Advancing Knowledge of ENSO in a Changing Climate

Advancing Knowledge of ENSO in a Changing Climate

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.

Visualization of the Pacific Ocean showing how sea surface temperatures differed from normal in October 2015
Satellite sea surface temperature departure for October 2015 over the Pacific. Orange-red colors indicate above normal temperatures, indicative of an El Niño condition. The 2015-16 El Niño was the first extreme El Niño of the 21st century and among the three strongest El Niños on record. Credit: NOAA National Environmental Satellite, Data, and Information Service (NESDIS)

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.

Photo showing flooded streets and people being rescued in Peru during the 1997–1998 extreme El Niño
Flooding in Peru during the 1997–1998 extreme El Niño. Credit: Courtesy of the University of Piura, Peru

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?

Illustration of potential foci for future ENSO research
List of potential foci of future ENSO research featuring some of the physical processes that require advancements in observing systems and climate model resolution and complexity. Credit: Karamperidou et al. [2020], Figure 21.4, in McPhaden et al. [2020]

ENSO is a complex phenomenon operating in a climate system that is highly nonlinear. Each El Niño and La Niña is different; their uniqueness invariably challenges both our understanding of the key mechanisms that give rise to them and our ability to predict them. For instance, the 2015-16 extreme El Niño evolved differently in space and time than the previous extreme El Niños in 1982-83 and 1997-98, highlighting the issue of “ENSO diversity” in which ENSO events come in different flavors. Our short observational record is insufficient to capture the real extent of event diversity which on the other hand is also difficult to simulate by many current state-of-the-art climate models.

Front cover of book El Niño Southern Oscillation in a Changing ClimateThese 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]; ORCID logo 0000-0002-8423-5805), NOAA Pacific Marine Environmental Laboratory, USA; Agus Santoso (ORCID logo 0000-0001-7749-8124), University of New South Wales, Australia; and Wenju Cai (ORCID logo 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.

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