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The El Ni帽o-Southern Oscillation (ENSO) is known to have a significant impact on climate across the Pacific, including Hawai'i, and adjacent continents. However, new research led by University of Hawai'i at M膩noa atmospheric scientists revealed that the Pacific Meridional Mode (PMM), another climate pattern that operates in the eastern Pacific Ocean, plays a major role in the variability of rainfall in Hawai'i.

Their study was published in the .

"Our study suggests that although El Ni帽o emerges as the primary driver of winter rainfall variability in Hawai'i, the Pacific Meridional Mode has a pivotal role in spring rainfall, particularly for Maui and the Island of Hawai'i," said Pao-Shin Chu, study co-author, professor of atmospheric sciences in the UH M膩noa School of Ocean and Earth Science and Technology, and Hawai'i State Climatologist.

"Importantly, our analysis disentangles the respective roles of ENSO and the PMM in driving rainfall variability across seasons and types of weather disturbance in Hawai'i," said lead author Bo-Yi Lu, who was an atmospheric sciences doctoral student in SOEST at the time of this research.

"These findings not only deepen our understanding of regional climate dynamics but also offer valuable insights for water resource management and disaster preparedness in Hawai'i and beyond."

During what is termed the "positive state" of the PMM, weaker trade winds in the northeast Pacific Ocean between Hawai'i and Baja California occur along with increased sea surface temperatures. During the "negative state," stronger trade winds and cooler surface temperatures prevail.

Chu and Lu performed diagnostic analyses using a combination of actual weather and sea surface observations, and weather model-generated data to determine how these patterns affect rainfall variation.

They determined that in spring, a positive PMM state precipitates extensive rainfall across the state. Specifically, greater rainfall throughout the islands occurs as cold fronts move through. Additionally, whether the positive state occurs in winter or spring, the result is that the leeward sides of the Hawaiian Islands experience an increase in extreme rainfall events, suggesting a heightened risk of floods.

Their analysis also showed that a negative state of the PMM corresponded with reduced daily rainfall over windward sides of the islands, potentially exacerbating drought occurrences.

As the state of Hawai'i experiences population growth, the demand increases for water for drinking, food production, agriculture, recreation, construction, medical uses and more.

"This uncertainty in interannual rainfall, together with the increasing demand for water, requires us to better understand the relationship between rainfall and climate variability," said Chu. "We aim for our research to empower our communities with climate and weather information."