Dynamical Forecasts of Tropical Pacific SST

contributed by Wanqiu Wang², Ming Ji², Arun Kumar² and Ants Leetmaa¹

¹Climate Prediction Center, National Centers for Environmental Prediction, NOAA, Camp Springs, Maryland

²Environmental Modeling Center, National Centers for Environmental Prediction, NOAA, Camp Springs, MarylandA 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). As with most AGCMs, this coupled model's atmospheric response to observed tropical Pacific SST is relatively reliable in the tropics and less so in the extratropics. The extratropical response is realistic during ENSO extremes, in terms of the shift of the probability density away from its climatological location. While skillful prediction of the extratropical atmosphere is a major goal, a prerequisite is the ability to predict ENSO itself--the tropical Pacific SST anomaly field. Such SST forecasts for the upcoming few seasons are presented here.

As has been the case since the original presentation of the NCEP model in this Bulletin in 1993, the expected forecast skill field in the tropical Pacific ocean has a horseshoe-shaped pattern with highest equatorial model skill near the date line and higher skill just north or south of the equator than immediately along it to the east of 165oW. Since 1993 several major improvements have occurred in the model, resulting in higher skill (Ji et al. 1995, 1996). The more recent versions use an ocean data assimilation system (Ji et al. 1995). The basic skill characteristics of the earlier versions of the model remain, including its geographical distribution and season dependence (e.g. a relative spring skill barrier). The approximate skill of the SST forecasts for SST in the Niño 3.4 region for the first four lead times is shown in Table 1, based on the 1982-97 period. NCEP model forecasts out to 6 months lead are now updated on a weekly basis and are available on Internet site <http://www.emc.ncep.noaa.gov/cmb/sst_forecast/>.

The NCEP coupled model forecasts for the SST anomaly field averaged over Jun-Jul-Aug 2003, Sep-Oct-Nov 2003, and Dec-Jan-Feb 2003/2004 are shown in Fig. 1. In this plot the systematic model bias based on hindcasts over the 1981-95 period has been removed. The forecasts are based on the mean of an ensemble of 16 individual cases. These cases were produced with four, one week apart, oceanic initial conditions ranging from early to late May 2003. Further, for each oceanic initial condition, four integrations with differing atmospheric initial conditions were made. The model forecasts below-normal SST anomalies for Jun-Jul-Aug 2003 in the central and eastern tropical Pacific. These anomalies are weakened substantially in Sep-Oct-Nov 2003. In Dec-Jan-Feb 2003/2004, the forecast SSTs are near normal over the entire tropical Pacific.

Figure 2 shows the time series of the forecasts of Niño 3.4 SST from the 16 individual members. The solid line represents the 16-member ensemble mean. All individual forecasts show cold anomalies in Jun-Jul-Aug 2003. Most of these anomalies get weakened with time starting Aug 2003 and become near normal in boreal winter of 2003/2004. The spread among the members representing the forecast uncertainty for this prediction system is roughly 0.75 degrees K.

References:

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.

Figure captions:

Table 1. Expected skill (expressed as a temporal correlation, X100) of the NCEP coupled model in predicting the SST anomaly in the Niño 3.4 region at 4 lead times for 12 overlapping 3-month target seasons. A 1-month lead is, for example, a forecast for JFM made at the end of December.

Fig. 1. NCEP coupled model SST anomaly forecast fields for Jun-Jul-Aug 2003, Sep-Oct-Nov 2003, and Dec-Jan-Feb 2003/2004. Each forecast is an average of 16-member ensemble (see text).

Fig. 2. Observed (single line at left) and predicted (multiple lines after the present date) SST anomalies for the Nino 3.4 region from NCEP coupled model. Four sets of model integrations, each for one of four ocean initial conditions of May 2003, are shown (corresponding legends are shown inside the figure). For each ocean initial condition, four integrations with differing atmospheric initial conditions were made. The thick solid line shows the 16-member ensemble mean.