Friday, July 31, 2015

Dry, dry, dry: High Haleakala increasingly arid

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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