Instability of the Chaotic ENSO: The Growth-Phase Predictability Barrier

Abstract

The local predictability of the El Niño?Southern Oscillation (ENSO) is examined 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 conditions. The 4.3-yr cycle represents the dominant near-recurrent behavior in this weakly chaotic regime, so analysis of this single cycle gives useful insights into the dynamics of the irregular oscillation. Growing and neutral time-dependent eigenmodes of the unstable cycle are computed. Disturbance growth analyses based on these eigenmodes, and on singular vectors computed in the unstable-neutral subspace, suggest that there is a predictability barrier associated with the growth phase of El Niño conditions. This barrier arises because the growth mechanism for disturbances to the cycle is nearly the same as the growth mechanism for the El Niño conditions themselves. The local amplification of disturbances during the growth phase is several times greater than the eigenmode amplification associated with time-dependent (Floquet) normal-mode instability of the cycle. It is suggested that the existence of an ENSO predictability barrier tied to the growth phase of El Niño conditions is likely a robust result, independent of the particular model.