Molokai's bog forest.

Molokai’s bog forest.

Historically, when El Niño events occur, Hawai‘i has experienced nearly six months of drought, from November to April., Conversely, during La Niña events rainfall has been greater than normal — building up Hawai‘i’s groundwater supply.

However, this has changed, according to a recent study by University of Hawai‘i scientists.

New research published last month in the Journal of Climate by scientists at UH Mānoa, Nanjing University of Information Science and Technology, and NOAA’s Honolulu National Weather Service Office determined the relationship between La Niña and rainfall in Hawai‘i has changed, and recent La Niña years have brought less-than-normal rainfall.

Molokai's bog forest.

Molokai’s bog forest.

Because the La Niña events have brought excess rainfall to the state in the past, the new information indicating decreased rainfall during recent La Niña events has important implications for agriculture, water resource management and more.

“Initially, this changing relationship between La Niña and Hawai‘i rainfall was brought to my attention by Kevin Kodama (co-author of the paper and hydrologist at NOAA’s Honolulu NWS),” said co-author Pao-Shin Chu, professor of Atmospheric Sciences at the UHM’s School of Ocean and Earth Science and Technology and Hawai‘i State Climatologist. “However, we had no idea when this shift occurred, nor any comprehensive analysis to understand the mechanisms for this change.”

Chu and co-authors analyzed data from 50 rain gauges managed by the National Weather Service throughout the state, which provided rainfall measurements from 1956 to 2010. A statistical analysis, called a changepoint analysis, determined that the shift — from excess rainfall to less-than-normal rainfall — during La Niña years occurred in 1983.

The researchers then compared factors affecting weather and climate between the two periods on either side of that shift — 1956 to 1982 and 1983 to 2010. This comparison uncovered changes in the large-scale atmospheric circulation patterns before and after the shift.