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 49 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), and in the March 1995 issue of this Bulletin (see also Jiang et al. 1995). Briefly, the timeseries is filtered first by SSA, 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, reconstructed, and observed, since 1994, using the SSA-MEM scheme for 3- 6- and 9-12-month lead.
The latest forecasts, for the next 12 months, using data through August 1999, are shown in Fig. 2. The method predicts a slow warming of Niño 3 anomalies throughout the year, remaining in the cold phase throughout the winter and crossing over to very mild warm anomalies in the spring of 2000.
The SSA-MEM forecast for the SOI from September 1999 through August 2000 is presented in Fig. 3. In accordance with the SSTA prediction the SOI is expected to decline throughout the year and attain slightly negative values in the coming Spring.
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Jiang, N., D. Neelin and M. Ghil, 1995: Quasi-quadrennial and quasi-biennial variability in the equatorial Pacific. Clim. Dyn., 12, 101-112.
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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 Captions :
Fig. 1. Forecasts of the Niño 3 SST anomalies (SSTA) using the SSA-MEM scheme. The blue (solid) line connects the observed Niño 3 SSTA (blue circles). The latest forecast starts from September of 1999 and is shown for (a) 3-month lead and (b) 6-month lead (c) 9-month lead and (d) 12-month lead. The green (thick solid) line connects the area-average of the MSSA-MEM forecasts and the red (thin solid) line indicates the SSA-MEM forecasts. The dashed lines are situated 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.
Fig. 2. The forecasts of the Niño 3 SSTA for the upcoming four seasons using the SSA-MEM scheme. The solid (blue) line connects the observed Niño 3 SSTA through August of 1999. The (red) diamonds indicate the SSA-MEM forecasts for the next 12 months. the vertical (violet) lines are the error bars.
Fig. 3. SSA-MEM forecast of the SOI for September 1999 through August 2000.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.