A new study from the University of Â鶹´«Ã½ at ²Ñ¨¡²Ô´Ç²¹ revealed that a lesser-known Pacific climate pattern can bring heavier spring rains—and a higher risk of flooding—to parts of Â鶹´«Ã½, while also playing a key role in worsening droughts during dry phases. An example of this happened in the spring (March–May) of 2018, in which this climate pattern contributed to an extensively rainy season.
Most people know about El Niño-Southern Oscillation (ENSO), which is known to have a significant impact on climate across the Pacific, including Â鶹´«Ã½, and adjacent continents. However, new research led by UH ²Ñ¨¡²Ô´Ç²¹ 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 Â鶹´«Ã½. Their study was published in the .
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.
Population growth, increased demand for water
As the state of Â鶹´«Ã½ 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. We aim for our research to empower our communities with climate and weather information,” said Pao-Shin Chu, study co-author, professor of in the UH ²Ñ¨¡²Ô´Ç²¹ (SOEST), and Â鶹´«Ã½ State Climatologist.
During the positive state of the PMM, weaker trade winds in the northeast Pacific Ocean between Â鶹´«Ã½ and Baja California occur along with increased sea surface temperatures. During the “negative state,” stronger trade winds and cooler surface temperatures prevail.
Chu and lead author Bo-Yi Lu, who was an atmospheric sciences doctoral student in SOEST at the time of this research, 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.
“Our study suggests that although El Niño emerges as the primary driver of winter rainfall variability in Â鶹´«Ã½, the Pacific Meridional Mode has a pivotal role in spring rainfall, particularly for Maui and the Island of Â鶹´«Ã½,” said Chu.
“Importantly, our analysis disentangles the respective roles of ENSO and the PMM in driving rainfall variability across seasons and types of weather disturbance in Â鶹´«Ã½,” said Lu. “These findings not only deepen our understanding of regional climate dynamics but also offer valuable insights for water resource management and disaster preparedness in Â鶹´«Ã½ and beyond.”
—By Marcie Grabowski