In the dynamic and complex climate system of the Pacific region, understanding the drivers of rainfall variability has long been a critical scientific pursuit, particularly for island communities such as those in Hawai‘i. Historically, the El Niño-Southern Oscillation (ENSO) has dominated discussions as the principal influence on seasonal climate fluctuations across the Pacific, including its effects on Hawaiian rainfall patterns. However, groundbreaking new research led by atmospheric scientists at the University of Hawai‘i at Mānoa reveals that another climate phenomenon—the Pacific Meridional Mode (PMM)—plays a substantial and previously underappreciated role in shaping precipitation variability in Hawai‘i. This revelation opens new pathways for predicting seasonal rainfall and managing water resources in a region frequently challenged by both drought and flooding.
The Pacific Meridional Mode is a climate pattern operational in the eastern Pacific Ocean, characterized by variations in sea surface temperatures and surface wind patterns. Unlike ENSO, which broadly oscillates between warm (El Niño) and cool (La Niña) phases on interannual timescales, the PMM’s fluctuations manifest prominently through changes in the strength of the northeast Pacific trade winds and corresponding surface water temperatures. Specifically, a positive PMM phase features weakened trade winds and anomalously warm sea surface temperatures extending from the central to the eastern Pacific. Conversely, a negative PMM phase is marked by strengthened trade winds and cooler surface waters. These states influence atmospheric circulation and, crucially, precipitation patterns far beyond their point of origin.
The University of Hawai‘i study, published in the Journal of Climate, employs a meticulously detailed diagnostic approach, combining observational data spanning atmospheric conditions and sea surface temperatures with advanced weather model simulations. This multifaceted analysis allowed the researchers to isolate and quantify the distinct influences of ENSO and the PMM on rainfall variability, dissecting seasonal nuances that had previously been conflated or overlooked. The team’s core insight reveals a seasonal dichotomy in the dominant climatic drivers of Hawaiian rainfall: ENSO exerts primary influence during the winter months, whereas the PMM emerges as a dominant force in the spring, especially impacting the islands of Maui and Hawai‘i.
During the spring season, the positive phase of the PMM triggers extensive rainfall across the Hawaiian archipelago. This precipitation increase correlates strongly with the passage of cold fronts, which deliver moisture to the islands’ ecosystems and communities. The linkage between PMM states and weather disturbances suggests that the positive PMM enhances atmospheric conditions conducive to frontal activity, effectively amplifying moisture transport. Consequently, this relationship elevates the frequency and intensity of rainfall events precisely when the region transitions toward drier months, carrying profound implications for regional hydrology and ecosystem resilience.
Moreover, the research highlights the impact of PMM phases on the spatial distribution of rainfall extremes. When the PMM is in a positive state during either winter or spring, the typically dry leeward sides of Hawaiian islands experience a marked increase in extreme rainfall events. The leeward side, usually sheltered from prevailing winds and relatively arid, becomes vulnerable to torrential downpours, raising the risk of flash floods and associated hazards such as landslides and infrastructure damage. In contrast, negative PMM phases correspond with reduced rainfall intensity on the windward sides, potentially exacerbating drought conditions commonly sustained by these regions, which depend on orographic precipitation generated by trade wind uplift.
This nuanced understanding of PMM’s modulation of rainfall variability enhances the predictive capacity for seasonal flooding and drought risks. Hawai‘i’s population is growing steadily, intensifying demands on freshwater resources for domestic consumption, agriculture, recreation, and industrial applications. Variability in regional precipitation directly affects water supply reliability, agricultural output, and urban planning. Water resource managers and disaster preparedness officials can leverage these scientific insights to refine forecasting models, optimize reservoir management, and develop proactive mitigation strategies that consider seasonal shifts driven by both ENSO and PMM.
Importantly, the disentanglement of ENSO and PMM effects challenges prior assumptions that heavily emphasized ENSO as the singular driver of rainfall variability. The researchers demonstrate that these two climate modes operate simultaneously yet differently across seasons, underscoring the intricacy of tropical Pacific climate dynamics. This dual influence necessitates a more sophisticated approach to climate modeling and hazard assessment that integrates multiple interacting climate drivers rather than attributing all variability to ENSO alone.
Beyond its direct relevance to Hawai‘i, the study’s findings resonate within the broader field of climate science by highlighting the substantial role of regional ocean-atmosphere coupling modes in shaping localized weather and climate phenomena. The PMM, long studied for its interactions with ENSO, is now recognized as a critical modulator of midlatitude and tropical Pacific climates, with implications for understanding climate teleconnections and variability patterns across the Americas and Oceania. This enhanced comprehension could inform global climate models and seasonal forecasting efforts more generally, improving preparedness for climate-related risks.
Technically, the research utilizes an array of data sources including satellite-based sea surface temperature records, surface wind measurements from buoys and meteorological stations, and outputs from numerical weather prediction models that simulate both atmospheric dynamics and oceanic responses. By integrating observational evidence with simulated climate scenarios, the researchers conducted a series of correlation and composite analyses that teased out the subtle but systematic fingerprints of the PMM on precipitation metrics across varying temporal windows and geographic locales within Hawai‘i.
The practical applications of this research extend to sectors far beyond hydrology. Ecosystem management, public health, agriculture, infrastructure development, and emergency response all stand to benefit from an improved understanding of precipitation drivers and their seasonal cycles. For instance, anticipating periods of increased flood risk on leeward coasts enables infrastructure planners to reinforce drainage systems and prepare emergency services for potential disasters. Conversely, recognizing drought-enhancing conditions associated with negative PMM phases on windward slopes informs irrigation planning and water conservation measures to safeguard crops and natural habitats.
Looking forward, the incorporation of PMM dynamics into regional climate prediction frameworks promises improved seasonal forecasting skill, particularly for spring rainfall variability that was previously harder to anticipate. The University of Hawai‘i team underscores the importance of sustained observational efforts and advanced model development to continue unraveling the complex interactions between the Pacific Ocean and atmosphere. As climate change proceeds to alter baseline conditions and variability patterns, understanding these natural modes of variability will be essential for adapting to new climate regimes and maintaining resilience in island communities.
The implications of this research extend to the cultural and societal fabric of Hawai‘i, where water is both a vital resource and a central element of heritage and livelihood. By empowering communities with richer, science-based climate information, the study supports efforts toward sustainable resource management and risk reduction in the face of climate uncertainty. It embodies the critical role of atmospheric science in connecting global climate processes with local impacts and human well-being.
In conclusion, the discovery of the Pacific Meridional Mode’s pivotal influence on Hawaiian rainfall variability marks a significant advance in climate science. It shifts long-held paradigms, illuminates seasonal rainfall drivers, and equips stakeholders with actionable knowledge. As agencies and communities strive to navigate the challenges of water security and extreme weather in the 21st century, this research lays a robust foundation for informed decision-making grounded in the interplay of oceanic and atmospheric forces that shape Hawai‘i’s unique environment.
Subject of Research: Not applicable
Article Title: Impact of the Pacific Meridional Mode on Hawaiian Rainfall Variability
News Publication Date: 16-Apr-2025
Web References:
https://journals.ametsoc.org/view/journals/clim/38/9/JCLI-D-24-0038.1.xml
References:
Lu, B.-Y., Chu, P.-S., et al. (2025). Impact of the Pacific Meridional Mode on Hawaiian Rainfall Variability. Journal of Climate. DOI: 10.1175/JCLI-D-24-0038.1
Image Credits: Heath Cajandig, licensed under CC BY 2.0.
Keywords: Pacific Meridional Mode, ENSO, Hawaiian rainfall variability, climate dynamics, sea surface temperature, trade winds, seasonal precipitation, flood risk, drought risk, atmospheric circulation, climate modeling, water resource management
