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Forecasts of Tropical Pacific SST

Using a Simple Coupled Ocean-Atmosphere Dynamical Model

contributed by Stephen Zebiak and Mark Cane

Lamont-Doherty Earth Observatory, Columbia Univ., Palisades, New York



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 April, July and October of 1998, respectively. The forecasts are actually ensemble means of forecasts from six consecutive months ranging from May to October 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 that the somewhat warm conditions that will remain from the current El Niño in April 1998 will dissipate and turn to cold by boreal summer, intensifying further by fall.

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 May to October 1997 are shown along with the ensemble mean used for Fig. 1. The spread among the individual ensemble members is not large through boreal winter 1998-99. For summer 1998 the spread is low and normal SST is forecast.

Research at Lamont demonstrated that the skill of the SST forecasts could be increased 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, with additional influence from the most recent SST data. The improved system also does this, but allows the observed SST data to participate less disruptively in the intialization process, allowing the model to continue doing what it was "setting up" to do without the observed SST influence. The resulting increases in skill occurred most strongly in the early part of a forecast run, but also at the 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 (called LDEO2) 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 somewhat warmer SST forecast than the traditional Lamont forecast, and the cooling seen in the traditional model does not occur in LDEO2 (Figs. 3, 4). The individual initial condition trajectories for LDEO2 (Fig. 3) show moderate spread among the ensemble mem-bers. Both versions of the model are initializing with SST anomalies in the eastern tropical Pacific below what were observed for October.

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. In Atmospheric 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 May to October 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 May to October 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).



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