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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).