An Overview of the El Niño, La Niña, and the Southern Oscillation Phenomena: Theory, Observations, and Modeling Links

As an important subject of study, the climate science encompasses all processes and phenomena in the atmosphere, ocean, ice, and land, displaying variability over a broad, although well defined, spectrum of spatial and temporal scales. The disparity of spatial and temporal scales of climate processes poses a major challenge in climate modeling, ranging from regional to planetary scales and from intraseasonal to climate changes over centuries. Moreover, despite the number of efforts to elucidate prominent phenomena, certain key features remain elusive, and so preclude actions toward anticipation and mitigation of hazardous impacts, with implications over diverse human activities. One interesting example is the El Niño, La Niña—Southern Oscillation (ENSO), which implies in a prominent large-scale coupled ocean–atmosphere oscillation across the equatorial Pacific. ENSO represents the dominant variability in the climate system on interannual to decadal timescales. The long-time behavior of the ENSO dynamics, though, is typically a complex interplay between periodicity and randomness. The classic approaches cannot precisely characterize the observed variability, which typically displays structured but aperiodic oscillations. The irregularity makes difficult to predict when the next extreme phase of ENSO is going to be manifested and the peak magnitude of the event has also been a difficult parameter to predict. Although the theory for ENSO has been widely explored along the past 40 years by observational and theoretical work involving different views, there is still lack of consensus between modelers and observers about what are the essential mechanisms for ENSO. Recent studies suggest that there exists El Niño (La Niña) diversity regarding the parameters related to its amplitude, trigger mechanisms, spatial patterns, and life cycle as well as its impacts on the globe. Thus, the objective of the present work is to discuss the development of the ideas toward what is known about ENSO today. Here, a review on theoretical fundamentals is presented from ENSO model hierarchy, basic mechanisms, ENSO irregularity, and interactions with other scales of variability along with a brief discussion of some recent observational results.