Friday, October 12, 2012

The dying Hawaiian trades now a long-term trend

The northeast trade winds are among the key features of Hawai`i’s excellent weather, but new research shows that the trade winds are declining.

(Image: Trade winds directly impact cloud and wind patterns in the Hawaiian Islands, as here, on O`ahu. Credit: Chris Ostrander, UHM/SOEST.)

The change is dramatic. Winds measured at Honolulu Airport over the past 38 years show that the average number of northeast trade wind days annually has dropped 28 percent, from 291 to 210 days a year.

That has massive impacts, not only for human comfort levels, but for the environment. Cloud formation and rainfall patterns are changing with the altered wind patterns.

Four professors at the University of Hawai`i School of Meteorology published the new study. They are Jessica A. Garza, Pao-Shin Chu, Chase W. Norton and Thomas A. Schroeder. The study is “Changes to the prevailing trade winds over the islands of Hawaii and the North Pacific,” in the Journal of Geophysical Research. The abstract is here.

They based their work on their review of 37 years of wind speed and direction data from both land and sea-based stations on and around the Hawaiian Islands. As one might expect, since the northeast trades affect so much of our climate, that reduce trade weather has had an impact.

“We have seen more frequent drought in the Hawaiian Islands over the last 30 years. Precipitation associated with the moisture-laden northeasterly trades along the windward slopes of the islands contributes much of the overall rainfall in Hawaii,” Chu said.

The new study builds on previous work that had similar findings. In 2007, the journal on Climate published a paper, “Global Warming and the Weakening of the Tropical Circulation,” by Gabriel Vecchi of the NOAA Geophysical Fluid Dynamics Laboratory and Brian Soden of the Rosenstiel School for Marine and Atmospheric Science at the University of Miami. That paper is here.

While the Garza-Chu study covers actual field-collected data, the Vecchi study deals with models of a warming atmosphere. They came up with similar results. Vecchi summarized: “All models simulated a weakening of the convective overturning of mass in the atmosphere as the climate warmed.”

A 2010 briefing paper by University of Hawai`i coastal geologist Chip Fletcher, of the School of Ocean and Earth Science and Technology, makes the case linking climate change and the observed changes around Hawai`i:

“Hawai‘i’s climate is changing in ways that are consistent with the influence of global warming. In Hawai‘i:  air temperature has risen; rainfall and stream flow have decreased; rain intensity has increased; sea level and sea surface temperatures have increased; and, the ocean is acidifying,” Fletcher wrote.

In urging Hawai`i to move aggressively to study and plan for the changes, Fletcher minced no words: “Scientists anticipate growing impacts to Hawai‘i’s water resources and forests, coastal communities, and marine ecology.”

© Jan TenBruggencate 2012