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

contributed by Dmitri Kondrashov, 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 Nino-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.

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, on those of N. Jiang, M. Ghil and J. D. Neelin for Nino-3 SST anomalies, starting from March 1995, and on those of A. Saunders, M. Ghil and J. D. Neelin from September 1997. 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 1996 to the present. The forecast for each point utilizes only the appropriate part of the record that precedes the initial forecast time. Forecasts from the end of 2001 (not included in the December 2001 ELLFB) were for near-normal conditions and recent forecasts have been trending slightly upward.

The current SSA-MEM forecast (Fig. 2) for Niño-3 SSTA is for a mild warming toward the end of 2002. The forecast SOI index (Fig. 3) is trending toward negative, although still within error bars of normal. The SOI forecast trend is generally consistent with the SSTA forecast, thus suggesting (see Ghil and Jiang 1998) an overall prediction of slightly warm conditions.

It is a pleasure to thank Amira Saunders for the almost 5 years during which she did these SSA-MEM forecasts and her help in passing the baton to Dmitri.

References:

Ghil, M., and N. Jiang, 1998: Recent forecast skill for the El Nino/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.

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.

Figure Captions:

Fig. 1: Forecasts of the area-averaged Niño-3 SST anomalies (SSTAs) using the SSA-MEM scheme. Real-time forecasts from 1997 on have been for lead times of (a) 3 months, (b) 6 months, (c) 9 months, and (d) 12 months. The solid blue line gives the observed SSTAs, data-adaptively filtered by SSA; the solid red line is the forecast; and the dashed black 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 1990-2000 time span.

Fig. 2: Forecast Niño-3 SSTAs for the upcoming four seasons. The solid blue line gives the observed SSTAs through Febuary 2002, data-adaptively filtered by SSA. The solid red 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 for the upcoming 4 seasons. The solid blue line is the SSA-filtered SOI. The solid red circles indicate the forecast for the next 4 seasons, with error bars.