Since the middle to late 1980s, forecasts of the Niño 3 SST anomaly have been regularly made at Lamont-Doherty Earth Observatory (LDEO) of Colum-bia University using a simple coupled ocean-atmosphere dynamical model (Cane et al. 1986, Cane and Zebiak 1987, Zebiak and Cane 1987). This represented the beginning of a strong movement toward physical approaches to the diagnosis and prediction of climate and its short-term fluctuations. Here we present a few details of this model's current forecasts of Niño 3 and the tropical Pacific basin. Forecasts using a new version of the model with improved initialization have been issued since the latter half of 1995; these will be highlighted following a discussion of the forecasts of the standard version of the model.

Figure 1 shows 6, 9 and 12 month lead SST anomaly forecast fields for the tropical Pacific Basin, verifying in January, April and July of 1998, respectively. The forecasts are actually ensemble means of forecasts from six consecutive months ranging from February to July of 1997. The forecasts are adjusted to have the same mean and standard deviation as observed data on an overall basis (as opposed to individually for each calendar month and lead time, as in Lamont's forecast tables that are not presented here). The forecasts are adjusted for systematic biases. One such bias is an underestimation of the amplitude of anomalies in the central (but not eastern) Pacific, which would cause anomaly maxima to be placed too far east or prevent the central Pacific from fully participating. A statistical correction using singular value decomposition (SVD) is used for this adjustment. The 6, 9 and 12 month lead forecasts shown in Fig. 1 indicate fairly neutral conditions (but with some warmth near the date line) for boreal winter 1997-98, and a cooling trend as 1998 progresses.

A closer look at the forecast integrations for the Niño 3 region in particular is provided in Fig. 2, where six individual SST forecasts beginning from 1-month-apart initial conditions from February to July 1997 are shown along with the ensemble mean used for Fig. 1. The spread among the individual ensemble members is moderate for boreal fall/winter 1997-98, with several runs indicating warmth and others suggesting neutral conditions. For summer 1998 the spread is low and normal SST is forecast.

Research at Lamont showed that the skill of the SST forecasts is increased significantly by improving the intialization system (Chen et al. 1995). The original system uses wind stress anomalies (derived at Florida State University) to initialize the forecast runs, without current analyzed SST data. The improved system allows observed SST data to participate more efficiently in the intialization process. The resulting skill increases not only in the early part of a forecast run but also at longer leads. The "spring barrier" in skill that is present in the original initialization scheme is substantially reduced using the improved system.

When the new initialization system is applied to the current SST forecast, the result is as shown in Figs. 3 and 4 (analogous to Figs. 2 and 1, respectively). The newer scheme produces a forecast roughly similar to the traditional Lamont forecast, but remains slightly cool throughout the next year (Fig. 4) rather than beginning close to normal and cooling as the year progresses. The individual initial condition trajectories for the new scheme (Fig. 3) show high agreement among the ensemble members for slightly cool conditions through the remainder of 1997 and the first 1 to 2 seasons of 1998. Both versions of the model are initializing with SST anomalies in the eastern tropical Pacific much below what were observed for July. This initialization behavior is being examined closely to determine why the models so far have not handled the current warm event as skillfully as they handled past events such as 1982-83 or 1986-87, or the cold event of 1988-89.

Cane, M., S.E. Zebiak and S.C. Dolan, 1986: Ex-perimental forecasts of El Niño. Nature, 321, 827-832.

Cane, M. and S.E. Zebiak, 1987: Prediction of El Niño events using a physical model. InAtmospheric and Oceanic Variability, H. Cattle, Ed., Royal Meteorological Society Press, 153-182.

Chen, D., S.E. Zebiak, A.J. Busalacchi and M.A. Cane, 1995: An improved procedure for El Niño forecasting:Implications for predictability. Science, 269, 1699-1702.

Zebiak, S.E. and M.A. Cane, 1987: A model El Niño-Southern Oscillation. Mon. Wea. Rev., 115, 2262- 2278.

Fig. 1. LDEO1 SST anomaly forecast fields for January, April and July 1998, made at 6, 9 and 12 month lead times, respectively. The forecasts are ensemble averages of 6 forecasts with 1-month-apart initial conditions ranging from February to July 1997. Adjustments for the mean and standard deviation are applied, based on lead time but independent of start time.

Fig. 2. Time series of forecasts of Niño 3 SST produced by the original Cane and Zebiak coupled model (LDEO1), for individual 1-month-apart initial conditions from February to July 1997 (dashed lines) and the ensemble mean (solid line). The thick solid line on left side shows the observed one month mean SST over the temporal range of the initial conditions.

Fig. 3. As in Fig.2 (time series of forecasts from individual initial conditions) except for the forecast made using the newer initialization procedure (LDEO2) (Chen et al. 1995).

Fig. 4. As in Fig. 1 (forecast anomaly fields) except for the forecast made using the newer initialization procedure (LDEO2) (Chen et al. 1995).