The El Niño Southern Oscillation (ENSO) is a year-to-year fluctuation of the coupled ocean-atmosphere system that originates in the tropical Pacific. Warm El Niño and cold La Niña phases of ENSO alter atmospheric and oceanic circulation, weather patterns, and extreme events, affecting human and natural systems around the world. ENSO variability and its profound social and economic consequences are occurring today in the context of a global climate system that is rapidly changing.
El Niño Southern Oscillation in a Changing Climate, a book recently published by AGU, presents the latest advances in ENSO research, including theories, models, observations, paleo-reconstructions, seasonal prediction, future projections, teleconnections, and some of the most pronounced environmental and ecological impacts. Here, the editors of the book give an overview of our current understanding of how ENSO might change in the future, and what research challenges remain.
Has ENSO already changed due to climate change?
Reliable instrumental records are still relatively short to separate climate change impacts on ENSO from natural variability. There are suggestions, however, based on paleo records and models, that changes in the ENSO cycle may have already occurred. Also, because of the warming background conditions on which ENSO events develop, some ENSO impacts are being amplified, as happened during the 2015-16 El Niño. That event resulted in the most extensive and prolonged global coral bleaching episode to date, and a record increase in tropical Pacific storm activity because of the warm underlying ocean temperatures on which it occurred.
How will ENSO change in the future?
Climate model projections suggest extreme El Niño and La Niña events may increase in frequency from about once every 20 years to once every 10 years by the end of the 21st century under aggressive greenhouse gas emission scenarios.
ENSO extremes, such as unusually intense precipitation in the eastern equatorial Pacific during an extreme El Niño, and large relative cooling over the central equatorial Pacific during an extreme La Niña are expected to be more commonplace.
These changes are linked to changes in the background climate, such as weakening of the atmospheric Walker Circulation, and to changes in the mix of the ocean-atmosphere processes that govern ENSO.
How will precipitation changes shift?
Rainfall extremes will shift eastward along the equator in the Pacific Ocean during El Niño events and westward during extreme La Niña events in a warmer climate. Outside the tropical Pacific, rainfall patterns may not change much, but the rainfall extremes may be more pronounced if extreme El Niños and La Niñas increase in frequency and amplitude.
Will the influence on tropical cyclones change?
It is expected that climate change in general will result in more intense major tropical storms though the overall number of storms may not change or even slightly decrease. We might expect that ENSO will also affect tropical storms in the future because it significantly impacts tropical cyclone genesis in the Atlantic, Pacific and Indian Oceans today. However, precisely how changes in ENSO will affect tropical storms is still very much an open research topic.
Why is it important to consider this issue now?
Historically, we know that the impacts of major ENSO events can be severe and we now know that climate change can significantly intensify those impacts. We also have experienced a series of extreme El Niño and La Niña events in recent decades, including the first extreme El Niño of the 21st century in 2015-16. As we move into unchartered territory of Earth’s climate amid unabated increases in greenhouse gas emissions, it is more urgent than ever before to better understand ENSO in a changing climate to prepare for what is ahead of us.
What is particularly novel about the material presented in your new book?
Understanding ENSO in a changing climate requires a comprehensive knowledge of its dynamics, predictability, teleconnections, and impacts, as well as accurate observations, models, and paleo-reconstructions. These and related topics are covered in the book by 98 leading ENSO researchers in 21 separate chapters. The book is the most comprehensive appraisal of our current understanding of these issues, which we hope will reach a wide audience of researchers, students, and non-specialists in related fields.
What are some of the unresolved questions where additional research, data or modeling is needed?
These present real challenges for operational climate forecasting, and for anticipating ENSO impacts in general, especially given the changing background climate. The way forward is to maintain and expand our oceanic and atmospheric observing systems and to enhance the spatio-temporal coverage of ENSO paleo-reconstructions.
In addition, we need to improve the full range of climate models, from conceptual and intermediate-complexity models that facilitate understanding of fundamental physical processes, to high-resolution earth-system models that can represent in rich detail the complex dynamics governing the ENSO cycle and its evolving character in a changing climate.
El Niño Southern Oscillation in a Changing Climate, 2020, ISBN: ISBN: 978-1-119-54815-7, list price, $249.95 (hardcover), $200 (e-book)
—Michael J. McPhaden ([email protected]; 
0000-0002-8423-5805), NOAA Pacific Marine Environmental Laboratory, USA; Agus Santoso ( 0000-0001-7749-8124), University of New South Wales, Australia; and Wenju Cai ( 0000-0001-6520-0829), Commonwealth Scientific and Industrial Research Organization, Australia
Editor’s Note: It is the policy of AGU Publications to invite the authors or editors of newly published books to write a summary for Eos Editors’ Vox.
