaiian tradewinds shifting NE to East, and that means warmer weather for us

It feels warmer in the Islands, and it is—in part because there are significant changes in our tradewind flow.

It is not that the trades have stopped blowing, but that theyʻre blowing from warmer water, which makes the breeze warmer.

State Climatologist  Pao-Shin Chu said wind data over the past 40 years show a definite shift in the flow of tradewinds. Theyʻre blowing more from the warmer waters east of us, and less from the cooler waters northeast of us.

Chu, a meteorologist at the University of Hawai`i, compared two sets of decades-long data for winds at Honolulu Airport. And while the two sets are not precisedly comparable, they both tell the same story—a shift from northeast trades to easterly trades.

What does that mean to the person on the street, or sitting in front of a fan at home, or selecting restaurants for the efficiency of their air conditioning?

"The wind from the northeast is cooler than with the easterly component," Chu said.

Chu first noted the change in a paper published in 2012 in the Journal of Geophysical Research. He has since reviewed updated numbers and said the trend continues.

That paper, by Jessica A. Garza, Chu, Chase W. Norton and Thomas A. Schroeder, is entitled "Changes of the prevailing trade winds over the islands of Hawaii and the North Pacific."  

The researchers looked at wind data from eight stations on land and from ocean buoys around the Islands. The data runs from 1973 to 2009. Here is a press release on that paper.

"The northeast trade frequency is found to decrease for all eight stations while the east trade winds are found to increase in frequency," the authors wrote.

Hawai`i gets its reputation for having a comfortable climate in part from the remarkable consistency of the trade wind flow. It is the most consistent wind field on the planet, the authors said.

When Chu recently reviewed a newer set of wind numbers, from 1980 to 2014, he found a compable result: The frequency of northeast trades drops while the frequency of easterly trades rises.

He said that at the beginning of the data set, there were 170 days of northeast trades, and they dropped to 150 by the end of the period.

Meanwhile, east trades increased from 95 to 120 days.
And there is other news in trade winds. Chinese researchers report that during the past century, trade wind speeds have increased in the western Pacific, but decreased in the eastern Pacific. (Hawai`i is kind of in the middle.)
That study "Long-term trend of the tropical Pacific trade winds under global warming and its causes," is by a team lead by Yang Li, an atmospheric scientist at Chinaʻs Chengdu University. 
University of Hawai`iʻs Chu said he has seen a slight weakening in Hawaiian tradewinds, but not enough to be statistically significant.

©Jan TenBruggencate 2019

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