Dynamically Based Forecasts

for Tropical Pacific SST through Fall 1998

Using a Hybrid Coupled Ocean-Atmospheric Model

contributed by Tim Barnett¹, David Pierce¹, Nicholas Graham¹ and Mojib Latif²

¹Scripps Institution of Oceanography, La Jolla, California

²Max Planck Institut fur Meteorologie, Hamburg, Germany

Forecasts of the tropical Pacific SST are presented here 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 1996 an improved version of the HCM, called HCM-3, was used instead of the original HCM-1.

The original HCM-1 ocean model, created at MPI (Latif 1987), is a fully nonlinear GCM bounded by 30N-30S latitude and by Asia and South America. It has 13 vertical levels, with 10 in 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 fields of 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 HCM 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 12-18 months, with best performance for the central equatorial Pacific and for winter forecasts (Barnett et al. 1993). The model was developed using data from 1965- 85, leaving 1986 onward for independent forecasting.

The improved HCM-3 (Pierce et al. 1997) is similar to the HCM-1 in most respects. The main differ-ence is in the ocean GCM used, which is the HOPE2 from the Max Planck Institute in Hamburg (Wolff and Maier-Reimer 1992). While the resolution is approxi-mately as in HCM-1, the numerical scheme is improved to reduce the numerical diffusion, especially in the vertical, resulting in a better representation of the main thermocline across the tropical Pacific. A MOS correct-or is still used, but the magnitude of the correction is generally only 1C or less--a marked improvement over HCM-1. Statistical atmospheres were constructed using both the FSU and the da Silva (da Silva et al. 1994) wind 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 construc-tion. The final model used the da Silva wind data.

The HCM-3 model produces better hindcasts than did HCM-1, with correlation skill scores exceeding 0.8 for 3-6 month lead times covering most of the tropical Pacific, dropping to 0.6 in the far west. The skill is also moderately high nearly to the South American coast. Independent sample forecast skills are approximately comparable to those of the LDEO and NCEP models. In similar fashion to the LDEO, skills for the 1980s and early 1990s are much higher than during the 1970s.

The Hybrid Coupled Model is predicting strong warm conditions (anomalies in excess of 2.5C) through the end of 1997 and into early 1998. The predicted warming fades in spring of 1998, and is replaced by cold conditions by the latter half of 1998. Note that historically, the Hybrid Coupled Model has over-predicted the strength of cold events following large warm events.

Caveat: The forecasts shown above are experimental in nature. The reader is forewarned that the methods/forecasts are 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 (HCM-3) forecast of the field of tropical Pacific SST anomaly (^oC) for DJF 1997-98, and MAM, JJA and SON 1998. Observed data up to Nov 29 1997 are used. Contour interval 0.25^oC; contours of magnitude less than 0.5C not shown.

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