Hawaiian Islands Should Expect Less And Less Rainfall

The University of Hawaiʻi has been paying close attention to Hawaiian rainfall patterns of late, and a new study has supported previous research, confirming that rainfall over the Hawaiian Islands has been declining since 1978, a trend that they believe will continue through the end of this century thanks in part to the changes resulting from global warming.

“For water resource and ecosystem management, and for other societal needs, we need to know whether this drying trend will continue this century,” said lead investigator Oliver Elison Timm at the International Pacific Research Center at UH Mānoa.

Image Credit: Tom Burke via Flickr (CC BY-NC 2.0)
Image Credit: Tom Burke via Flickr (CC BY-NC 2.0)

There is a fundamental difficulty when attempting to peer into the future of Hawaiʻi’s rainfall patterns. The islands are so diverse, that often dry and wet areas lie only a mile or less apart, meaning that there are no climate models with the necessary resolution to provide accurate measurements of Hawaiian weather conditions in the future.

Not to be dissuaded from finding the answers they needed, the University of Hawaiʻi team devised a method called ‘statistical downscaling’ which makes use of observations between large scale variables taken from global climate models (GCM) to create a statistical relationship which provides local variables.

In this instance, University of Hawaiʻi researchers first looked at the effects of the general drying trend on local heavy-rain days by reanalysing observations taken between 1978 and 2010 from 12 rain-gauge stations spread across the Hawaiian Islands. They were subsequently able to identify the typical atmospheric circulation patterns in the North Pacific that resulted in heavy rains over the Islands.

“The patterns we saw did not surprise us,” recalled 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.”

From there, the researchers used the circulation patterns that were linked to heavy rains to develop a statistical model that estimates the number of heavy rain events during a given year. Their model showed that large circulation patterns over the mid-latitude and tropical North Pacific have already shifted since 1978, as a result, minimising the number of weather disturbances that are capable of reaching the Islands during the rainy season.

“We can’t predict individual rain events with our method,” clarified Thomas W. Giambelluca, Professor in the Department of Geography at UH Mānoa, “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.”

The scientists were subsequently able to combine the results of their statistical model outlined above together with a “cutting-edge” global climate model which included the projected increases in greenhouse gases through to the end of this century to conclude that a trend towards drier winter seasons with fewer heavy-rain days is likely over the remaining decades of the 21st 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. We do not know yet how further warming will impact extreme heavy downpours.”

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