Hawaiian Islands May See Less Rainfall
April 24, 2013

Rain In Hawaii On The Decline

April Flowers for redOrbit.com - Your Universe Online

It has happened almost imperceptibly, but rainfall over the Hawaiian Islands has been in decline since 1978. According to a team of scientists at the University of Hawaii, Manoa (UHM) and the University of Colorado, Boulder, this trend is likely to continue with global warming through the end of this century. Though the findings of this study, published in the early online issue of the Journal of Geophysical Research, supports prior UHM work, it is still unclear if this drying trend will continue.

"For water resource and ecosystem management, and for other societal needs, we need to know whether this drying trend will continue this century," says Oliver Elison Timm at the International Pacific Research Center, UHM.

Current climate models, even the most cutting edge, don't have enough resolution to capture the diverse rainfall pattern over Hawaii where wet and dry areas often lie only a mile or even less apart.

The team created a method called "statistical downscaling" to work around this problem. By reanalyzing observations from 1978 to 2010 at 12 rain-gauge stations spread throughout the islands, the researchers got a take on the effects of the general drying trend on local heavy-rain days. They were able to identify typical atmospheric circulation patterns in the North Pacific that favor heavy rains over Hawaii by studying hundreds of weather patterns during such days.

"The patterns we saw did not surprise us," recalls Elison Timm. "For example, we found that the typical winter Kona storms with moist air-flow from the South often produce torrential rains in the islands."

The team developed a statistical model to estimate the number of heavy rain events during a year using those weather patterns linked to heavy rains, finding that the large circulation patterns over the mid-latitude and tropical North Pacific have already shifted since 1978. This means that fewer weather disturbances reach the Islands during the November through April rainy season.

"We can't predict individual rain events with our method," clarifies Professor Thomas W. Giambelluca, Department of Geography, UHM, "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."

Data from the new statistical model was combined with cutting-edge climate models driven with the projected increase in greenhouse gases until the end of this century to reveal that we can expect the recent trends 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," cautions 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."