A non-simple coupled ocean-atmosphere model has been developed for use for long-lead 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 itself--the 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 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 Niño 3 and Niño 4 regions. 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 with a refinement of the flux climatology and the installation of a MOS correction for the stress anomalies produced by the atmospheric model. Skills consequently improved as the high skill area extended farther eastward into the western part of the Niño 3 region. Figures 2-1 and 2-2 of the September 1994 issue show a comparison of hindcast skills between the improved (CMP9) and the previous model versions as a function of lead time for Niño 3 and Niño 4.

In spring 1995, another improvement was implemented, resulting in the CMP10 version (Ji et al. 1996). While CMP9 contained a negative feedback procedure for coupling the anomalous net heat flux, CMP10 used anomaly coupling for the net heat flux forcing. Mean skills were not as different between CMP9 and CMP10 as they were for CMP6 and CMP9; however, CMP10 behaved 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.

The most recent improvement of the NCEP coupled model was completed during summer 1996, and is used in the forecasts presented here. This current version, called CMP12, has the highest hindcast skills in the model's history over the central and eastern equatorial Pacific over 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, as well as model improvements such as better mixing in the ocean model and more representative anomalous e-p (evaporation-precipitation) 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 forecasts are ensemble averages of 9 individual runs, each using different initial atmospheric conditions based on 1-day-apart transient atmospheric states resulting from integrations using 1-day-old start times successively staggered by 1 day (i.e., each dependent on the previous outcome, and the first dependent on a 15-day-old start time).

The CMP12 coupled model forecasts for the SST anomaly field averaged over Sep-Oct-Nov 1996 (no lead), Dec-Jan-Feb 1996-97 (3 months lead) and Mar-Apr-May 1997 (6 months lead) are shown in Fig. 1, where the systematic model bias for hindcasts over the 1981-95 period has been subtracted. This forecast is the mean of an ensemble of 9 individual cases as described above. The forecast shows a tendency for warming between fall 1996 and spring 1997, bringing the anomaly into mildly positive territory. The outlook described by this forecast is warmer than that of the same CMP12 model 5 to 6 weeks ago (beginning of August), the time of the transition from CMP10 when both the model versions were forecasting continuation of neutral to somewhat cool conditions. Fig. 2 shows the Niño 3 and Niño 3.4 forecasts in the form of a time series for the three lead times used to form the 3-month forecast averages used in Fig. 1. The upward-trending SST forecast is clear.

The observed anomalous SST and subsurface equatorial temperature field for the week centered on August 28 (Fig. 3) show negative subsurface sea temperature anomalies in much of the eastern and east-central tropical Pacific Basin. Positive subsurface anomalies appear in the western Pacific and in the central Pacific at roughly 150 m. These warm sea temperatures have not progressed eastward over the last 3 months, indicating a pause in the anticipated transition to warm conditions farther east at the surface over the coming year (Smith et al. 1995).

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, 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.

Fig. 1. NCEP (formerly NMC) coupled model SST anomaly forecast fields for Sep-Oct-Nov 1996, Dec-Jan-Feb 1996-97, and Mar-Apr-May 1997. The CMP12 version of the model is used. Each forecast is an average of 9 individual ensemble members, each based on a different one-day-apart initial atmospheric condition (see text).

Fig. 2. NCEP coupled model SST anomaly forecast time series for Niño 3 and Niño 3.4 for Jul-Aug-Sep 1996 through Mar-Apr-May 1997 at 1-month increments. The broken line in each panel represents the SST anomaly forecast (^oC), and the solid line the observed SST anomaly. Each forecast is an average of 9 individual ensemble members, each based on a different one-day-apart initial atmospheric condition (see text).

Fig. 3. Equatorial depth-longitude section of ocean temperature anomaly with respect to the 1983-92 mean for the week centered on August 28, 1996.