In past 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 ocean-atmosphere model (HCM) developed jointly at Scripps Institution and the Max Planck Institute for Meteorology (MPI) (Barnett et al. 1993). In this issue of the Bulletin, the forecast produced by an improved version of the HCM is presented and compared with the forecast of the original HCM. The nature of the original and improved versions of the HCM, differing mainly in the ocean GCM, is next briefly discussed.

The original ocean model, created at MPI (Latif 1987), is a fully nonlinear GCM bounded by 30oN and 30oS 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 CCA-like regression model, using historical observed data fields of anomalous SST and the corresponding wind stress. The coupling process includes a MOS-like 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. Over the 1965-93 period the model demonstrated statistically significant predictive skill out to 12 to 18 months, with best performance for the central equatorial Pacific and for winter forecasts (The skill distribution is shown in Barnett et al. 1993 and in the March 1994 issue of this Bulletin.) The model was developed using data from 1965-85, 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^.


The HCM3

A new and improved version of the HCM, called HCM3, is now developed. The HCM3 is based on the same strategy used in the original HCM (to be called HCM1) described above. The main difference is in the ocean GCM used. The ocean model is the HOPE2 from the Max Planck Institute in Hamburg (Wolff and Maier-Reimer, 1992). The model resolution is approximately the same as before. However, the numerical scheme has been improved to significantly reduce the numerical diffusion, especially in the vertical. The result is a much better representation of the main thermocline across the tropical Pacific. A MOS corrector is still used, but in most cases and areas the magnitude of the correction is 1C and generally less--a distinct improvement over the old model. Statistical atmospheres were constructed using both the FSU and the da Silva (da Silva et al. 1994) data sets. Model performance was independent of which set was used, as long as a 3 to 5 month smoother was applied to the wind stress prior to model construction. The final model used the da Silva data for the wind field.

The model performs much better in the hindcast mode than did its predecessor. Hindcast correlational skill scores exceeding 0.8 for 3 to 6 month lead times now cover virtually the entire tropical Pacific dropping to 0.6 in the far western Pacific where the old model had negative skills. The skill also remains high almost to the South American coast, a region that also had negative skill in the old model version. Preliminary evaluation of independent sample forecast skills show they are about comparable to those of the Lamont and NCEP (formerly NMC) models. As was found with the Lamont model, the forecast skills for the 1980s and early 1990s in HCM3 are much higher (exceeding 0.8 over a large region) than were the skills during the 1970s.


Current Forecast

The HCM1 began in April 1995 to forecast a major warm event for the winter of 1996-97; this was an 18 month lead forecast. Subsequent HCM1 forecasts, repeated every month, changed little until March 1996. Then they began to diminish in magnitude. At the same time, the HCM1 began to initialize more and more poorly, in that the model SST field began differing from the observed SST at t=0 to a greater extent in spinning up the model with the observed SST. The current HCM1 forecast for December 1996 (Fig. 1, top), although still calling for a mild warm event, represents a small reduction in the event magnitude over that of the forecast made last month. Under these conditions, we place low confidence in the forecast.

The current HCM3 forecast (Fig. 1, bottom) is also for a slight warming of the tropical Pacific to be in place by the coming winter. However, the magnitude of the warming, roughly 0.3 to 0.6C in an east-west band spanning the central and eastern equatorial Pacific, is within the standard error of the HCM3 forecasts. Hence, the forecast is most properly represented as a call for "normal" conditions in the equatorial Pacific this winter.

Caveat: The forecasts shown below 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 ENSO-related predictability: Part 1 - Prediction of equatorial Pacific sea surface temperatures with a hybrid coupled ocean-atmosphere model. J. Climate, 6, 1545-1566.

Da Silva, A.M., C.C.Young and S. Levitus, 1994: Atlas of surface marine data 1994, Vol. 1: Algorithms and procedures. NOAA Atlas NESDIS 6, U.S. Department of Commerce, 83 pp.

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

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

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.

Pierce, D., J. Ritchie and T.P. Barnett, 1997: An improved hybrid coupled model for tropical SST prediction. In preparation.

Wolff, J.-O. And E Maier-Reimer, 1992: HOPE, the Hamburg ocean primitive equation model. 81 pp. Available from Max Planck Institut fur Meteorologie, Hamburg, Germany.

Fig. 1. Scripps/MPI hybrid coupled model forecast of the field of tropical Pacific SST anomaly (oC) for December, 1996. The top panel shows the forecast using the original model (HCM1) and the bottom panel shows the forecast using the new and improved HCM model version (HCM3). Observed data up to August 1996 are used, the final day of which is shown beneath each respective panel.