Study finds new blueprint for El Niño diversity

An international study involving Monash climate scientists has identified key mechanisms that cause El Niño events to differ amongst each other.

The study was published this week in the prestigious journal Nature. Scientists have long recognized that not every El Niño is alike. Some are weak, others are strong. Some occur in the central Pacific, others in the east.

“We use to think of El Niño being a fairly well explained by simple conceptual models,” said study co-author Dr Dietmar Dommenget, from the Monash School of Earth, Atmosphere and Environment.

“However, as the number of El Niño events we have observed have increased, its complex behaviour has become more apparent, revealing interesting interactions with the atmosphere and other ocean basins,” he said.

Study co-author Dr Shayne McGregor, also from the Monash School of Earth, Atmosphere and Environment, said the study was important because it synthesized our current understanding of the dynamical processes underlying El Nina and La Nina events and their role in controlling ENSO complexity.

This synthesis led the research team to identify that the complexity and irregular occurrence of El Niño and La Niña events appear to be underpinned by the co-existence of two coupled atmosphere-ocean oscillations, with different spatial characteristics and different frequencies.

“Our study reveals that there is a hidden structure in the seemingly chaotic and unpredictable occurrence of El Niño events,” said Axel Timmermann, Director of the IBS Center for Climate Physics (ICCP) at Pusan National University and lead author of the study.

El Niño events are characterized by an unusual warming of the central to eastern equatorial Pacific, which can last up to one year. Many events subsequently transition to a La Niña (cold) state, with a typical duration of 1-2 years. El Niño events, which tend to peak in boreal winter, typically lead to a drying of Southeastern Asia and the Western tropical Pacific whilst enhancing rainfall near the eastern Pacific shores, in countries such as Ecuador and Peru. El Niño’s remote “ripple effects” can not only be found in the atmosphere, but also in ocean currents, ecosystems, the occurrence of natural disasters, global markets and national economies.

Climate scientists have long recognised that not every El Niño is alike. Some are weak, others are strong. Some occur in the central Pacific, others in the east. These differences will determine which areas will be hit hardest by climatic extremes and which ones will be spared. Predicting El Niño events accurately requires a deeper understanding of its diversity or as some scientists call it — its “flavors”.

To elucidate the origin of El Niño diversity, a group of 40 climate scientists from 11 countries met in October last year at the IBS Center for Climate Physics and Pusan National University, South Korea. Analyzing large amounts of climate observations and computer model simulations, the team unravelled the mechanism behind El Niño’s capricious behavior. When the upper tropical Pacific Ocean stores more heat, El Niño events tend to peak in the Eastern Pacific and during boreal winter, whereas a cooler upper ocean system leads preferably to the development of Central Pacific El Niño events, which exhibit a weaker seasonal coupling.

By running El Niño computer model simulations for different temperature, wind and ocean current configurations the research team found that Eastern Pacific El Niño events are characterized by a return time of 3-7 years, whereas Central Pacific events tend to recur on average every 2-3 years (Figure 1). The different character of these modes is determined by how strongly atmosphere and ocean interact with each other. The theoretical results presented in this study are consistent with observational datasets. In reality however, the co-existing Eastern and Central Pacific warm/cold swings are far from periodic. The tropical Pacific climate systems requires constant excitation, either through random weather events or through atmospheric circulation changes induced by temperature changes in the Indian and Atlantic Oceans. These interactions are an important source for El Niño irregularity, and limit how far ahead Tropical Pacific climate anomalies can be predicted.

“Finally we can attribute the observed complexity of the most powerful natural climate engine to the co-existence of two oscillatory states of the underlying mathematical equations” said Fei-Fei Jin, professor for Atmospheric Sciences at the University of Hawai‘i, USA and co-author of this study.  

Schematic illustration of the two atmosphere/ocean oscillations that contribute to the complexity of the El Niño-Southern Oscillation and its impacts on various sectors of society. Identified by computer models, these oscillations with periods of 2-3 and 3-7 years are associated with central and eastern equatorial Pacific warming, respectively. Citation: Timmermann et al., (2018): El Niño-Southern Oscillation Complexity, Nature, DOI: 10.1038/s41586-018-0252-6.

“More comprehensive climate modeling studies have to be conducted now to test our new ideas in a more realistic setting and to determine whether we can expect shifts in El Nino characteristics in our warming world,” described Jeongseong Kug, one of the coordinating lead authors of the study and Professor at Pohang University of Science and Technology, South Korea.

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