Forecasts of Tropical Pacific SST Using a Comprehensive Coupled

Ocean-Atmosphere Dynamical Model

contributed by Ming Ji, Arun Kumar and Ants Leetmaa

National Centers for Environmental Prediction, NOAA, Camp Springs, Maryland

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 most realistic 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 in many other dynamical as well as statistical predictive models.

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 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 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 CMP12 coupled model forecasts for the SST anomaly field averaged over Jul-Aug-Sep 1997 (no lead), Oct-Nov-Dec 1997 (3 months lead) and Jan-Feb-Mar 1998 (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 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 1997.

The forecast shows further warming even relative to the significant warming that has already been observed during March-June 1997. By northern fall 1997, and continuing through winter 1997-98, all of the equatorial Pacific east of the date line is to exceed 1.5C and much of the region from Niño 3.4 eastward is to exceed 2C. Figure 2 shows the Niño 3 and Niño 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. In this format the continued warming trend in the SST forecast is also clear, as is the fact that the mo-del has been predicting warming for at least 6 months.

The observed anomalous SST and subsurface equatorial temperature field for the week centered on June 4 (Fig. 3) is dominated by positive sea temperature anomalies from the surface to about 150 m depth in the eastern, central, and much of the western tropical Pacific Basin. Positive subsurface anomalies of 5C or more exist in the eastern and central Pacific at roughly 50-125 m, underlying weaker positive anomalies at the surface. Compared with 3 months ago, the positive anomalies in all but the extreme west have greatly strengthened, while negative subsurface anomalies near 100-175 m depth have appeared in the extreme west. The strong SST anomaly increases in the central and eastern tropical Pacific might have been expected on the basis of the empirically based 1 to 1.5 year lag between the anomaly in the subsurface western Pacific and the future SST farther east (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 coupled model SST anomaly forecast fields for Jul-Aug-Sep 1997, Oct-Nov-Dec 1997, and Jan-Feb-Mar 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, each based on a different one- to two-week-apart initial ocean condition ranging from early April through early June 1997 (see text).

Fig. 2. NCEP coupled model SST anomaly forecast time series for Niño 3 and Niño 3.4 for Jun, Jul, Aug 1997 (as 1-month means; top panels); Sep, Oct, Nov (middle panels); and Dec, Jan, Feb 1997-98 (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, respectively, and each produced by forecasts from two to three individual 1 to 2-week-apart initial conditions per month.

Fig. 3. Equatorial depth-longitude section of ocean temperature anomaly with respect to the 1983-92 mean for the week centered on June 4, 1997. Dashed contours denote negative anomalies.

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