A non­simple coupled ocean­atmosphere model has been developed for use for long­lead climate forecasts in the Coupled Model Project at NOAA's National Centers for Environmental Prediction (NCEP) (Ji et al. 1994a,b). The NCEP Medium Range Forecast (MRF) atmospheric model is used with a dynamic Pacific Basin ocean model originated at the Geophysical Fluid Dynamics Laboratory. The MRF has a reduced spatial resolution and is tuned for more realistic tropical circulation. The ocean thermal field, including SST and subsurface temperature, is initialized using an ocean data assimilation system (Ji et al. 1995). Research has shown that when observed SST fields are prescribed, this coupled model's atmospheric response is fairly reliable in the tropics but considerably less so in the extratropics. The extratropical response is best during the warm or cold phase of ENSO as reflected in the SST. Much attention has in fact been given to the prediction of ENSO itself­­the tropical Pacific SST anomaly. Such a forecast is presented here.

In the September and December 1993 issues of this Bulletin, the expected forecast skill of the coupled model version used in 1993 (called CMP6) was shown. A horseshoe­shaped spatial pattern of maximum model skill was noted, with highest equatorial skill near the date line and higher skill just north or south of the equator than immediately along it to the east of 165^oW. Mean skill for the NiZo 3 and NiZo 4 regions was shown as a function of season and lead time. The model generally outperformed persistence by a substantial margin. A seasonal dependence in skill was noted, where forecasts were affected by a "spring barrier" as found both in other dynamical as well as statistical predictive models.

Starting with the forecasts presented in the September 1994 issue, the NCEP coupled model was upgraded in several ways, including a refinement of the flux climatology and the installation of a MOS correction for the stress anomalies produced by the atmospheric model. Thus, skills became higher than those cited above; e.g. the high skill horseshoe "thickened" along the equator, extending farther eastward into the western part of the NiZo 3 region. Figures 2-1 and 2­2 of the September 1994 issue show a comparison of hindcast skills between the newer (called CMP9) and the previous model versions as a function of lead time for NiZo 3 and NiZo 4, respectively.

During the last few months, a still newer version of the NCEP model has been developed, called CMP10 (Ji et al. 1996). A major difference between CMP9 and CMP10 is in the heat flux coupling: CMP9 contained a negative feedback procedure for coupling the anomalous net heat flux, while CMP10 uses anomaly coupling for the net heat flux forcing. While mean skills are not as different between CMP9 and CMP10 as they are for CMP6 and CMP9, CMP10 behaves more realistically for high amplitude SST anomalies. CMP9, with its negative feedback mechanism, ran the danger of damping strong ENSO events too much and/or too soon. A still further improved model version, called CMP12, is soon to be operational.

The CMP9 and CMP10 coupled model forecasts for the SST anomaly field averaged over Jun-Jul-Aug 1996 (no lead), Sep-Oct-Nov 1996 (3 months lead) and Dec-Jan-Feb 1996-97 (6 months lead) are shown in Fig. 1, where the systematic model bias for hindcasts over the 1983­95 period has been subtracted. This forecast is actually the mean of an ensemble of 7 to 11 individual cases, each based on a different one- to two­week­apart initial ocean condition ranging from early April through early June 1996. The CMP10 forecast suggests a continuation of cool conditions in the eastern half of the basin through winter, although only in a meridionally narrow band, and the winter forecast indicates warmth near the dateline. The CMP9 forecast calls for near to slightly above normal SST throughout the period. Fig. 2 shows the NiZo 3 forecast in the form of a time series for the three lead times used to form the 3-month forecast averages used in Fig. 1 for both versions of the model.

The observed anomalous SST and subsurface equatorial temperature field for the week centered on June 5 (Fig. 3) show a region of negative subsurface sea temperature anomalies in the eastern tropical Pacific Basin. Positive subsurface anomalies appear in the western Pacific, which may presage a return to warm conditions farther east at the surface in 12-18 months (Smith et al. 1995). While near normal SST prevails in the central basin, mild positive anomalies lie underneath.

Ji, M., A. Kumar and A. Leetmaa, 1994a: A multi­season climate forecast system at the National Meteorological Center. Bull. Am. Meteor. Soc., 75, 569­577.

Ji, M., A. Kumar and A. Leetmaa, 1994b: An experimental coupled forecast system at the National Meteorological Center: Some early results. Tellus, 46A, 398-418.

Ji, M., A. Leetmaa and J. Derber, 1995: An ocean analysis system for seasonal to interannual climate studies. Mon. Wea. Rev., 123, 460-481.

Ji, M., A. Leetmaa and V.E. Kousky, 1996: Coupled model forecasts of ENSO during the 1980s and 1990s at the National Meteorological Center. J. Climate, 9, in press.

Smith, T.M., A. G. Barnston, M. Ji and M. Chelliah, 1995: The impact of Pacific Ocean subsurface data on operational prediction of tropical Pacific SST at the NCEP. Wea. Forecasting, 10, 708-714.

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