Instability of the chaotic ENSO: The growth-phase predictability barrier

The local predictability of the El Nino Southern Oscillation (ENSO) is exam ined by the analysis of the evolution of small disturbances to an unstable 4.3-yr ENSO cycle in the Cane-Zebiak model forced by perpetual July conditi ons. The 4.3-yr cycle represents the dominant near-recurrent behavior in th is weakly chaotic regime, so analysis of this single cycle gives useful ins ights into the dynamics of the irregular oscillation. Growing and neutral t ime-dependent eigenmodes of the unstable cycle are computed. Disturbance gr owth analyses based on these eigenmodes, and on singular vectors computed i n the unstable-neutral subspace, suggest that there is a predictability bar rier associated with the growth phase of El Nino conditions. This barrier a rises because the growth mechanism for disturbances to the cycle is nearly the same as the growth mechanism for the El Nino conditions themselves. The local amplification of disturbances during the growth phase is several tim es greater than the eigenmode amplification associated with time-dependent (Floquet) normal-mode instability of the cycle. It is suggested that the ex istence of an ENSO predictability barrier tied to the growth phase of El Ni no conditions is likely a robust result, independent of the particular mode l.