Lecture 3 ENSO’s irregularity and phase locking

That El Nino is aperiodic is seen, for example, in a nino3 time series (Fig. 26). The ENSO delayed oscillator mechanism may result in either self-sustained oscillations, or in a steady state solution that has a damped oscillatory mode. In the later case, oscillations may be excited by external stochastic (weather) noise. In the scenario in which ENSO is self-sustained, it may be irregular due to low-order chaos. In the stochastically driven scenario, ENSO’s irregularity is simply an outcome of the stochastic forcing. Whether ENSO is damped or selfsustained depends on the ocean-atmosphere coupled instability, also referred to as the coupling strength. For this purpose, the coupling strength may be defined for example as the response of the atmospheric wind stress per unit change in the thermocline depth. Note that this coupling is the product of (at least) three different coupling coefficients: the response of SST to thermocline depth changes, the response of the atmospheric heating to SST changes, and the response of the wind stress to the atmospheric heating. A stronger coupling implies self-sustained possibly chaotic oscillation, while a weak coupling implies damped oscillations.