Evidence for the Delayed Oscillator Mechanism for ENSO: The “Observed” Oceanic Kelvin Mode in the Far Western Pacific

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

Observed surface winds from 1961 through September 1992 are used to force a reduced gravity shallow water ocean model. Results from the hindcast of ocean variability are found to be consistent with the results Li and Clarke presented in a previous study of sea level and zonal wind variability in the tropical Pacific. In this note the apparent discrepancies are reconciled between “delayed oscillator” theory and calculated lag correlations between the observationally based records of western boundary Kelvin mode amplitude (ηW) and zonal wind forcing. Evidence for sensitivity in these “delayed oscillator” lag correlations is presented from a variety of sources, including: the hindcast data, output from the standard version of the Zebiak and Cane coupled ocean-atmosphere model, and through three cases with idealized time series of ENSO variability. The authors demonstrate that the low lag correlations for ηW leading the zonal wind forcing by 1 to 1½ years is not inconsistent with the hypothesized role of western boundary reflections as the ultimate termination mechanism for ENSO anomalies. The lack of regularity in the system studied guarantees a degraded correlation for ηW leading the zonal wind by 1 to 1½ years. This lack of regularity is not contained in, nor explained by, delayed oscillator theory.

A robust feature in all of the records examined in this work is the existence of upwelling Kelvin signals in the far western equatorial Pacific Ocean due to the developing warm (ENSO) events. The amplitude of the observed Kelvin signals in the western Pacific is sufficient to terminate the developing ENSO events via the delayed oscillator mechanism.

Abstract

Observed surface winds from 1961 through September 1992 are used to force a reduced gravity shallow water ocean model. Results from the hindcast of ocean variability are found to be consistent with the results Li and Clarke presented in a previous study of sea level and zonal wind variability in the tropical Pacific. In this note the apparent discrepancies are reconciled between “delayed oscillator” theory and calculated lag correlations between the observationally based records of western boundary Kelvin mode amplitude (ηW) and zonal wind forcing. Evidence for sensitivity in these “delayed oscillator” lag correlations is presented from a variety of sources, including: the hindcast data, output from the standard version of the Zebiak and Cane coupled ocean-atmosphere model, and through three cases with idealized time series of ENSO variability. The authors demonstrate that the low lag correlations for ηW leading the zonal wind forcing by 1 to 1½ years is not inconsistent with the hypothesized role of western boundary reflections as the ultimate termination mechanism for ENSO anomalies. The lack of regularity in the system studied guarantees a degraded correlation for ηW leading the zonal wind by 1 to 1½ years. This lack of regularity is not contained in, nor explained by, delayed oscillator theory.

A robust feature in all of the records examined in this work is the existence of upwelling Kelvin signals in the far western equatorial Pacific Ocean due to the developing warm (ENSO) events. The amplitude of the observed Kelvin signals in the western Pacific is sufficient to terminate the developing ENSO events via the delayed oscillator mechanism.

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