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

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  • The local predictability of the El Nin˜o–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 Nin˜o conditions. This barrier arises because the growth mechanism for disturbances to the cycle is nearly the same as the growth mechanism for the El Nin˜o 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 Nin˜o conditions is likely a robust result, independent of the particular model.
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  • Samelson, Roger M., Eli Tziperman, 2001: Instability of the Chaotic ENSO: The Growth-Phase Predictability Barrier. J. Atmos. Sci., 58, 3613–3625.
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  • 58
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  • 23
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  • This work was partially supported for Roger Samelson by the Office of Naval Research, Grant N00014-98-1-0813, and the National Science Foundation, Grant OCE98-96184, and for Eli Tziperman by the Israeli Academy of Sciences.
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