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 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 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 Mar-Apr-May, Jun-Jul-Aug and Sep-Oct-Nov 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 16 individual cases, each based on a different one- week-apart initial ocean condition ranging from early to late February 2000. The forecasts show a dissipation of the currently moderately strong La Niña conditions during spring and summer of 2000. Figure 2 shows the time series of the forecasts of Niño 3.4 SST by sets of ensembles initiated from ocean conditions ranging over four consecutive weeks in February 2000. For each week, model integrations began from four differing atmospheric initial conditions. The solid line represents the 16-member ensemble mean. While all 16 runs indicate a return of the Niño 3.4 SST to normal by mid-summer, the spread among the members represents the uncertainty which may be described as plus or minus roughly 0.5 to 0.6 degrees K. By summers end most but not all of the runs show near normal to very slight positive SST anomalies. The other forecasts shown in this bulletin indicate a variety of possible scenarios of ENSO state through the middle and latter half of 2000. The subsurface equatorial sea temperature (not shown) still indicates a good supply of below-normal water temperature beneath the surface in the east-central and eastern part of the basin, capable of maintaining negative SST anomalies if the low level trades remain stronger than normal as they have in the central equatorial Pacific. All the while, warm subsurface water gradually builds in the western Pacific, especially from 100 to 150 m deep. During the April through June period, when the interannual standard deviation of the SST is lowest, the ENSO state is most subject to change and is relatively least predictable. By mid-June this uncertainty should be noticeably reduced.

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	68	70	79	84	88	90

Figure Captions

Fig. 1. NCEP coupled model SST anomaly forecast fields for Mar-Apr-May, Jun-Jul-Aug, and Sep-Oct-Nov 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.

Fig. 2. Observed (single line at left) and predicted (multiple lines after present date) SST anomalies in the Niño 3.4 region from NCEP coupled model. Four model integrations are shown for each of four ocean initial conditions spanning the approximately 1-month recent period as shown in the legend inside the figure. Within each ocean initial condition four integrations were run, each for differing atmospheric initial conditions. The thick solid line shows the 16-member ensemble mean.