Scientists at UH Manoa have observed a decrease in the frequency of northeast tradewinds and an increase in eastern tradewinds over the past nearly four decades, according to a recent study published in the Journal of Geophysical Research.
For example, northeast tradewind days, which occurred 291 days per year 37 years ago at the Honolulu International Airport, now only occur 210 days per year.
Analyzing 37 years of wind speed and direction, and sea level pressure data from land-based weather stations, buoys and reanalysis data, were Jessica Garza, a Meteorology Graduate Assistant at the School of Ocean and Earth Science and Technology (SOEST) at UH Manoa; Pao-Shin Chu, Meteorology Professor and Head of the Hawaii State Climate Office; and Chase Norton and Thomas Schroeder.
Persistent northeast tradewinds are important to the Hawaiian Islands since they affect wave height, cloud formation, and precipitation over specific areas of the region. When trades fail to develop, the air can become dormant, and unpleasant weather can develop.
Furthermore, Chu explained that the trades are the primary source of moisture for rain, and that a dramatic reduction could fundamentally change Hawaii’s overall climate. “We have seen more frequent drought in the Hawaiian Islands over the last 30 years,” he noted. “Precipitation associated with the moisture-laden northeasterly trades along the windward slopes of the islands contributes much of the overall rainfall in Hawaii.”
According to the National Drought Mitigation Center’s State Drought Monitor, nearly 50% of land in Hawaii has experienced some degree of drought during the past year. While previous research has focused primarily on changes in tradewind intensities, this work, along with Chu’s 2010 study, is among the first to show changes in tradewind frequencies.
“In 2010, we only studied the tradewind changes at four major airports in Hawaii (Honolulu, Kahului, Hilo and Lihue). In the current paper, we expanded our study to include four ocean buoys in the vicinity of Hawaii and a large portion of the North Pacific,” Chu commented.
In the future, these scientists will be using model simulated data to further understand the dynamics of rainfall and tradewinds, and estimate future patterns.
Funding for this work was provided to the Joint Institute for Marine and Atmospheric Research (JIMAR) by the National Oceanic and Atmospheric Administration (NOAA).
Garza, Chu, Norton, and Schroeder (2012), Changes to the prevailing trade winds over the islands of Hawaii and the North Pacific, Journal of Geophysical Research, 117, D11109, doi:10.1029/2011JD016888.