The Role of the Western Boundary in the ENSO Cycle: Experiments with Coupled Models

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  • 1 Climate Research Division, Scripps Institution of Oceanography, La Jolla, California
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Abstract

Coupled models of the Pacific ocean–atmosphere system have been shown to produce oscillations in the model coupled system that resemble the observed El Niño–Southern Oscillation (ENSO) cycle in many respects. The tendency for the coupled models to oscillate is due to delayed negative feedback resulting from the reflection of Rossby wave activity from the western boundary. In the real ocean the reflection process is difficult to observe. Furthermore, among other potential complications affecting the generation and propagation baroclinic waves, the real western boundary is ragged with unknown reflective characteristics rather than the efficiently reflecting boundaries found in the oceanic components of the simple coupled models that produce ENSO-like oscillations. Thus, the validity of the delayed negative feedback–coupled oscillator concept remains open to question.

We present the results of experiments with two coupled models, one simple and one more complex, in which realistic western equatorial boundary data were prescribed. The working hypothesis for these experiments is that, if western boundary reflections play the role envisioned under the coupled oscillator scenario in the real world, then the models should produce correctly timed ENSO activity. The results of these experiments strongly support this hypothesis and are consistent with the systematic role of western boundary reflections in the ENSO cycle and the validity of the delayed negative feedback/coupled oscillator concept.

Abstract

Coupled models of the Pacific ocean–atmosphere system have been shown to produce oscillations in the model coupled system that resemble the observed El Niño–Southern Oscillation (ENSO) cycle in many respects. The tendency for the coupled models to oscillate is due to delayed negative feedback resulting from the reflection of Rossby wave activity from the western boundary. In the real ocean the reflection process is difficult to observe. Furthermore, among other potential complications affecting the generation and propagation baroclinic waves, the real western boundary is ragged with unknown reflective characteristics rather than the efficiently reflecting boundaries found in the oceanic components of the simple coupled models that produce ENSO-like oscillations. Thus, the validity of the delayed negative feedback–coupled oscillator concept remains open to question.

We present the results of experiments with two coupled models, one simple and one more complex, in which realistic western equatorial boundary data were prescribed. The working hypothesis for these experiments is that, if western boundary reflections play the role envisioned under the coupled oscillator scenario in the real world, then the models should produce correctly timed ENSO activity. The results of these experiments strongly support this hypothesis and are consistent with the systematic role of western boundary reflections in the ENSO cycle and the validity of the delayed negative feedback/coupled oscillator concept.

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