A unified oscillator model for the El Nino-Southern Oscillation

The delayed oscillator, the western Pacific oscillator, the recharge-discha rge oscillator, and the advective-reflective oscillator have been proposed to interpret the oscillatory nature of the El Nino-Southern Oscillation (EN SO). All of these oscillator models assume a positive ocean-atmosphere feed back in the equatorial eastern and central Pacific. The delayed oscillator assumes that the western Pacific is an inactive region and wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate. The western Pacific oscillator emphasizes an active role of the western Pacific in ENSO. The recharge-discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate. The advective-reflective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries. Motivated by the existence of these different oscillat or models, a unified oscillator model is formulated and derived from the dy namics and thermodynamics of the coupled ocean-atmosphere system. Consisten t with ENSO anomaly patterns observed in the tropical Pacific, this oscilla tor model considers sea surface temperature anomalies in the equatorial eas tern Pacific, zonal wind stress anomalies in both the equatorial central Pa cific and the equatorial western Pacific, and thermocline depth anomalies i n the off-equatorial western Pacific. If the western Pacific wind-forced re sponse is neglected, thermocline and zonal wind stress anomalies in the wes tern Pacific are decoupled from the coupled system, and the unified oscilla tor reduces to the delayed oscillator. If wave reflection at the western bo undary is neglected, the unified oscillator reduces to the western Pacific oscillator. The mathematical form of the recharge-discharge oscillator can also be derived from this unified oscillator. Most of the physics of the ad vective-reflective oscillator are implicitly included in the unified oscill ator, and the negative feedback of wave reflection at the eastern boundary is added to the unified oscillator. With appropriate model parameters chose n to be consistent with those of previous oscillator models, the unified os cillator model oscillates on interannual timescales.