Heavy rainstorms have become more numerous on Hawaii island over the past 50 years while becoming less frequent in Leeward Oahu and Central Maui, according to a recent study by a pair of University of Hawaii researchers who tie the phenomenon to climate change.

By 1960, for example, a storm that dumped 12 inches of rain on the Big Island occurred once every 20 years. By 2009, however, a storm that strong was striking the island every three to five years, the study found, and the "new" storm was pouring down more than 16.5 inches of rain.

In a paper published in the International Journal of Climatology, Ying Chen, a UH-Manoa graduate student at the time of the study, and Pao-Shin Chu, professor of atmospheric sciences at UH-Manoa, analyzed extreme rain events and the frequency with which they occur on three islands — Oahu, Maui and Hawaii island.

"In the past, the frequency of heavy rainfall events was assumed to be fairly constant," Chu, who is also head of the Hawaii State Climate Office, said in a press release posted Wednesday on UH’s website.

But the study points out that the climate is changing and there’s a need to rethink flood control standards and other rain-related guidelines ranging anywhere from filling potholes to designing dams and reservoirs.

He said there are myriad repercussions for ecological systems, property, buildings, hazard planning and more.

"The planet is changing," Chu said Saturday. "You should not assume the weather will remain steady as before. You need to revolutionize your thinking."

The study offers clues about why and how the frequency of rainfall extremes has changed. Major rain events were more frequent during La Nina years, the researchers found, while in El Nino years the opposite occurred.

La Nina occurs when cooler than normal sea surface temperatures are found in the central and eastern Pacific Ocean near the equator off the west coast of South America, while with El Nino warmer than normal sea surface temperatures occur in the same area.

While heavy rainfall events have become more frequent over the last 50 years on the easternmost island in Hawaii, the opposite was seen for Oahu and Maui to the west.

Chu said the study results indicate there’s an east-to-west regional difference in how precipitation patterns are responding to a changing climate. But he cautioned that the number of rain gauges used was limited to 24 weather stations on the three islands.

The stations on Oahu were situated only in the leeward sections of the island, and the Maui weather stations were at the center of the island in Central and Upcountry Maui.

While an approximate frequency of storms was not calculated for Oahu and Maui, a clear trend was seen, he said — for example, the 20-year storm recorded at one Leeward Oahu rain station dropped in intensity from about 9 inches in 1960 to about 5 inches in 2009.

For future work, Chu said he hopes to analyze data from additional stations to paint a more detailed picture of changing rain patterns across the islands.