Forecasts of Niño-3 SST Anomalies and SOI Based on

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.

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 SSA-reconstruction to produce the forecasted 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 warming of Niño-3 SST anomalies attaining modest positive anomalies that remain within error bars of normal conditions, until end of the calendar year, and decay afterwards. Correspondingly, the forecast SOI index shows a gradual decline until December 2000 to a very mild negative value, and then tends back toward normal conditions. The agreement between these forecasts suggests that although the cold event terminated it still seems unlikely that a strong event will develop during the coming year.

References:
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.

Fig. 1. Forecasts of the area-averaged Niño-3 SST anomalies (SSTAs) using the SSA-MEM scheme. The solid blue line connects the observed Niño-3 SSTAs (diamonds). The latest forecast starts from June 2000 and is shown forlead times of (a) 3 months, (b) 6 months, (c) 9 months, and (d) 12 months. In each figure the solid (red) line connects the individual forecasts (circles). The dashed lines are situated each at a distance of one standard deviation from the SSA-MEM forecasts. The standard deviation value is bsed on forecast verification over the 1984- 2000 time span.

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

Fig. 3. SSA-MEM forecast of the SOI from June 2000 through May 2001. The black circles are the monthly SOI values based on a 5-month runningmean without the seasonal cycle. The solid (blue) line is the SSA-filtered SOI. The last 12 circles (red) indicate the forecast for the next 12 months.