(one of my happiness today)
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A review of ENSO theories
Chunzai Wang
National Science Review, Volume 5, Issue 6, November 2018, Pages 813–825,
Abstract
The El Niño and the Southern Oscillation (ENSO) occurrence can be usually explained by two views of (i) a self-sustained oscillatory mode and (ii) a stable mode interacting with high-frequency forcing such as westerly wind bursts and Madden-Julian Oscillation events. The positive ocean–atmosphere feedback in the tropical Pacific hypothesized by Bjerknes leads the ENSO event to a mature phase. After ENSO event matures, negative feedbacks are needed to cease the ENSO anomaly growth. Four negative feedbacks have been proposed: (i) reflected Kelvin waves at the ocean western boundary, (ii) a discharge process due to Sverdrup transport, (iii) western-Pacific wind-forced Kelvin waves and (iv) anomalous zonal advections and wave reflection at the ocean eastern boundary. These four ENSO mechanisms are respectively called the delayed oscillator, the recharge–discharge oscillator, the western-Pacific oscillator and the advective–reflective oscillator. The unified oscillator is developed by including all ENSO mechanisms, i.e. all four ENSO oscillators are special cases of the unified oscillator. The tropical Pacific Ocean and atmosphere interaction can also induce coupled slow westward- and eastward-propagating modes. An advantage of the coupled slow modes is that they can be used to explain the propagating property of interannual anomalies, whereas the oscillatory modes produce a standing oscillation. The research community has recently paid attention to different types of ENSO events by focusing on the central-Pacific El Niño. All of the ENSO mechanisms may work for the central-Pacific El Niño events, with an addition that the central-Pacific El Niño may be related to forcing or processes in the extra-tropical Pacific.
https://academic.oup.com/nsr/article/5/6/813/5126370
(to quote from the paper)
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Bjerknes [1] hypothesized that a positive ocean–atmosphere feedback process causes ENSO. Given an initial warm sea-surface temperature (SST) anomaly in the equatorial eastern Pacific, the warm SST anomaly reduces the east–west SST gradient and hence weakens the Walker circulation [2,3], producing the westerly wind anomaly in the equatorial central Pacific. The westerly wind anomaly in turn drives the ocean circulation change that further enhances the SST anomaly. As a result of the positive feedback, the tropical Pacific reaches a warm state, i.e. El Niño. After El Niño matures, negative feedbacks are required to turn El Niño from a warm phase to a cold phase, which was called La Niña [4].
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https://academic.oup.com/nsr/article/5/6/813/5126370