Hawai’i, a tropical paradise known for its lush landscapes and rich biodiversity, is also prone to varying levels of rainfall throughout the year. While El Niño-Southern Oscillation (ENSO) has long been recognized as a significant driver of climate variability across the Pacific, new research reveals that another crucial climate pattern, the Pacific Meridional Mode (PMM), plays a vital role in shaping Hawai’i’s rainfall patterns.

Led by University of Hawai’i at Manoa atmospheric scientists, this groundbreaking study published in the Journal of Climate sheds light on the impact of PMM on spring rainfall, particularly for Maui and the Big Island. According to Dr. Pao-Shin Chu, co-author and Hawaii State Climatologist, “Our study suggests that although El Niño emerges as the primary driver of winter rainfall variability in Hawai’i, the Pacific Meridional Mode has a pivotal role in spring rainfall.”

The PMM operates in two distinct states: positive and negative. During the positive state, weaker trade winds prevail, accompanied by increased sea surface temperatures. Conversely, stronger trade winds and cooler surface temperatures are observed during the negative state.

Researchers Bo-Yi Lu and Dr. Chu employed diagnostic analyses using actual weather data, sea surface observations, and weather model-generated information to understand how these PMM patterns influence rainfall variations across Hawai’i.

Their findings indicate that a positive PMM state in spring leads to increased rainfall throughout the islands as cold fronts move through. This phenomenon is particularly pronounced on the windward sides of the islands, where the increased rainfall can exacerbate the risk of flooding. The leeward sides, however, tend to experience an increase in extreme rainfall events.

Interestingly, whether the positive PMM state occurs in winter or spring, the result is an elevated risk of floods on the leeward sides of the Hawaiian Islands. Conversely, a negative PMM state corresponds with reduced daily rainfall over windward sides, potentially worsening drought occurrences.

As Hawai’i’s population grows, so does the demand for water resources. This increased pressure underscores the need to comprehend the intricate relationship between rainfall and climate variability. As Dr. Chu emphasizes, “This uncertainty in interannual rainfall, together with the increasing demand for water, requires us to better understand the relationship between rainfall and climate variability.”

By shedding light on the pivotal role of PMM in Hawai’i’s rainfall patterns, this research aims to empower communities with climate and weather information, ultimately contributing to more informed decision-making for disaster preparedness and resource management.

The world’s oceans are experiencing an unusual and rapid warming trend, but not uniformly so. According to a recent study led by climate scientist Dr Kevin Trenberth, two distinct bands of ocean heat upsurge around the globe, one in the southern hemisphere and another in the north. These bands are surprisingly close together, at approximately 40 degrees latitude.

The first band, stretching from 40 to 45 degrees south, is heating at an alarming rate, with particularly pronounced effects observed near New Zealand, Tasmania, and the Atlantic waters east of Argentina. In contrast, the second band is situated around 40 degrees north, with significant warming evident in waters east of the United States in the North Atlantic and east of Japan in the North Pacific.

“This pattern stands out starkly,” Dr Trenberth remarks, emphasizing that such a distinctive heating trend is unusual when analyzing climate data. The implications of these findings are substantial, as oceanic heat contributes to an array of issues, including disrupted marine ecosystems, increased atmospheric water vapor (a potent greenhouse gas), and the intensification of severe weather patterns.

Researchers employed an unprecedented volume of atmospheric and oceanic data to assess 1-degree latitude strips of ocean down to a depth of 2000 meters from 2000 to 2023. Their analysis revealed not only the two primary heat bands but also notable warming in regions from 10 degrees north to 20 degrees south, encompassing much of the tropics.

The absence of significant heating near 20 degrees latitude, however, is a striking anomaly, especially considering it spans both hemispheres. As Dr Trenberth notes, “What’s unusual here is that we’re not seeing warming in this area.”

This research, co-authored by Lijing Cheng and Yuying Pan from the Chinese Academy of Sciences, John Fasullo from NCAR, and Michael Mayer from the University of Vienna and the European Centre for Medium-Range Weather Forecasts, highlights a critical need to reassess our understanding of climate patterns in light of this new information.

The article discusses a recent study led by University College London researchers that suggests using existing large planes, like the Boeing 777F, to inject particles into the atmosphere to reflect sunlight and cool the planet. This approach, known as stratospheric aerosol injection, is a geoengineering technique that has been previously researched but assumed to require specially designed aircraft flying at high altitudes.

The study found that injecting sulphur dioxide particles at an altitude of 13 km above the polar regions could meaningfully cool the planet, albeit less effectively than at higher altitudes closer to the equator. This approach would require using three times the amount of aerosol and would have increased side effects like acid rain.

However, climate change is a serious problem, and it’s essential to understand all options for policy-makers to make informed decisions. The researchers used simulations in the UK’s Earth System Model 1 (UKESM1) to estimate the impact of stratospheric aerosol injection at different altitudes, latitudes, and seasons.

Injecting 12 million tonnes of sulphur dioxide a year at 13 km would cool the planet by around 0.6°C, which is roughly the same amount added to the atmosphere by the eruption of Mount Pinatubo in 1991. This strategy is not as effective as injecting particles at higher altitudes but could begin sooner.

The study’s lead author noted that any stratospheric aerosol injection would need to be introduced gradually and reduced gradually to avoid catastrophic impacts from sudden warming or cooling, and would not eliminate the need for emissions reductions. The researchers emphasized that long-term climate stability can only be achieved with net-zero greenhouse gas emission reductions.