Forecasts of Niño-3 SST Anomalies and SOI Based on Singular Spectrum Analysis Combined with the Maximum Entropy Method

contributed by Amira Saunders, Michael Ghil and J. David Neelin

Department of Atmospheric Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California

Singular spectrum analysis(SSA: Vautard and Ghil 1989) and the maximum entropy method (MEM: Penland et al.1991) are combined to produce long-lead forecasts of sea-surface temperature (SST) anomalies averaged over the Niño-3 area and of the Southern Oscillation Index (SOI). The forecast is for up to one year ahead based on data from January 1950 to present. The Niño-3 averaged SSTA anomaly is calculated based on the extended Kaplan dataset (Kaplan et al.,1998).

This forecast follows up on earlier forecasts using combined SSA-MEM methodology for the SOI index by C. Keppenne and M. Ghil, starting in the March 1992 issue of this Bulletin, and on those of N. Jiang, M. Ghil and J. D. Neelin for Niño-3 SST anomalies, starting with the March 1995 issue. Detailed information on the forecast method is given by Keppenne and Ghil (1992) and in the March 1995 issue of this Bulletin (also Jiang et al. 1995). Briefly, the time series is filtered by SSA so that only the statistically significant low-frequency components are retained. Next, MEM is applied to advance these components in time. The extended components are then used in the SSA-reconstruction to produce the forecast values.

Figure 1 shows the method's Niño-3 SSTA forecasts for lead times of 3, 6, 9 and 12 months, from 1994 to the present. The forecast for each point utilizes only the appropriate part of the record that precedes the initial forecast time.

The SSA-MEM method predicts a slight warming of Niño-3 SST anomalies. Modest positive anomalies remain within error bars of normal conditions through the summer of 2001. The forecast of the SOI index supports this, as it also shows near-normal values through winter and spring 2001. The agreement between these two forecasts suggests that a strong warm event developing this winter seems highly unlikely. For fall 2001, the Niño-3 forecast announces a growing warm phase. The forecast of positive SOI for fall 2001, however, disagrees with this and thus lowers confidence in the 12-month lead forecast (Ghil and Jiang, 1998).

References:

Ghil, M., and N. Jiang, 1998: Recent forecast skill for the El Niño/Southern Oscillation. Geophys. Res. Lett.,25, 171-174.

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

Kaplan, A., M. Cane, Y. Kushnir, A. Clement, M. Blumenthal, and B. Rajagopalan,1998: Analyses of global sea surface temperature 1856-1991, Journal of Geophysical Research, 103, 18,567-18,589.

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

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.

Vautard, R., and M. Ghil, 1989: Singular spectrum analysis in nonlinear dynamics with applications to paleoclimatic time series. Physica D, 35, 395-424.

Fig. 1 . Forecasts of the area-averaged Niño-3 SST anomalies (SSTAs) using the SSA-MEM scheme. Forecasts from 1994 are for lead times of (a)3 months,(b)6 months, (c)9 months, and (d) 12 months. The dashed lines are situated each at a distance of one standard deviation from the SSA-MEM forecasts. The standard deviation value is based on forecast verification over the 1984-2000 time span.

Fig. 2. Forecast Nino-3 SSTAs for the upcoming four seasons using the SSA-MEM scheme. The solid line connects the observed Niño-3 SSTAs through August 2000. The dotted line and circles indicate the SSA-MEM forecasts for the next 4 seasons, together with the respective error bars (vertical lines).

Fig.3. SSA-MEM forecast of the SOI from September 2000 through August 2001. The circles are the monthly SOI values based on a 5-month running mean. The solid line is the SSA-filtered SOI. The last 12 circles indicate the forecast for the next 12 months.