Almost imperceptibly, rainfall over the Hawaiian Islands has been declining since 1978, and this trend is likely to continue with global warming through the end of this century, according to a team of scientists at the University of Hawaiʻi at Mānoa and the University of Colorado at Boulder.
This latest Hawaiʻi rainfall study, published in the March 13, 2013, early online issue of the Journal of Geophysical Research, supports previous work conducted at the University of Hawaiʻi.
Drying trend uncertain
What has been unclear, however, is whether this drying trend will continue. As of now, not even cutting edge climate models have enough resolution to capture the diverse rainfall pattern over Hawaiʻi, where dry and wet areas often lie only a mile or even less apart.
“For water resource and ecosystem management, and for other societal needs, we need to know whether this drying trend will continue this century,” said UH Mānoa International Pacific Research Center Assistant Researcher and lead investigator Oliver Elison Timm.
Heavy rain predictor
To work around this problem, the team devised a method called statistical downscaling. They first got a take on the effects of the general drying trend on local heavy rain days by reanalyzing observations from 1978 to 2010 at 12 rain-gauge stations spread throughout the islands. Studying hundreds of weather patterns during such days, they identified the typical atmospheric circulation patterns in the North Pacific that favor heavy rains over Hawaiʻi.
Using those weather patterns linked to heavy rains, the team developed a statistical model that estimates the number of heavy rain events during a year. 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.
“We can’t predict individual rain events with our method, but it gives us a very good estimate of the number of heavy rain events in a given season based on the large-scale atmospheric circulation patterns,” said Thomas W. Giambelluca, UH Mānoa geography professor.
“It is extremely difficult to take all the uncertainties into account and our overall result may not apply to all sites in Hawaiʻi,” said 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. We do not know yet how further warming will impact extreme heavy downpours.”
