The periodic variations in the temperature of the water and air in the Pacific Ocean known as the El Niño and La Niña phenomenon (El Niño-Southern Oscillation), are a climatic-meteorological force that affects a large part of the planet. The El Niño phenomenon has existed for thousands of years but various scientific teams consider it very likely that human activities such as the emission of greenhouse gases or natural processes such as volcanic eruptions are affecting this system, although they have not been able to determine the processes and its reach.
A new study on this matter whose results are published this week in the journal Nature indicates that the atmospheric component called the Walker circulation (Walker cell) has changed its behavior in the Pacific during the industrial era in ways that were not known until now.
The international team of authors also discovered, on the other hand, that volcanic eruptions can cause the Walker circulation to temporarily weaken, inducing conditions in which the El Niño phenomenon is considered activated.
The results provide important information about how El Niño and La Niña events may change in the future, probably becoming more frequent.
"How and why is this disruption taking place?" asks co-author Samantha Stevenson, an associate professor in the Bren School of Environmental Science and Management at the University of California, Santa Barbara.
Earth's rotation causes warm surface water to pool on the western side of ocean basins. In the Pacific, this induces wetter conditions in Asia, with low-lying trade winds blowing west across the sea. High-altitude easterly winds create an atmospheric circulation (the Walker circulation) that drives weather patterns in the tropical Pacific and far beyond.
"The tropical Pacific has a huge influence on global climate," said Sloan Coats, study co-author and assistant professor of earth sciences at the University of Hawaii Mānoa School of Earth and Ocean Sciences and Technology. "Understanding how it responds to volcanic eruptions, anthropogenic aerosols, and greenhouse gas emissions is critical to confidently predicting climate variability."
These effects leave biological and geological signatures. The team used data from ice cores, trees, lakes, corals, and caves to investigate long-term weather patterns in the Pacific over the past 800 years.
Certain conditions favor the absorption of heavier or lighter versions of an element, called an isotope, into structures such as carbonate skeletons, sediments, and tree rings. The researchers used sophisticated statistics to analyze the ratios of different types of oxygen and hydrogen. This allowed them to track how the Walker circulation changed in the past and compare trends before and after the increase in greenhouse gases.
They observed that the time it takes for the Walker circulation to switch between El Niño and La Niña-like phases has slowed slightly during the industrial age. "That means we could see more of these multi-year La Niña or El Niño events in the future as airflow over the Pacific Ocean changes more slowly between the two phases," Falster said. This could exacerbate the associated risks of drought, fire, rain and flooding.
They also found that volcanic eruptions affected Walker's circulation. "After a volcanic eruption, we see a very consistent weakening of the Walker circulation," said co-author Bronwen Konecky, an assistant professor at Washington University in St. Louis. This causes El Niño-like conditions after eruptions.
"Our study provides long-term context for a fundamental component of the atmosphere-ocean system in the tropics," said Coats, whose expertise covers climate variability over the past 2,000 years. "Understanding how the Walker circulation is affected by climate change will allow communities across the Pacific and beyond to better prepare for the challenges they might face in the coming decades."
Understanding the effect of climate change on the Walker circulation is also important for creating reliable predictions. "If we don't know what happened in the real world, then we won't know if the models we're using to project future changes, [...] impacts and risks are giving us the right picture," Samantha Stevenson explained.
