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 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 48 years of observed data.

This forecast follows up on earlier forecasts using combined SSA-MEM methodology by C. Keppenne and M. Ghil for the SOI index, 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. 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: Keppenne and Ghil 1993) combined with MEM is documented in the March 1995 issue of this Bulletin (see also 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 quasi-quadrennial (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 3- 6- 9- and 12-month lead. 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 1984-93 time span The latest forecasts, for the next 1-4 seasons, using data through May 1998, are shown in Fig. 2. Figure 3 shows the forecast SOI for the next 12 months becoming positive and steadily increasing throughout the year towards a moderate peak.

The model is predicting only a very small La Niña in early 1999. This must be treated with some caution, in that the model has been consistently lagging the observations during the rapid changes of the past warm event, as may be expected from a model that contains only low-frequency components. Current observations are already colder than the model prediction for 3 months from now. We interpret the forecasts as being likely to underpredict the apparent currently developing cold phase, but they do suggest that it lacks support from low-frequency components of the system and thus is less likely to be a large one.

References:

Ghil, M. And N. Jiang, 1997: Recent forecast skill for the El Niqo/Southern Oscillation. Geophys. Res. Lett., 102, in press.

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: An application to subannual variability in atmospheric angular momentum. Intl. J. Bif. & Chaos, 3, 625-634.

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.

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

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

Figure. 1. Forecasts of the area-averaged Niño-3 SST anomalies (SSTA) using the SSA-MEM (green solid line) and MSSA-MEM (thick dashed red line) schemes. The thick blue line connects the observed (blue circles) Niño-3 SSTA. The forecasts, are shown for (a) 3-month lead and (b) 6-month lead (c) 9-month lead and (d) 12-month lead. The thin dashed lines are each located at a distance of one standard deviation from the SSA-MEM forecasts.

Figure. 2. The forecasts (open circles) of the Niño-3 SSTA for the upcoming 4 seasons using the SSA-MEM scheme. The solid line connects the observed Niño-3 SSTA(full circles) through August 1998.

Figure. 3. SSA-MEM forecast of the SOI for September 1998 through August 1999. The circles are the monthly SOI values based on a 5-month running mean without the seasonal cycle. The solid (blue) line is the SSA-filtered SOI. The dashed (red) line indicates the forecast for the next 12 months.