Forecasts of Tropical Pacific SST Using a Simple
Coupled Ocean/Atmosphere Dynamical Model
contributed by Stephen Zebiak and Mark Cane
LamontDoherty Earth Observatory, Columbia
Univ., Palisades, New York
Since the middle to late 1980s, forecasts of the Nino
3 SST anomaly have been regularly made at LamontDoherty Earth Observatory
of Columbia University using a simple coupled oceanatmosphere 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 shortterm fluctuations.
Here we present a few details of this model's current forecasts of Nino
3 and the tropical Pacific basin. Forecasts using a new version of the
model with improved initialization are now additionally available; these
will be highlighted following a discussion of the forecasts of the standard
version of the model.
Figure 1 shows forecasts of the SST anomaly in Nino 3
for 3 to 15 months lead using data through April 1996, and the observed
Nino 3 SST over the past 2 years. These forecasts are actually ensemble
means of forecasts from six consecutive months ranging from November 1995
to April 1996. Forecasts are adjusted to have the same mean and standard
deviation as the observed data for each calendar month and lead time. The
vertical bars show the error standard deviations. These do not necessarily
become larger with increasing lead time, because even with an unchanging
expected fraction of SST variance explained by the forecasts, the natural
interannual SST variability changes with season. (The vertical axis is
in anomalous EC as opposed to standardized anomaly in which case the size
of the error bars would reflect only the skill of the forecasts.) In this
case the forecasts describe a continuation of somewhat below normal Nino
3 conditions, especially in fall 1996, with some return toward normal during
A closer look at the forecast integrations is provided
in Fig. 2, where six individual SST forecasts beginning from 1monthapart
initial conditions from November 1995 to April 1996 are shown along with
the ensemble mean which is used in Fig. 1. The forecasts shown in Fig.
2 may not correspond exactly to those shown in Fig. 1, because in Fig.
2 the forecasts are adjusted to have the same mean and standard deviation
as observed data only on an overall basis rather than for each calendar
month and lead time. The spread among the individual ensemble members is
reasonably small from the initialization times through 1996, but becomes
larger in 1997.
Figure 3 shows 6, 9 and 12 month lead SST anomaly forecasts
for the tropical Pacific Basin, verifying in October 1996, and January
and April of 1997, respectively. These forecasts are adjusted to have the
same mean and standard deviation as observed data on an overall basis,
so they are comparable with those shown in Fig. 2 but not necessarily Fig.
1. Like those for other target periods or for just Nino 3, the forecasts
are adjusted for systematic biases, such as an underestimation of 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 carried out to do the adjustment. The 6, 9 and 12
month lead forecast shown in Fig. 3 indicate a continuation of coolish
conditions through winter 1996-97, with some normalization in early 1997.
Recent research at Lamont has shown that the skill of
the SST forecasts can be increased significantly by improving the intialization
system (Chen et al. 1995). The existing system has used wind stress anomalies
(derived at Florida State University) to initialize the forecast runs,
without current analyzed SST data. A newly developed system allows observed
SST data to participate in the intialization process. Skill is found to
increase not only in the early part of a forecast run but at intermediate
and long leads as well. The Aspring barrier@ in skill that is present in
the existing initialization scheme is substantially reduced using the improved
When the new initialization system is applied to the current
SST forecast, the result is as shown in Fig. 4. The improved scheme produces
a forecast generally similar to the customary Lamont forecast, although
the negative anomalies continue into 1997 without abatement. The individual
initial condition trajectories for the new scheme (Fig. 5) show relatively
less warming in 1997, and much lower spread, than for the customary scheme.
Cane, M., S.E. Zebiak and S.C. Dolan, 1986: Ex-perimental
forecasts of El Nino. Nature, 321, 827832.
Cane, M. and S.E. Zebiak, 1987: Prediction of El Nino
events using a physical model. In Atmospheric and Oceanic Variability,
H. Cattle, Ed., Royal Meteorological Society Press, 153182.
Chen, D., S.E. Zebiak, A.J. Busalacchi and M.A. Cane,
1995: An improved procedure for El Nino forecast-ing: Implications for
predictability. Science, 269, 1699-1702.
Zebiak, S.E. and M.A. Cane, 1987: A model El NinoSouthern
Oscillation. Mon. Wea. Rev., 115, 2262 2278.
Fig. 1. Forecasts for the SST anomaly (oC)
in the NiZo 3 region (5oN5oS, 90o150oW),
based on the simple coupled model of Cane and Zebiak. Filled squares at
the midpoints of the vertical forecast boxes represent the predictions,
and the vertical boxes (lines) show the one (two) error standard deviations.
The solid line represents the observed three month mean SST anomaly in
NiZo 3 up to the most recently available data. The bars show forecasts
for 1 month mean SST anomalies at leads of 3, 6, 9, 12 and 15 months. See
text for additional detail.
Fig. 2. Time series of forecasts of NiZo 3 SST
from the Cane and Zebiak coupled model, for individual 1monthapart initial
conditions from November 1995 to April 1996 (dashed lines) and the ensemble
mean (solid line) used to form Fig. 1. However, an overall adjustment for
the mean and standard deviation is used rather than lead and seasonspecific
adjustments as was done for Fig. 1. The thick solid line on left side shows
the observed one month mean SST over the temporal range of the initial
Fig. 3. Cane and Zebiak coupled model SST anomaly
forecast fields for October 1996, and January and April 1997, made at 6,
9 and 12 month lead times, respectively. The forecasts are ensemble averages
of 6 forecasts with 1monthapart initial conditions ranging from November
1995 to April 1996. Adjustments for the mean and standard deviation are
applied, based on lead time but independent of start time.
Fig. 4. As in Fig. 3 except for the forecast made
using the new initialization procedure (LDEO2) (Chen et al. 1995).
Fig. 5. As in Fig. 2 except for the forecast made
using the new initialization procedure (LDEO2) (Chen et al. 1995).
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