The El Niño–Southern Oscillation (ENSO) is currently a hot topic, with predictions growing for a “Super El Niño” that could lead to widespread extreme weather events. It’s more important than ever to understand ENSO dynamics and effects, which makes the new special collection from AMS Journals, “The Atypical 2014–2024 ENSO Decade,” particularly timely. The research discussed in this collection originated from a committee of CLIVAR (Climate and Ocean: Variability, Predictability and Change), which is a project of the World Climate Research Programme (WCRP). It examines a decade that featured a strong El Niño developing in 2014–15 and growing into the last Super El Niño in 2015–16, followed by a weak double-dip La Niña from 2016 to 2018 and then a weak El Niño in the Central Pacific in 2018–19. An uncommon triple-dip La Niña then occurred from 2020 to early 2023, followed by a notable El Niño event in 2023–24. These notable fluctuations in ENSO phases were accompanied by diverse patterns, intensities, and durations, resulting in an active and atypical ENSO decade.
Andrea Taschetto of the University of New South Wales is the lead organizer of the special collection; she spoke to AMS Headlines about its origins, some findings, and its importance given current conditions.
How did you get involved with working on this special collection?
Over the past decades we’ve seen ENSO events that challenged our expectations in different ways. 2014–24 was a decade full of ENSO diversity, including strong eastern Pacific El Niño (2015–16, 2023–24), weak central Pacific El Niño (2018–19), and coastal El Niño (2017) types, as well as a weak La Niña (2017–18) and a rare triple-dip La Niña event (2020–23), leaving no room for neutral events. Individually, those events are not unprecedented in the historical record. However, it is a unique sequence of ENSO diversity occurring in a rapidly warming climate, which has sparked interest among the CLIVAR community in investigating how unusual this decade has been.
The idea for the special collection originated from discussions among CLIVAR members. CLIVAR is one of the core projects of WCRP. The Special Collection is a joint effort by the CLIVAR Pacific Region Panel, of which I’m a cochair, together with the Research Foci on Marine Heatwaves in the Global Ocean and Tropical Basin Interaction, and the CLIVAR Scientific Steering Group. The organizers include Sophie Cravatte (Laboratory of Space Geophysical and Oceanographic Studies, French National Research Institute for Sustainable Development), Antonietta Capotondi (University of Colorado), Yuko Okumura (The University of Texas at Austin), Magdalena Balmaseda (ECMWF), Lei Zhang (South China Sea Institute of Oceanology), and Agus Santoso (CLIVAR Office and Ocean University of China).
Forecasting many of these events has also been challenging, and they led to impacts that did not always match what we would normally expect from traditional statistical knowledge. That is important because ENSO is the largest year-to-year climate phenomenon that influences seasonal rainfall, weather systems, and extremes such as droughts, floods, and heat waves in many regions across the globe. As global temperatures continue to rise, understanding how ENSO behavior and its impacts may be changing is crucial to improving climate predictions and helping communities better prepare for climate adaptation and minimize risks of ENSO-related impacts.
How did this collection come together?
After discussions among CLIVAR members, it became evident that several ENSO experts shared the feeling that the past decade had been unusually interesting in some aspects of ENSO, particularly from the forecasting and impact points of view. So there was mutual interest to investigate whether the past decade exhibited atypical ENSO behavior. We then sought insights and surveyed the interest of the scientific community to contribute studies to the special collection, and we received substantial positive responses. This confirmed the interest and the need to further examine ENSO over the past decade.
We invited experts working across different areas of ENSO science and opened a call for contributions looking at the many aspects of these recent events that challenged our understanding of ENSO. The collection includes studies on unusual ENSO evolution, changes in variability, the processes driving these events, ocean–atmosphere connections, climate impacts across marine and terrestrial systems, and the ongoing challenges of prediction and forecasting.
What’s been especially exciting is seeing the range of perspectives and new ideas coming from researchers around the world. Together, the papers highlight just how complex ENSO is, and how much there is still to learn about how it may behave in a changing climate.
Are there any overarching findings from the studies in the collection that you think are especially significant? Specifically, are there findings that help explain the atypical ENSO decade?
One of the interesting things emerging from the collection is that there is no single explanation for why the past decade posed a challenge to ENSO predictions and impacts. In fact, it is difficult to conclude whether the past decade was atypical compared to the past ones for many reasons, especially due to the short observational record.
The papers currently published in the special collection discuss a combination of interacting factors across the atmosphere, ocean, and broader climate system. The topics range from the physical processes driving the ENSO life cycle to interactions with other modes of climate variability, teleconnections and regional impacts, ocean biogeochemistry, forecasting challenges, and future climate projections. Together, they showcase an ENSO system influenced by both natural variability and a changing climate background. The findings of these papers suggest that even if ENSO itself behaves in traditional ways, its impacts may not because of a warming background. As the climate continues to warm, the relationships between ENSO and regional climate patterns are also evolving, making prediction and risk assessment more complicated than in the past.
So while we now have much better observations and forecasting systems than in the past, which allow us to identify features and processes that may have gone unnoticed in previous decades, our record is too short to confidently conclude that ENSO behavior this decade was particularly unusual compared with previous decades. Many of the studies in the special collection highlight how important it is to maintain and expand the observing system because long-term observations are crucial for understanding how ENSO is changing in a warming climate.
What kind of relevance might this collection have for a broad audience?
The collection highlights examples of how climate variability can be harder to predict in a warming world. ENSO is a dominant source of seasonal predictability, accounting for a considerable proportion of rainfall, temperature, and extremes in many regions across the world.
Changes in its behavior can have important societal implications, as ENSO is often associated with increasing the chances for droughts, floods, and heat waves, which impact agriculture, health, water resources, power production, and other economic sectors.
One of the key messages from this collection is that there is no longer a “classical” picture of ENSO nowadays. The past decade has surprised scientists, both in how ENSO events evolved and in the impacts they produced. Our understanding of ENSO has largely been built from the historical record, but as the climate continues to warm, that baseline is changing, too. This means our scientific understanding, forecasting approaches, and expectations also need to keep evolving. And given the current El Niño forecast, these questions are becoming even more timely and important.
What is the biggest surprise or surprises the collection has uncovered so far?
It is probably just how challenging it has been to predict ENSO behavior, and particularly its impacts over the past decade. The background conditions in other ocean basins can have an important influence on ENSO behavior and its global impacts. And as the climate warms, their relationship with ENSO may change. This means that ENSO itself matters, but in addition to that, the mean state in which ENSO events occur is equally important.
Does the collection lack an area of study that you would like to see included?
While the collection covers broad aspects of ENSO, there are still a few areas of emerging science that we would like to see more contributions to. One area is the application of AI and machine learning approaches to improve ENSO predictions and understand complex interactions in the climate system. We would also welcome more studies on biogeochemical impacts, as there has been growing interest from the scientific community in the consequences for marine ecosystems, carbon cycling, and fisheries. Finally, studies using observations from the Tropical Pacific Observing System (TPOS) are also welcome. These are just a few of the areas where many important open questions remain.
Submissions to the special collection on “The Atypical 2014–2024 ENSO Decade” are currently being accepted. Visit the Call for Papers page for more information.
