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Less Rainfall for Hawai‘i

UH Mānoa climate research:  less rainfall expected for Hawai‘i

The latest Hawai‘i rainfall study, published March 13 in an early online issue of Journal of Geophysical Research, supports the findings of earlier research at the University of Hawai‘i at Mānoa, but it goes beyond analyzing historical trends and looks at a critical issue:  whether the drying trend that Hawai‘i has been experiencing since 1978 will continue.  The research team, led by Oliver Elison Timm at the International Pacific Research Center (IPRC), finds that the drying trend is indeed likely to continue through the end of the century.

Modeling Hawai‘i’s climate presents many challenges because of Hawai‘i’s complex topography and the coarse resolution of global climate models.  To meet the challenge, the team used statistical downscaling—a method for relating local observations (usually at the station-level) to features of large-scale global models for the same time period. These statistical relationships are then applied to global climate models for the future to see how local climate may be affected.

“The patterns we saw did not surprise us,” recalled Dr. Elison Timm, referring to the historical data. “For example, we found that the typical winter Kona storms with moist air-flow from the South often produce torrential rains in the islands.”

Photo courtesy of the International Pacific Research Center
Photo courtesy of the International Pacific Research Center

They found that the large circulation patterns over the mid-latitude and tropical North Pacific have already shifted since 1978 so that fewer weather disturbances reach the Islands during the rainy season from November through April.  Combining information from their statistical model and cutting-edge climate models driven with the projected increase in greenhouse gases, the scientists conclude that we can expect the recent trend towards drier winter seasons with fewer heavy-rain days to continue through the end of this century.

“It is extremely difficult to take all the uncertainties into account and our overall result may not apply to all sites in Hawai‘i,” cautioned Senior Researcher Henry Diaz from the University of Colorado. “We are just beginning to understand the details of how climate change will affect the Hawaiian Islands.”

The project was supported by grants from the U.S. Fish and Wildlife Service through the Pacific Island Climate Change Cooperative (PICCC) and the Pacific Islands Climate Science Center. Additional funding was provided jointly by the U.S. Army Corps of Engineers, Honolulu District, and the Commission on Water Resource Management, Hawai‘i Department of Land and Natural Resources.

Citation:  Oliver Elison Timm, Mami Takahashi, Thomas W. Giambelluca, and Henry F. Diaz, 2013: On the Relation between Large-Scale Circulation Pattern and Heavy Rain Events over the Hawaiian Islands: Recent Trends and Future Changes. Journal of Geophysical Research, (early online-release in March 2013, http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50314/abstract)

Cover Photo:  A waterfall in Ka‘au Crater, O‘ahu, Hawai‘i. (Source: Victoria Keener)

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