A non-simple coupled ocean-atmosphere model has been developed for use in long-lead climate forecasting in the Coupled Model Branch of the Environmental Modeling Center (EMC) 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). It has been shown that when observed SST fields are prescribed, the coupled model's atmospheric response is fairly reliable in the tropics but considerably less so in the extratropics, as with most AGCMs. The extratropical response is most realistic during the warm or cold phase of ENSO. The nature of the model's mid-latitude atmospheric response to the ENSO state has been treated in some detail in Livezey et al. (1997). However, while better prediction of the extratropical atmosphere is an ultimate goal, much attention has been given to the more basic prediction of ENSO itself-the tropical Pacific SST anomaly field. Such an SST 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 horseshoe-shaped 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. The model generally outperformed persistence by a substantial margin in forecasting the Nino 3 and Nino 4 regions. A seasonal dependence in skill was noted, where forecasts were affected by a "spring barrier" as found in many other dynamical and statistical predictive models. As discussed in Barnston et al. (1994), the skill of CMP6's forecasts of tropical Pacific SST was competitive with that of other dynamical and statistical models in early 1994.

Starting with the forecasts presented in the September 1994 issue, the model was upgraded with a refinement of the flux climatology and the installation of a MOS (for "model output statistics") correction for the stress anomalies produced by the atmospheric model. Skills consequently improved and the high skill area extended farther eastward into the western part of the Nino 3 region (see Figures 2-1 and 2-2 of the September 1994 issue). The newer model version was named CMP9. In spring 1995, another improvement was implemented, resulting in the CMP10 version (Ji et al. 1996). The CMP9 contained a negative feedback procedure for coupling the anomalous net heat flux; CMP10, by contrast, used anomaly coupling for the net heat flux forcing. While mean skill differences between CMP9 and CMP10 were not as marked as between CMP6 and CMP9, CMP10 behaved more realistically for high amplitude SST anomalies. CMP9, with its negative feedback mechanism, sometimes damped strong ENSO events too much and/or too soon.

The most recent improvement of the NCEP coupled model was completed during summer 1996, and is used in the current forecasts. The resulting model version (CMP12) has the highest hindcast skills in the model's history over the central and eastern equatorial Pacific for 1981-95, exceeding 0.8 correlation skill in much of that area for 4-6 month lead hindcasts. Upgrades in CMP12 include improvements in the data assimilation system, and model improvements such as better mixing in the ocean model and more representative anomalous evaporation-precipitation (e-p) flux forcing in the coupling. CMP12 forecasts out to 6 months lead are now updated on a weekly basis and are available on Internet site http://nic.fb4.noaa.gov:8000/research/climate.html.

The CMP12 coupled model forecasts for the SST anomaly field averaged over Jun-Jul-Aug, Sep-Oct-Nov and Dec-Jan-Feb 1998-99 are shown in Fig. 1, where the systematic model bias for hindcasts over the 1981-95 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 two-week-apart initial ocean condition ranging from early April through early June 1998. The forecasts show the onset of a moderate La Nina with the approach of winter 1998-99. This forecast set is substantially colder than that issued in the March issue of this Bulletin. In fact, the observed conditions have already returned to normal to slightly below normal, as the low-level easterly trades along the equator returned to normal strength as of early June. Figure 2 shows the Nino 3 and Nino 3.4 forecasts in the form of time series for the three lead times used to form the 3-month forecast averages used in Fig. 1. The demise of the El Nino and the onset of cool conditions late in the year are apparent. The forecasts in Fig. 2 seem less cold than those in Fig. 1. This may be because the Nino 3 and Nino 3.4 regions extend 5^o north and south of the equator, while the coolest water is expected to be concentrated in a narrower band along the immediate equator.

The observed anomalous SST and subsurface equatorial temperature field for a recent 7-day period (Fig. 3) shows that the previously strong positive sea temperature anomalies in the eastern tropical Pacific Basin are being replaced by negative anomalies which had been limited to the subsurface until the last several weeks. The surfacing of this cold water materialized as a result of the re-emergence of easterlies along the equatorial Pacific. Some warmer-than-normal water is now found in the far west Pacific near a depth of 100 m. This may be the first sign of a warm episode that will follow the period of below-normal SST that is now just beginning (see the statistical study of Smith et al. 1995).

References:

Barnston, A.G., H.M. van den Dool, S.E. Zebiak, T.P. Barnett, M. Ji, D.R. Rodenhuis, M.A. Cane, A. Leetmaa, N.E. Graham, C.F. Ropelewski, V.E. Kousky, E. A. O'Lenic and R.E. Livezey, 1994: Long-lead seasonal forecasts--Where do we stand? Bull. Amer. Meteor. Soc., 75, 2097-2114. 

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

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, 3105-3120. 

Livezey, R.E., M. Masutani, A. Leetmaa, H. Rui, M. Ji and A. Kumar, 1997: Teleconnective response of the Pacific-North American region atmosphere to large central equatorial Pacific SST anomalies. J. Climate, 10, 1787-1820.

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.

Figure Captions

Fig. 1. NCEP coupled model SST anomaly forecast fields for Jun-Jul-Aug, Sep-Oct-Nov, and Dec-Jan-Feb 1997-98. The CMP12 version of the model is used. Each forecast is an average of about 13 individual ensemble members, each based on a different mean of an ensemble of 7 to 11 individual cases based on a different one- to two-week-apart initial ocean condition ranging from early April through early June 1998 (see text). These forecasts were made June 3, 1998.

Fig. 2. NCEP coupled model SST anomaly forecast time series for Nino 3 and Nino 3.4 for three 1-month-apart running 3-month means starting with Apr-May-Jun 1998 (top panels), starting with Jul-Aug-Sep, Aug-Sep-Oct (middle panels), and starting with Nov-Dec-Jan 1998-99 (bottom panels). The broken line in each panel represents the SST anomaly forecast (^oC), and the solid line the observed SST anomaly. The predictions represent the mean of three ensemble mean forecasts, each for one of the 3 most recent months. These forecasts were made June 3, 1998. 

Fig. 3. Equatorial depth-longitude section of ocean temperature anomaly with respect to the 1983-92 mean for a 7-day average centered on 20 May 1998. Dashed contours denote negative anomalies.