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As with many natural phenomena, scientists look to past climate to understand what may lie ahead as Earth warms. By assessing so-called 'flavors' of El Ni帽o events in past climate records and model simulations, researchers have a clearer picture of El Ni帽o patterns over the past 12,000 years and are able to more accurately project future changes and impacts of this powerful force. The study, by scientists at the University of Hawai'i at M膩noa and University of Colorado Boulder, was published recently in Nature Communications.

"We used a unique set of climate model simulations that span the Holocene, the past 12,000 years, and accounted for changes in the frequency of El Ni帽o flavors, the three preferred locations in which the peak of warming during different El Ni帽o events occur鈥攅astern Pacific, central Pacific, and coastal," said Christina Karamperidou, lead author of the study and associate professor of atmospheric sciences at the UH M膩noa School of Ocean and Earth Science and Technology (SOEST). "Doing this allowed us to reconcile conflicting records of past El Ni帽o behavior."

El Ni帽o is the primary factor affecting variability in water temperature and trade wind strength in the Pacific. Typically, researchers look for indicators of El Ni帽o events in ancient, preserved material such as coral skeletons, Peruvian mollusk shells or lake sediment from the tropical Andes because locked within are indicators of past temperature and rainfall across Paci铿乧.

"However, depending on where the samples are taken from鈥攅astern Pacific, central Pacific, or near the South American coast鈥攖he frequency of El Ni帽o events appears to exhibit different patterns," said Karamperidou. "Records from the eastern Pacific show an intensification of El Ni帽o activity from early to late Holocene, while records from the central Pacific show highly variable El Ni帽o throughout the Holocene."

The new set of climate model simulations developed by Karamperidou and co-author Pedro DiNezio, associate professor at the University of Colorado Boulder, are the first to allow the study of changes in the frequency of El Ni帽o flavors during the past 12,000 years. This enabled the researchers to test a that Karamperidou and colleagues posed in 2015鈥攖hat paleoclimate records across the Pacific could be explained by changes in El Ni帽o flavors.

"Indeed, we showed that Eastern Pacific events have increased in frequency from early to late Holocene, while Central Pacific and Coastal events have decreased in frequency, resulting in changes in the hydroclimate in the tropical Pacific," said Karamperidou. "Importantly, we showed that it is not only their frequency, but also the strength of their impact that changes, which is important for interpreting records of past climate."

Surprising impact of coastal El Ni帽o

Additionally, this is the first study into the response of coastal El Ni帽o events to climate changes. During these events the sea surface warming is confined off the coast of South America while the conditions in the rest of the Pacific basin are normal or colder than normal.

"These coastal events have supersized impacts with severe flooding and disasters in countries like Peru and Ecuador," said Karamperidou. "In fact, we showed in another recent that even though these events are not felt around the globe like the more widely known Eastern and Central Pacific events, a better understanding of the mechanisms that drive them is essential for understanding the drivers of the other two flavors, as well."

Connections to Hawai'i's rainfall, hazards

El Ni帽o events have significant impacts on Hawai'i's rainfall, trade wind strength, the probability of hurricane formation and drought, and the type of El Ni帽o event matters for these impacts.

"This information is important for water resource managers among others to better prepare for Hawai'i regional climate," said Karamperidou. "So, it is imperative that we gain a better understanding of the mechanisms of these flavors, and also improve their representation in climate models and assess their projected changes under future climate conditions."

This work offers new knowledge on how El Ni帽o may respond to climate change and thus can help reduce these uncertainties in global climate models and therefore, predictions of El Ni帽o impacts.