An Overview of Understanding ENSO

During the past decades, significant progress has been made in the understanding of El Niño-Southern Oscillation (ENSO). ENSO has been viewed as a self-sustained and naturally oscillatory mode or a stable mode triggered by stochastic forcing. Whatever the case, El Niño involves Bjerknes' positive ocean-atmosphere feedback that culminates with warm sea surface temperature (SST) anomalies in the equatorial eastern and central Pacific. After an El Niño reaches its mature phase, negative feedbacks are required to terminate the growth of warm SST anomalies. Four major negative feedbacks have been proposed: wave reflection at the ocean western boundary, a discharge process due to Sverdrup transport, a western Pacific wind-forced Kelvin wave of opposite sign, and anomalous zonal advection. These negative feedbacks may work in varying combinations to terminate El Niño, and reverse it into La Niña. The paper reviews the current understanding of ENSO physics.