HONOLULU — The frequency of tradewinds has decreased on average for the past 37 years, according to a study released Sunday by the University of Hawai‘i. Lihu‘e currently experiences trade winds about 52 percent of the year, but if the
HONOLULU — The frequency of tradewinds has decreased on average for the past 37 years, according to a study released Sunday by the University of Hawai‘i. Lihu‘e currently experiences trade winds about 52 percent of the year, but if the downward trend continues, it could lead to higher annual drought conditions.
A study published in the Journal of Geophysical Research reports that scientists at UH Manoa recorded 37 years of tradewinds data. It follows previous studies on changes in tradewind intensities to show changes in frequencies.
“We don’t know what causes this change,” said Pao-Shin Chu, a University of Hawai‘i Manoa meteorology professor and state climatologist at the School of Ocean and Earth Science and Technology.
“This study does not try to address the causes but points out the changes in the trade winds, and to show they are losing their frequency dramatically,” said Chu, who is also head of the Hawai‘i State Climate Office.
The data was collected from land-based weather stations at the four major airports in Honolulu, Kahului, Hilo and Lihu’e, and from four ocean buoys at points around the Hawaiian islands. It reports that northeast tradewinds occurred 291 days in 1973 and 210 days in 2010.
The stations measured wind speed, direction and sea level pressure data around the Hawaiian Islands. The report observed a decrease in the frequency of northeast tradewinds and an increase in eastern tradewinds.
“You never can be sure over the land stations, so we use buoy data to support the analysis,” Chu said. “They both show a decrease in northeast trade frequency.”
The source of the northeasterly trade winds is from tropical high pressure systems that exist north of the Hawaiian islands. Chu said these high pressure systems are an atmospheric phenomenon on a large scale that covers the north Pacific Ocean.
The trades approach the islands with cool ocean air and effect wave height and cloud formation. These clouds move upward against mountains barriers to the island major source of precipitation.
“This is why windward side of the mountain has more clouds and precipitation,” Chu said. “This is very important and provides a major source of rainfall to the island.”
Lihu‘e has an increase in east trade winds days amounting to around 77 days per year. Chu said the slight increase comes with the decrease of the prevailing north east tradewinds — but does not offset them.
When trade winds fail to develop, the air can become dormant and unpleasant weather can develop, defined by light winds, high humidity and a variation in rainfall distribution, according to the study press release.
Similar systems include the Bermuda high pressure front, and the Aleutian low pressure system. In recent decades scientist have recorded more data regarding inter-annual variations in wind speed and direction caused by synoptic disturbances and climatic events such as El Niño and La Niña.
“We just want to show that based on historical record there is a downward trend in tradewind frequency,” Chu said.
The data is based on an eight-point compass, with cardinal points of the of north, east, south and west, as well as the ordinal directions of northeast, southeast, southwest and northwest.
The report defines trade wind as the northeast and east direction intervals of the eight-point compass, with a wind observation between 22.5 and 67.5 degrees. The east trade wind is defined from 67.6 to 112.5 degrees.
Chu said a dramatic reduction in trade winds could fundamentally change Hawai‘i’s overall climate. The islands have experienced an increase in drought conditions over the past 30 years, he said.
The press release noted a National Drought Mitigation Center figure that said 50 percent of land in Hawai‘i has experienced some degree of drought during the past year.
The report lead author was Jessica Garza, a UH Manoa Meteorology Graduate Assistant, who is now teaching in the California system. Other scientists who contributed to the report include Chase Norton and Thomas Schroeder.
Funding for the study was provided to the Joint Institute for Marine and Atmospheric Research by the National Oceanic and Atmospheric Administration. Future studies will use data simulations to further understand the dynamics of rainfall and tradewinds and estimate future patterns.
• Tom LaVenture, staff writer, can be reached at 245-3681 (ext. 224) or tlaventure@thegardenisland.com.
