The El Niño Southern Oscillation (ENSO) is characterized by being irregular or nonperiodic and asymmetric between El Niño and La Niña with respect to amplitude, pattern, and temporal evolution. These observed features suggest the importance of nonlinear dynamics and/or stochastic forcing. Both nonlinear deterministic chaos and linear dynamics subject to stochastic forcing and/or to nonnormal growth were introduced to explain the irregularity of ENSO, but no consensus has been reached to date given the short observational record. As a dominant source of stochastic forcing, westerly wind bursts play a role in triggering, amplifying, and determining the irregularity and asymmetry of ENSO, which are best treated as part of the deterministic dynamics or as a multiplicative noise forcing. Various nonlinear processes are responsible for the spatial and temporal asymmetry of El Niño and La Niña, which includes nonlinear ocean advection, nonlinear atmosphereocean coupling, statedependent stochastic noise, tropical instability waves, and biophysical processes. In addition to the internal nonlinear processes, a capacitor effect of the Indian and Atlantic Oceans and atmospheric and oceanic teleconnections from extratropical Pacific could also contribute to the temporal and amplitude asymmetry of ENSO. Despite significant progress, most stateoftheart models are still lacking in simulation of the spatial and temporal asymmetry of ENSO.
|Title of host publication||Geophysical Monograph Series|
|Publisher||John Wiley and Sons Inc.|
|Number of pages||20|
|Publication status||Published - 2020|
|Name||Geophysical Monograph Series|
Bibliographical noteFunding Information:
S.‐I. An was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF‐2017R1A2A2A05069383, NRF‐2018R1A 5A1024958) and appreciates J.‐W. Kim for drawing Figure 7.5. E. Tziperman was supported by the NSF climate dynamics program, grant AGS‐1826635, and by a Harvard‐UTEC collaborative grant, and would like to thank the Weizmann Institute for its hospitality during parts of this work. Y. Okumura was supported by the US National Oceanic Atmosphere Administration (NA17OAR 4310149) and National Science Foundation (OCE1756883). T. Li was supported by NSFC grant 41630423, NSF grant AGS-2006553, and NOAA grant NA18OAR4310298.
© 2021 American Geophysical Union. Published 2021 by John Wiley & Sons, Inc.
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