Q. zyxwvutsrqpo J. R. Meteorol. zyxwvuts SOC. zyxwvutsrqpo (1998), 124, pp. 1985-2004 The annual cycle of persistence in the zyxw El Niiio/Southern Oscillation By CHRISTOPHER TORRENCE* and PETER J. WEBSTER University of Colorado at Boulder, USA (Received 25 October 1996; revised 19 December 1997) SUMMARY A spring ‘predictability barrier’ exists in both zyxwvu data and models of the El NiiiolSouthern 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 sea- level 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. KEYWORDS: El Niiio/Southern Oscillation Interdecadal variability Predictability zyx 1. INTRODUCTION During the 1930s, Sir Gilbert Walker outlined the characteristics of the ‘Southern Oscillation’, consisting of a global-scale ‘see-saw’ of pressure, rainfall, and temperature anomalies centered on the tropical Pacific Ocean (Walker and Bliss 1932, 1937). Walker’s attempts to ‘foreshadow’ the strength of the Indian summer monsoon were based on both the high correlations of his ‘Southern Oscillation Index’ (SOI) with the Indian monsoon, and the seasonal persistence of the oscillation. Unfortunately, Walker discovered that the persistence in the SO1 is much higher following the June-August monsoon season, than preceding the monsoon. Furthermore, the correlations between the Southern Oscillation and the Indian monsoon decreased considerably after about 1925, leading to speculation that whatever usefulness Walker’s model might have possessed was now gone (Normand 1953; Treloar and Grant 1953). After a thirty-year quiescent period, both in research and in Indian monsoon and Southern Oscillation variability, Troup (1965) re-examined the evidence and found that, while there had been some secular changes since 1920, all of Walker’s earlier correlations remained valid. Walker’s index still showed large persistence from season to season, with the least persistence across the March-May season. In examining this seasonal change in persistence, Walker and Bliss (1932) concluded that “conditions in the southern winter exercised greater influence on subsequent seasons than did those in the southern summer”. Much of the research in the last thirty years has been on analysing the Southern Oscil- lation as part of the larger El Niiio/Southern Oscillation (ENSO) phenomenon, involving basin-wide migrations of both pressure and sea surface temperature (SST) anomalies (Ras- musson and Carpenter 1982).The recognition of the ENSO as a coupled ocean-atmosphere phenomenon has enabled the development of simple models capable of useful predictions up to nine months in advance (Cane et al. 1986). * Corresponding author: Advanced Study Program, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA. 1985