The annual cycle of persistence in the El Nino Southern Oscillation

A spring 'predictability barrier' exists in both data and models of the El Nino/Southern Oscillation (ENSO) phenomenon. In statistical analyses this barrier manifests itself as a drop-off in monthly persistence (lagged correlation) while in coupled ocean-atmosphere models it appears as a decrease in forecast skill. The 'persistence barrier' for ENSO indices is investigated using historical sea surface temperature and sealevel pressure data. Simple statistical models are used to show that the persistence barrier occurs because the boreal spring is the transition time from one climate state to another, when the 'signal-to-noise' of the system is lowest and the system is most susceptible to perturbations. The strength of the persistence barrier is shown to depend on the degree of phase locking of the ENSO to the annual cycle. The phase locking of the ENSO to the annual cycle, as well as the ENSO variance, is shown to vary on interdecadal time-scales. During 1871-1920 and 1960-90 the ENSO variance was high, while during 1920-50 it was low. Using wavelet analysis, this interdecadal variability in ENSO is shown to be correlated with changes in Indian summer monsoon strength. Finally, the change in persistence-barrier strength between 1960-79 and 1980-95 is related to changes in the phase locking of ENSO to the annual cycle. These changes in persistence and phase locking appear to be related to the increased forecast skill seen from recent coupled ocean-atmosphere models.