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