Dominant contribution of atmospheric nonlinearities to ENSO asymmetry and extreme El Niño events

Abstract

Extreme El Niño events have outsized impacts and strongly contribute to the El Niño Southern Oscillation (ENSO) warm/cold phase asymmetries. There is currently no consensus on the respective importance of oceanic and atmospheric nonlinearities for those asymmetries. Here, we use atmospheric and oceanic general circulation models that reproduce ENSO asymmetries well to quantify the atmospheric nonlinearities contribution. The linear and nonlinear components of the wind stress response to Sea Surface Temperature (SST) anomalies are isolated using ensemble atmospheric experiments, and used to force oceanic experiments. The wind stress-SST nonlinearity is dominated by the deep atmospheric convective response to SST. This wind-stress nonlinearity contributes to ~ 40% of the peak amplitude of extreme El Niño events and ~ 55% of the prolonged eastern Pacific warming they generate until the following summer. This large contribution arises because nonlinearities consistently drive equatorial westerly anomalies, while the larger linear component is made less efficient by easterly anomalies in the western Pacific during fall and winter. Overall, wind-stress nonlinearities fully account for the eastern Pacific positive ENSO skewness. Our findings underscore the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events and, more generally, ENSO asymmetry.

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

“…Here, we have explicitly diagnosed oceanic nonlinearities from OGCM realistic experiments, concluding that their contribution to extreme El Niño amplitude in the eastern Pacific is weak. This finding corroborates the results of Srinivas et al (2024), who indicated that most of the eastern Pacific SST anomalies positive skewness can be attributed to the wind stress nonlinear response to SST. This is also consistent with prior studies emphasizing atmospheric nonlinearities in extreme El Niño events (e.g., Takahashi et al, 2018) and ENSO asymmetry (e.g., Choi et al, 2013;Frauen & Dommenget, 2010;Geng et al, 2019;Kang & Kug, 2002;Ohba & Ueda, 2009;Yu & Fedorov, 2022).…”

“…Oceanic nonlinearities barely contribute to La Niña Geophysical Research Letters 10.1029/2024GL108813 events and most El Niño events, except in 1997-1998, 1982-1983 and 2015-2016, where they respectively contribute to 5.7%, 16.7% and 14.7% of the November-January Niño3 SST anomalies. Although these contributions are not completely negligible, they are much weaker than the 40%-50% contribution from atmospheric nonlinearities highlighted by Srinivas et al (2024). Our result hence indicates that the oceanic response to ENSO forcing is predominantly linear, even during extreme El Niño events.…”

“…In principle, the former gives the sum of odd and the latter of even terms in the Taylor expansion of T (τ′). In practice, they yield the linear and quadratic contributions, respectively (Figure S5; Srinivas et al, 2024). We hence term them linear and nonlinear parts of the oceanic response to wind stress.…”

“…In summary, the extent to which oceanic and atmospheric nonlinearities respectively contribute to extreme El Niño events remains uncertain. Our previous study (Srinivas et al, 2024) suggested that atmospheric nonlinearities predominantly drive the amplitude difference between moderate and extreme El Niño events. In that study, atmospheric nonlinearities were isolated through paired ensemble simulations using an Atmospheric General Circulation Model (AGCM) forced with opposite SST anomalies, which were subsequently used to force simulations with an Ocean General Circulation Model (OGCM).…”

“…ENSO asymmetries stem from nonlinearities within the coupled ocean-atmosphere system (e.g., An et al, 2020). The nonlinear dependence of tropical convection to SST results in precipitation and wind anomalies that are eastward-shifted and stronger during El Niño compared to La Niña (e.g., Graham & Barnett, 1987;Hoerling et al, 1997;Srinivas et al, 2022), thereby contributing to ENSO asymmetry (Choi et al, 2013;Frauen & Dommenget, 2010;Geng et al, 2019;Kang & Kug, 2002;Ohba & Ueda, 2009) or specifically to extreme events (Srinivas et al, 2024;Takahashi et al, 2018). Westerly Wind Bursts (WWBs), are more frequent, intense, and eastward-shifted during warm events (Eisenman et al, 2005;Puy et al, 2015).…”