Singular spectrum analysis (SSA: Vautard and Ghil 1989; Ghil and Vautard 1991; Plaut et al. 1995) and the maximum entropy method (MEM: Burg 1968; Penland et al. 1991) are used here for long-lead forecasts of the sea-surface temperature (SST) anomalies averaged over the Niño 3 area and the Southern Oscillation Index (SOI). The forecast is for up to one year ahead, based on the last 45 years of observed data. More detailed information on the forecast method based on single-channel SSA combined with MEM is given by Keppenne and Ghil (1992), while multi-channel SSA (M-SSA: Kimoto et al. 1991; Keppenne and Ghil 1993; Plaut and Vautard 1994) combined with MEM is documented in the March 1995 issue of this Bulletin (Jiang et al. 1995). Briefly, the time series is filtered first by SSA (if univariate) or M-SSA (if multivariate), so that the statistically significant components are retained, specifically the quasiquadrennial (QQ) and the quasi-biennial (QB) components of ENSO variability (Rasmusson et al. 1990; Keppenne and Ghil 1992; Jiang et al. 1995). Then MEM is applied to advance these components in time.

Figure 1 shows area-averaged Niño 3 SSTAs, forecast and observed, since 1990, using the SSA- and MSSA-MEM schemes for a 3-, 6-, 9- and 12-month lead. The last forecast, for the next 1-4 seasons, using data through October 1996, is shown in Fig. 2. The vertical bars are one standard deviation in length, based on forecast verification over the 1984-93 time span. The forecasts indicate that the presently near-normal to slightly warmer than normal conditions in Niño 3 will continue to be warmer than normal through fall 1997.

Figure 3 shows the SSA-MEM forecast for the SOI from November 1996 through October 1997. The SOI is expected to remain close to its mean, but above it, through early 1997. The present SOI forecast does not agree with the Niño-3 SSTA forecast inasmuch as the two indices are not anticorrelated, the way they usually are. This reduces somewhat our confidence in the forecast, which otherwise agrees with those of the statistical CCA method (Barnston and Ropelewski 1992) and the NCEP coupled model (Ji et al. 1994).

Barnston, A.G. and C.F. Ropelewski, 1992: Prediction of ENSO episodes using canonical correlation analysis. J. Climate, 7, 1316-1345.

Burg, J.P., 1968: Maximum entropy spectral analysis. Modern Spectrum Analysis, 34-48. IEEE Press.

Ghil, M. and R. Vautard, 1991: Interdecadal oscillations and the warming trend in global temperature time series. Nature, 350, 324-327.

Jiang, N., D. Neelin and M. Ghil, 1995: Quasi-quadrennial and quasi-biennial variability in the equatorial Pacific. Clim. Dyn., 12, 101-112.

Keppenne, C.L. and M. Ghil, 1992: Adaptive filtering and prediction of the Southern Oscillation Index. J. Geophys. Res., 97: 20449-20454.

Keppenne, C.L. and M. Ghil, 1993: Adaptive filtering and prediction of noisy multivariate signals Adaptive filtering and prediction of noisy multivariate signals: An application to subannual variability in atmospheric angular momentum. Intl. J. Bif. & Chaos, 3, 625-634.

Kimoto, M., M. Ghil and K.C. Mo, 1991: Spatial structure of the 40-day oscillation in the Northern Hemisphere extratropics. Proc. 8th Conf. Atmos. & Oceanic Waves & Stability. Amer. Met. Soc., Boston, 115-116.

Penland, C., M. Ghil and K.M. Weickmann, 1991: Adaptive filtering and maximum entropy spectra, with application to changes in atmospheric angular momentum. J. Geophys. Res., 96, 22,659-22,671.

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Plaut, G.R., M. Ghil and R. Vautard, 1995: Interannual and interdecadal variability in 335 years of central England temperature. Science, 268, 710-713.

Rasmusson E.M., X. Wang and C. F. Ropelewski, 1990: The biennial component of ENSO variability. J. Mar. Sys., 1, 71-96.

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Fig. 1. Forecasts of the area-averaged Niño-3 SST anomalies (SSTA) using the SSA-MEM (star) and MSSA-MEM (circle) schemes. The solid line indicates the observed Niño-3 SSTA. The latest forecast starts from July 1996. Shown for: (a) 3-month, (b) 6-month, (c) 9-month and (d) 12-month lead.

Fig. 2. The forecasts of the Niño-3 SSTA for the upcoming 4 seasons using the SSA-MEM scheme. The solid line indicates the observed Niño-3 SSTA.

Fig. 3. SSA-MEM forecast of the SOI for September 1996 through August 1997. The circles are the monthly SOI values based on a 5-month running mean without the seasonal cycle and the solid line is the SSA-filtered SOI. The dashed line indicates the forecast for the next 12 months.

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