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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 of Columbia 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 forecasts of the SST anomaly in Niño 3 for 3 to 15 months lead using data
through October 1996, and the observed Niño 3 SST over the past 2 years. These forecasts are
actually ensemble means of forecasts from six consecutive months ranging from May to October
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 C 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 below normal Niño 3 conditions for spring 1997,
returning to normal by win-ter 1997-98 and becoming slightly warm in early 1998.
A closer look at the forecast integrations is provided in Fig. 2, where six individual SST forecasts
beginning from 1-month-apart initial conditions from August 1996 to January 1997 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 fairly
small from the initialization times through most of 1997, becoming larger thereafter. Some
warming is indicated for winter 1997-98, followed by stronger warming in late 1998.
Figure 3 shows 6, 9 and 12 month lead SST anomaly forecast fields for the tropical Pacific Basin,
verifying in July and October 1997, and January 1998, 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 Niño 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 used for this
adjustment. The 6, 9 and 12 month lead forecasts shown in Fig. 3 indicate moderately cold
conditions for summer 1997, weakening to slightly cool by fall and slightly warm by winter
1997-98.
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 "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
Fig. 4. The improved scheme produces a forecast generally similar to the traditional Lamont
forecast at the shorter lead times, but by late fall 1997 the two runs diverge as the new version of
the model prolongs the negative anomalies without significant abatement through early 1998. The
individual initial condition trajectories for the new scheme (Fig. 5) show cool conditions persisting
through winter 1997-98, followed by warming during 1998. A low spread among the ensemble
members is noted compared with that of the original scheme.
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. Forecasts for the SST anomaly (oC) in the Niño 3 region (5oN-5oS, 90o-150oW), 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
Niño 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 Niño 3 SST produced by the Cane and Zebiak coupled model,
for individual 1-month-apart initial conditions from August 1996 to January 1997 (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 season-specific 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 conditions.
Fig. 3. Cane and Zebiak coupled model SST anomaly forecast fields for July and October 1997,
and January 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 August 1996 to
January 1997. 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).