Recharge Oscillator Model Research Articles

Testing Simple Models of ENSO

The realistic simulation of El Nino-Southern Oscillation (ENSO) by the University of California, Los Angeles (UCLA), coupled atmosphere-ocean general circulation model (CGCM) is used to test two simple theoretical models of the phenomenon: the recharge oscillator model of Jin and the delayed oscillator model of Schopf, Suarez, Battisti, and Hirst (SSBH). The target for the simple models is provided by the CGCM results prefiltered with singular spectrum analysis to extract the leading oscillatory mode. In its simplest form, the Jin model can be reduced to two first ordinary differential equations. If the parameters of the model are fit in this reduced form, it appears to capture the period of the CGCM oscillatory mode. If the Jin model is instead fit using the individual physical balances that are used to derive it, substantial misfits to the CGCM are encountered. The SSBH model can likewise be expressed either in a condensed form or a larger set of individual physical balances with highly analogous results. It is shown that the misfits in both simple models can be greatly reduced by introducing a spinup timescale for wind stress relative to eastern equatorial Pacific SST. In the CGCM, this spinup time appears to be associated with a combination of atmospheric and ocean mixed layer processes in a way consistent with the ''mixed mode'' regime discussed by Syu and Neelin, which is not included in the Jin and SSBH models. These appear indis- tinguishable in this analysis, although the latter is more sensitive to fitting. This paper provides a bridge between work on ENSO by theoreticians and numerical modelers. The CGCM results validate the conceptual framework of the simple models by demonstrating that they can provide a plausible representation of ENSO with realistic sets of parameters. The results also suggest that, in terms of realistic ENSO variability, the framework of the simple models can be made substantially more complete by including the adjustment time between wind stress and eastern Pacific SST required by the coupled spinup of the atmosphere and the ocean mixed layer processes outside this region.

Collective Role of Thermocline and Zonal Advective Feedbacks in the ENSO Mode*

The vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current are known to be essential for the equatorial SST anomaly associated with the El Nino-Southern Oscillation (ENSO). In the coupled model, these two processes are referred to as the thermocline feedback and the zonal advective feedback, respectively. Using a version of a recharge oscillator model for ENSO obtained from the stripped-down approximation of the Cane-Zebiak-type model, it is demonstrated that these two feedbacks, which are linked dynamically through the geostrophic approximation, tend constructively to contribute to the growth and phase transition of ENSO. However, these two feedbacks control the leading coupled mode in different ways. The thermocline feedback leads to a coupled mode through the merging of the damped SST mode and ocean adjustment mode, whereas the zonal advective feedback tends to destabilize the gravest ocean basin mode. With both of these feedbacks, the leading modes of the coupled model still can be traced back to these different origins under moderate changes in the model setup. The main consequence of these sensitivities is that the growth rate and frequency of the ENSO mode may be sensitive to slight changes in basic-state parameters, which control the strength of these feedbacks.

Thermocline and Zonal Advective Feedbacks Within the Equatorial Ocean Recharge Oscillator Model for ENSO

Through the dynamical coupling between ocean and atmosphere, the vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current constitute the so‐called the thermocline and zonal advective feedbacks that are essential for El Niño/Southern Oscillation (ENSO) coupled dynamics. In this paper, we demonstrate that these two feedbacks are dynamically linked because of the geostrophic balance between the upper ocean zonal current and the meridional gradient of the thermocline. Both feedbacks thus play important and similar roles in the growth and phase transition of ENSO. We further propose a new version of the conceptual recharge oscillator model for ENSO by including these two feedbacks. The new model retains the simplest possible form of a harmonic oscillator, yet presents a more complete description of slow physics for ENSO. Moreover, it also provides the reconciliation between the biased emphases in the ENSO theories oh the thermocline and zonal advective feedbacks.

An Equatorial Ocean Recharge Paradigm for ENSO. Part I: Conceptual Model

A new conceptual model for ENSO has been constructed based upon the positive feedback of tropical ocean‐ atmosphere interaction proposed by Bjerknes as the growth mechanism and the recharge‐discharge of the equatorial heat content as the phase-transition mechanism suggested by Cane and Zebiak and by Wyrtki. This model combines SST dynamics and ocean adjustment dynamics into a coupled basinwide recharge oscillator that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. Over a wide range of the relative coupling coefficient, this recharge oscillator can be either self-excited or stochastically sustained. Its period is robust in the range of 3‐5 years. This recharge oscillator model clearly depicts the slow physics of ENSO and also embodies the delayed oscillator (Schopf and Suarez; Battisti and Hirst) without requiring an explicit wave delay. It can also be viewed as a mixed SST‐ocean dynamics oscillator due to the fact that it arises from the merging of two uncoupled modes, a decaying SST mode and a basinwide ocean adjustment mode, through the tropical ocean‐atmosphere coupling. The basic characteristics of this recharge oscillator, including the relationship between the equatorial western Pacific thermocline depth and the eastern Pacific SST anomalies, are in agreement with those of ENSO variability in the observations and simulations with the Zebiak‐Cane model.

An Equatorial Ocean Recharge Paradigm for ENSO. Part II: A Stripped-Down Coupled Model

Abstract The conceptual recharge oscillator model intuitively established in Part I is derived from a dynamical framework of a Cane–Zebiak type model for tropical ocean–atmosphere interaction. A two-strip approximation to the equatorial ocean dynamics and one-strip approximation to the SST dynamics are employed to obtain a stripped-down coupled model that captures the main physics of the Cane–Zebiak type model. It is shown that the conceptual recharge oscillator model can be obtained from the stripped-down coupled model with a two-box approximation in the zonal direction or a low-frequency approximation to filter out high-frequency modes. Linear solutions of the stripped-down model are analytically solved and the dependence of coupled modes on various model parameters is delineated. In different parameter regimes, the stripped-down coupled model describes a coupled-wave mode and a mixed SST–ocean-dynamics mode that results from the merger of a nonoscillatory ocean adjustment mode with an SST mode. These t...

A Limit Theorem for the Solutions of Differential Equations with Random Right-Hand Sides

A Limit Theorem for the Solutions of Differential Equations with Random Right-Hand Sides