GEOPHYSICAL MONOGRAPH SERIES, VOL. 147, PP. 21-48, 2004
Understanding ENSO physics—a review
Since the TOGA program, and in particular the maintenance of its observing system in the tropical Pacific, significant progress
has been made in the understanding of 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 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 seasonal cycle can contribute to the irregularity and phase-locking of ENSO, and the intraseasonal variability can be a source of both ENSO's variability and irregularity. Tropical Pacific decadal-multidecadal variability and warming trends may modulate ENSO. Many mechanisms have been proposed to explain tropical Pacific decadal-multidecadal variability, and they are categorized by their tropical origins and tropical-extratropical connections. Mechanisms of tropical origins include stochastic forcing, interactions between the seasonal and interannual cycles, internal nonlinearity, asymmetry between El Niño and La Niña, and local oceanatmosphere interaction, while those of tropical-extratropical connections involve oceanic bridges, wave propagation, and atmospheric bridges. Difficulties and uncertainties of studies on low-frequency variability and interpretation of warming trends, global warming, and ENSO are also discussed.
Citation: Wang, C., and