The response of a low-frequency mode of climate variability, El Niño-Southern Oscillation, to stochastic forcing is studied in a high-dimensional model of intermediate complexity, the fully-coupled Cane-Zebiak model [ZC87], from the spectral analysis of Markov operators governing the decay of correlations and resonances in the power spectrum.Noise-induced oscillations excited before a supercritical Hopf bifurcation are examined by means of complex resonances, the reduced Ruelle-Pollicott (RP) resonances, via a numerical application of the reduction approach of the first part of this contribution [CTND19] to model simulations.The oscillations manifest themselves as peaks in the power spectrum which are associated with RP resonances organized along parabolas, as the bifurcation is neared. These resonances and the associated eigenvectors are furthermore well described by the small-noise expansion formulas obtained by [Gas02] and made explicit in the second part of this contribution [TCND19]. Beyond the bifurcation, the spectral gap between the imaginary axis and the real part of the leading resonances quantifies the diffusion of phase of the noise-induced oscillations and can be computed from the linearization of the model and from the diffusion matrix of the noise. In this model, the phase diffusion coefficient thus gives a measure of the predictability of oscillatory events representing ENSO. ENSO events being known to be locked to the seasonal cycle, these results should be extended to the non-autonomous case.More generally, the reduction approach theorized in [CTND19], complemented by our understanding of the spectral properties of reference systems such as the stochastic Hopf bifurcation, provides a promising methodology for the analysis of low-frequency variability in high-dimensional stochastic systems.
KeywordsRuelle-Pollicott resonances · Stochastic Bifurcation · Markov matrix · ENSO
IntroductionComplex and unpredictable behavior of trajectories is observed for many physical systems. This can be due to interactions with many degrees of freedom which can be modeled by a stochastic forcing or to nonlinear coupling resulting in chaotic trajectories. As a result, prediction beyond a certain horizon is hopeless and one focuses instead on the statistical evolution of the system. This loss of predictability manifests itself by the evolution of an ensemble of trajectories becoming independent on its initial condition after a given time, the mixing time. This notion of mixing in state space is in turn closely linked to the correlation function of a pair of observables, which assigns to any positive time lag the correlation between the first observable and the lagged version of the second, and thus gives a measure of the statistical dependence A. Tantet