Dynamical Forecasts of Tropical Pacific SST

contributed by Tony Barnston¹, Ming Ji², Arun Kumar², Wanqiu Wang² 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, Maryland

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). As with most AGCMs, this coupled model's atmospheric response to observed tropical Pacific SST is fairly reliable in the tropics but considerably less so in the extratropics. The extratropical response is realistic during ENSO extremes, in terms of the shift of the probability density function 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 165^oW. 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/images/cmb.fsst.gif>.

The NCEP coupled model forecasts for the SST anomaly field averaged over Dec-Jan-Feb 1999-2000, Mar-Apr-May and Jun-Jul-Aug of 2000 are shown in Fig. 1, where the systematic model bias for hindcasts over the 1981-95 period has been removed. This forecast is 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 September through early December 1999. The forecasts show the continuation moderate La Niña conditions through winter 1999-2000, followed by dissipation in spring and virtual disappearance in summer of 2000. The subsurface equatorial sea temperature (not shown) still indicates a good supply of below-normal water temperature beneath the surface in the eastern part of the basin. However, warm subsurface water has gradually has building in the western Pacific, especially from 100 to 150 m deep, and is slightly nosing underneath the cold water just eastward of the dateline.

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.

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.

LEAD	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ	DJF
1-mon	90	87	80	76	78	79	80	84	88	91	94	92
2-mon	89	84	77	72	73	74	76	81	86	89	92	91
3-mon	88	81	74	68	68	70	72	75	83	87	90	90
4-mon	88	79	68	64	63	66	66	68	70	84	88	90

Figure Captions

Fig. 1. NCEP coupled model SST anomaly forecast fields for Dec-Jan-Feb, Mar-Apr-May and Jun-Jul-Aug 2000. 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 1- to 2-week-apart initial ocean condition ranging over the last three months (see text). These forecasts were made at the time shown at the bottom of the figure.