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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 winter
1996-97, returning to normal by fall 1997.
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 May to October 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 small to moderate from the
initialization times through most of 1997, becoming large in 1998. A gradual warming during 1997 is forecast to lead
to further warming in 1998.
Figure 3 shows 6, 9 and 12 month lead SST anomaly forecast fields for the tropical Pacific Basin, verifying in
April, July and October 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 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 carried out to do the adjustment. The 6, 9 and 12 month lead forecasts shown in Fig. 3
indicate moderately cold conditions for spring 1997, gradually weakening to slightly cool by summer and near-normal
by fall.
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, although the negative
anomalies continue through fall 1997 without abatement. The individual initial condition trajectories for the new scheme
(Fig. 5) show slightly cool conditions persisting through 1997 and 1998, with much lower spread than for 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 forecast-ing:
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 from the Cane and Zebiak coupled model, for individual
1-month-apart initial conditions from May to October 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
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 April, July and October 1997, 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 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).