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.