Why Do Oceanic Nonlinearities Contribute Only Weakly to Extreme El Niño Events?

• Published in: Geophysical research letters Vol. 51; no. 11
• Main Authors: Liu, Fangyu, Vialard, Jérôme, Fedorov, Alexey V., Éthé, Christian, Person, Renaud, Zhang, Wenjun, Lengaigne, Matthieu
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.06.2024; American Geophysical Union; American Geophysical Union (AGU); Wiley
• Subjects: Atmospheric processes; Climate variations; Dynamic stability; El Nino; El Nino events; El Nino phenomena; El Nino-Southern Oscillation event; ENSO asymmetries; extreme El Niño; General circulation models; Heat budget; Instability waves; Instability waves (fluids); La Nina; Mixed layer; Nonlinearity; Ocean models; Oceanic general circulation model; oceanic nonlinearities; Sciences of the Universe; Sea surface; Sea surface temperature; Skewness; Southern Oscillation; Surface temperature; Tropical environments; Vertical mixing; Wind stress
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL108813

Extreme El Niño events have outsized global impacts and control the El Niño Southern Oscillation (ENSO) warm/cold phases asymmetries. Yet, a consensus regarding the relative contributions of atmospheric and oceanic nonlinearities to their genesis remains elusive. Here, we isolate the contribution of oceanic nonlinearities by conducting paired experiments forced with opposite wind stress anomalies in an oceanic general circulation model, which realistically simulates extreme El Niño events and oceanic nonlinearities thought to contribute to ENSO skewness (Tropical Instability Waves (TIWs), Nonlinear Dynamical Heating (NDH)). Our findings indicate a weak contribution of oceanic nonlinearities to extreme El Niño events in the eastern Pacific, owing to compensatory effects between lateral (NDH and TIWs) and vertical processes. These results hold across different vertical mixing schemes and modifications of the upper‐ocean heat budget mixed layer criterion. Our study reinforces previous research underscoring the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events. Plain Language Summary The El Niño‐Southern Oscillation (ENSO) is the primary driver of year‐to‐year climate variations in the tropics and beyond. Originating from air‐sea interactions in the tropical Pacific, ENSO oscillates between warm (El Niño) and cold (La Niña) phases, modulating sea surface temperature in the central and eastern equatorial Pacific. Occasionally, El Niño events intensify into “super” El Niño events, causing widespread impacts globally. Utilizing a state‐of‐the‐art oceanic model, our research challenges previous results suggesting a strong oceanic contribution to the amplitude difference between “normal” and “super” El Niño events. Instead, our findings reveal that potential oceanic influences on “super” El Niño events tend to offset each other. This is consistent with recent research highlighting the crucial role of atmospheric processes in the transformation from a “normal” to a “super” El Niño. Key Points A state‐of‐the‐art ocean model reproduces extreme El Niño events and the corresponding nonlinear oceanic processes realistically Contributions from oceanic nonlinearities are isolated using paired simulations forced by opposite wind stress anomalies Effects of oceanic nonlinearities on extreme El Niño events are small, due to compensation between lateral and vertical processes