The ENSO-Spectrum: A Result of Deterministic Chaos

or Stochastic Forcing?

]in-Sang van STORCH ,

Institute 0 Meteorology, University Hamburg, Germany

Bin W ANG and Zhua W ANG

School 0 Ocean and Earth Science and Technology, University 0 Hawaii at Manoa, Honolulu, HI 96822, USA

Abstract -The spectrum of ENSO is studied using both observational data and integrations with coupled models of various complexities. The observed ENSO spectrum is characterized by a w -2-s1ope ( where w is frequency) at high frequencies, a spectral plateau at low frequencies, and a broad spectral maximum at about one cycle every 3 to 4 years. This smooth spectral shape is reproduced by the coupled GCMs, but not by the simplified coupled models. The simple models produce spectra with significant peaks and a w -4- slope at high frequencies. The roles of two mechanisms, deterministic chaos and stochastic forcing, in de- termining the ENSO spectrum are studied using a simple non-linear dynamical model, which captures the low-order dynamics of ENSO and produces, in the presence of the annual cycle, chaotic' behaviors. The two mechanisms affect the spectrum in different ways. Even though deterministic chaos Is able to spread variance over a large range of time scales, It alone cannot produced the w-2-slope in the non-linear model considered. In contrary, a strong enough stochastic forcing will ultimately produce a w -2-slope at high fre" quencies. The physical meaning of a strong stochastic forcing is discussed. The unrealistic spectra ob- tainedwith the simple models suggest the limited utility of simple models in making probability forecasts of ENSO.