In several issues of this Bulletin (e.g. March and June of 1994, September 1995), we introduced forecasts of the tropical Pacific SST using a hybrid coupled ocean­atmosphere model (HCM) developed jointly at Scripps Institution and the Max Planck Institute for Meteorology (MPI) (Barnett et al. 1993). The ocean model, created at MPI (Latif 1987), is a fully nonlinear GCM bounded by 30^oN and 30^oS latitude and by Asia and South America. It has 13 vertical levels, 10 of which are within the top 300 m. The seasonal cycle is governed by a Newtonian heat flux and observed wind stress (Goldenberg and O'Brien 1981). The vertical mixing scheme is dependent upon the Richardson number (Pacanowski and Philander 1981). The atmospheric model is statistical, deriving the wind stress forcing for the ocean GCM using the GCM's SST. This is done with a CCA­like regression model, using historical observed data fields of anomalous SST and the corresponding wind stress. The coupling process includes a MOS­like statistical correction of the SST fields produced by the ocean GCM. The hybrid coupled model is initialized with wind stress fields derived from observed SST data; thus, it is indirectly "spun up" with SST information. Considering the entire 1965­93 period, the model has demonstrated statistically significant predictive skill out to 12 to 18 months, with best performance for the central equatorial Pacific and for winter forecasts (Barnett et al. 1993, and the March 1994 issue of this Bulletin show the skill distribution). The model was constructed using data from the 1965­85 period, leaving 1986 and later for independent forecasting. Partly because of the MOS correction scheme, there is no need for bias correction. However, a temporal phase postprocessor is applied to the forecasts as a function of their lead time.

Figure 1 shows the HCM forecast of tropical Pacific SST anomaly field for December 1996, using data through most of May 1996. It suggests that the currently slightly below normal SST along the equator in the eastern half of the basin will warm considerably by early winter, especially in the central region where a 2EC anomaly is forecast. The historical skill in this part of the equatorial Pacific is comparatively good, lending some confidence to the warming trend. The model has been forecasting warming in some form (i.e. varying in exact location, timing and intensity) for winter 1996-97 for the last 10 months, which might be regarded as an additional indication of reliability.

Caveat: The forecasts shown above are experimental in nature. The reader is forewarned that the methods/forecasts are very new and subject to future change and improvement.

Acknowledgment: This work is supported by NOAA and the National Science Foundation's Climate Dynamics Division.

Barnett, T.P., M. Latif, N. Graham, M. Flugel, S. Pazan and W. White, 1993: ENSO and ENSO­related predictability: Part 1 ­ Prediction of equatorial Pacific sea surface temperatures with a hybrid coupled ocean­atmosphere model. J. Climate, 6, 1545­1566.

Goldenberg, S.D. and J.J. O'Brien, 1981: Time and space variability of tropical Pacific wind stress. Mon. Wea. Rev., 109, 1190­1207.

Latif, M., 1987: Tropical ocean circulation experiments. J. Phys. Oceanogr., 17, 246­263.

Pacanowski, R.C. and S.G.H. Philander, 1981: Parameterization of vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr., 11, 1443-1451.

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