Summers high on Haleakala
have been getting steadily drier for the past 25 years, according to detailed quarter-century study of the weather on Maui’s dominant mountain.
And that’s just one of the
notable trends in the climate of the mountain.
And it is another indicator
that Hawai`i can expect a drier future: that natural environments and human
water availability will be significantly impacted over time. More on other
research in this area below.
The complex study notes that
Haleakala has multiple micro-environments: “ecosystems can range from desert,
to tropical rainforest, to alpine shrubland over very short distances.”
A key feature of Hawaiian
high mountain rainfall patterns is the tradewind inversion. Tradewinds blow
across the Pacific, and are driven upward as they hit Island slopes. The
rising, cooling air tends to promote rainfall. The paper describes it this way;
“On the windward side of the island, trade winds push moist air up the eastern
slopes of the mountain, cooling air to the dew point, causing water vapor to
condense, forming clouds.”
But this pattern is blocked
by the tradewind inversion, a layer of dry, clear and generally warmer air. On
Hawaiian high peaks, you can often look down through clear air to the tradewind
inversion layer, where the clouds are. The bottom of the inversion layer is the
top of the cloud layer.
One result is that there is
often far less rainfall on the highest slopes, which are protected from
tradewind showers by the inversion layer. They tend to be virtual deserts
compared to the rainforests on the middle slopes.
The researchers found that
weather has been changing, and specifically, “a significant drying trend is apparent
at all of the stations located above 1000 m,” or about 3,300 feet. And that is
likely the result of a much more frequent tradewind inversion presence, they
say.
To get this data, a team of
researchers established 11 climate monitoring stations high on the slopes of
the volcanic mountain starting in June 1988. The complex stations collected “solar
radiation, net radiation, relative humidity, wind speed, temperature, precipitation
and soil moisture, and derived variables including potential evapotranspiration,
vapor pressure deficit, soil heat flux and daytime cloud attenuation of
sunlight.”
The new report, “Climatology of Haleakala” was prepared by Ryan J. Longman, Thomas W. Giambelluca and Michael
A. Nullet, all of the University of Hawai`i Geography Department, and Lloyd
Loope, retired researcher with the USGS Pacific Island Ecosystems Research
Center. It is Technical Report 193 of
the UH Pacific Cooperative Studies Unit.
“The goal of this report is
to bring awareness of the climate diversity that exists along the slopes of
Haleakalā Volcano and to the changes that are occurring there,” the authors
write.
One of their messages: “Changes
in moisture may affect vegetation characteristics, promote the spread of invasive
species, and decrease water recharge to the aquifers. These changes are amplified by the time scales
by which they are occurring. The faster the climate changes, the less time
native species will have to react to these changes and the area in which these
species have a competitive advantage may shrink.”
There’s a lot more in this
study, and if you’re interested, click on the link above. The full paper is available
free online.
The suggestion that rainfall
is dropping over time isn’t new. This report is just another brick in that
wall.
Earlier this year, Pao-Shin
Chu of the University of Hawai`i’s Department of Meteorology published a studyin the International Journal of Climatology, which linked winter drought to El
Nino events.
Tim Hurley’s story on the
study in the Honolulu Star-Advertiser quoted Chu as saying, “"The planet
is changing. You should not assume the weather will remain steady as before.
You need to revolutionize your thinking."
A 2014 report, “Climate ChangeImpacts in Hawai`i,” also sees reduced rainfall.
It is a dense report, readily
available online, that includes recitations of many of the impacts of climate
change. With regard to water, there is
some spooky stuff.
“Streamflow records also show
a decline in base flow over the last century by 20-70%, depending on the
watershed, suggesting a decrease in groundwater level,” it says.
“Hawai‘i has experienced
longer droughts in recent years, as all the populated islands show an
increasing trend in length of dry periods during 1980-2011, as compared
with1950 -1970.”
And the tradewinds, which
drive a lot of our rainfall? “Prevailing northeasterly trade winds, which drive
orographic precipitation on windward coasts, have decreased in frequency since
1973 in Hawai‘i.”
Still, there’s a lot of
uncertainty. Some models suggest O`ahu and Kaua`i Counties should get drier and
that Maui and the Big Island should get a little more rain due to climate
change. (Although the Haleakala study cited above doesn't show that.) Other models suggest drier winters but slightly wetter summers
statewide.
But so far, dry tends to be
the trend.
© Jan TenBruggencate 2015
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