  Anomalies during an El Nino are dominated by a single, irregularly
oscillating, mode. Equatorial dynamics has been linked to delayed-oscillator
models of this mode. Usually, the El Nino mode is regarded as an unstable mode
of the coupled atmosphere system and the irregularity is attributed to noise
and possibly chaos. Here a variation on the delayed oscillator is explored. In
this stochastic-oscillator view, El Nino is a stable mode excited by noise. It
is shown that the autocorrelation function of the observed NINO3.4 index is
that of a stochastic oscillator, within the measurement uncertainty. Decadal
variations as would occur in a stochastic oscillator are shown to be comparable
to those observed, only the increase in the long-term mean around 1980 is
rather large. The observed dependence of the seasonal cycle on the variance and
the correlation is so large that it can not be attributed to the natural
variability of a stationary stochastic oscillator. So the El Ni\~{n}o
stochastic-oscillator parameters must depend on the season. A forecast model
based on the stochastic oscillator with a variance that depends on the season
has a skill that approaches that of more comprehensive statistical models: over
the period 1982-1993, the anomaly correlation is 0.65 for two-season lead
forecasts.
