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Pacific Climate Pattern Found to Influence Hawai‘i's Spring Rainfall

Climate and Weather

Business Honor
14 May, 2025

New study reveals Pacific Meridional Mode’s key role in seasonal rainfall shifts.

Scientists from the University of Hawai‘i at Manoa have discovered that a lesser-known climate pattern, called the Pacific Meridional Mode (PMM), plays a key role in Hawai‘i’s spring rainfall. While El Nino is already known to influence winter weather in the region, this new research shows the PMM also affects rainfall, especially on the islands of Maui and Hawai‘i.

The study, recently published in the Journal of Climate, was led by atmospheric scientists Pao-Shin Chu and Bo-Yi Lu. Chu, a professor at UH Manoa and Hawai‘i State Climatologist, explained that the PMM’s effect on rainfall is strong in spring, bringing either more or less rain depending on the PMM’s state.

In a "positive" PMM state, northeast Pacific trade winds weaken and sea surface temperature increases. This causes cold fronts to move through the islands, bringing more rain. Heavy rain also becomes possible for otherwise arid leeward zones, increasing flood potential.

By contrast, in a "negative" PMM state, more powerful trade winds and cooler sea temperatures decrease rainfall, particularly on the windward sides of the islands. This can increase the chances of droughts.

Lead author Lu noted that knowing both El Nino and the PMM enables scientists to more accurately forecast seasonal shifts in the weather. To investigate these trends and their effects, researchers employed computer models and actual meteorological data. Understanding how climatic trends impact rainfall is becoming increasingly crucial as Hawai‘i’s population expands and the demand for clean water rises.