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Forecasts of Tropical Pacific SST Using a Dynamical

Ocean Model Coupled to a Statistical Atmosphere

contributed by Magdalena Balmaseda1, David Anderson2 and Michael Davey3

1European Centre for Medium Range Weather Forecasts, Reading, UK

2Atmospheric, Oceanic and Planetary Physics, Oxford University, UK

3Hadley Center, UK Meteorological Office, Bracknell, UK

An intermediate dynamical ocean-empirical atmosphere coupled model is being used in the Atmos-pheric, Oceanic and Planetary Physics Department at the University of Oxford to predict SST anomalies in the tropical Pacific. The model, whose detailed description and performance are documented in Balmaseda et al. (1994), consists of a tropical Pacific ocean model with two active layers coupled to a statistical model that relates SST anomalies, heat content (HC) anomalies, and surface wind stress anomalies. The anomalies are relative to monthly climatology. The ocean model is first forced by observed wind stress (based on data from Florida State University [FSU]; Goldenberg and O'Brien 1981) during the period 1961- 91. The output of this simulation run is used to build the statistical atmospheric model, which assumes that the wind stress anomalies are a linear function of the first 6 principal components (PCS) of the model SST and HC anomalies, with seasonal variation.

The hindcast skill of the model was tested using a set of 252 hindcast experiments, each of 24 months duration, with initial conditions taken from the simula-tion run at one month intervals during the period 1970 -91. The correlation skill (sk) and root mean square error (RMSE) for hindcasts of SST anomalies versus observed values have been calculated using ensemble means of hindcasts from three consecutive months.

The model shows good reliability in the central equatorial Pacific--in particular, in Niño 3 and Eq2 (130-170oW, 5oN-5oS). Figures 7-1a and 7-1b in the December, 1994 issue of this Bulletin show the corre-lation and RMSE skills for model hindcasts of the SST anomalies in the Niño 3 and Eq2 regions. Correlation skill at 6 months lead is about 0.59 for Niño 3 and about 0.62 for Eq2, while at 12 months lead they are about 0.50 and 0.55, respectively. The error in the initial conditions is quite high because only wind information is used to obtain the model initial state.

Figure 1 shows current forecasts of SST anomalies for 6 and 9 month lead times. The vertical bars are two RMSE in length, based on the 1970-91 period. The forecasts for Niño 3 (left column) and Eq2 (right column) at 6 months lead show an increase of SST anomaly. However, the model was late in predicting the onset of the warming in mid-1997 and has underesti-mated its high amplitude. The forecasts for 9 months lead indicate a dissipation of the warming by July 1998.

The last four individual forecasts, with initial conditions from July to October 1997, are shown in Fig. 2 for Niño 3 and Eq2. The thick solid lines are the observed SST anomalies, and the thin solid lines show the control run from which the initial conditions are taken. The trajectories of the individual forecasts are in fairly good agreement for Eq2 and someone less agreement for Niño 3. Continued anomalous warmth is indicated for the rest of 1997 and winter 1997-98, following by a reversal to cool conditions by late 1998.

The model has had a problem with initial SST conditions (forced by wind stress) that are quite different from the observed SST anomalies. For example, in the present case the initial SST is not warm enough. This puts the forecast on the wrong track right from the beginning. Lately, we have tried to overcome this problem by having the coupled model predict the tendency of the SST rather than the actual values:

F(t) = Obs(t0) + [FMDL(t) - FMDL(t0)]

where the bracketed term on the right side is the change in the model forecast from the initial time (t0) to time t, the first term on the right is the observed SST value at t0, and F(t) is the resulting forecast.

The correlation skill and the RMSE for Niño 3 and Eq2 as a function of lead time were shown in Fig. 3 of the March 1997 issue of this Bulletin for the new model as compared to the original model and persistence. The new forecasts are more skillful than the previous forecasts for leads up to 10 months, and always better than persistence.

The current forecasts using the new version of the Oxford model is shown in Fig. 3, and the forecasts using each of the most recent 1-month-apart initial conditions is shown in Fig. 4. These forecasts indicate greater warming than the forecasts of the original model, and the initial conditions are more realistic. The forecast did not begin showing very strong warmth until such warmth appeared in the observations. The model predicts a some further intensification of the persent anomalies during winter 1997-98, followed by weaken-ing in late spring 1998.

Balmaseda, M.A., Anderson, D.L.T. and M.K. Davey, 1994: ENSO prediction using a dynamical ocean model coupled to statistical atmospheres. Tellus, 46A, 497-511.

Goldenberg, S.D. and J.J. O'Brien, 1981: Time and space variability of tropical Pacific wind stress. Mon Wea. Rev., 109, 1190-1207.

Fig. 1. Oxford coupled model forecasts of the SST anomalies in the Niño 3 (left column) and Eq2 (130-170oW; right column) regions for 6 and 9 month leads. The latest forecast was initialized from October 1997 data. The vertical bars represent the RMSE-based 68% confidence intervals for the relevant lead time, based on predictions for 1970-91. Each prediction is the average of forecasts from three consecutive months. Correlation skill (sk) and RMSE values (fractions of observed standard deviation, or std) are indicated in each panel. Thick lines indicate observed SST anomalies.

Fig. 2. Individual initial condition predictions of SST anomalies in the Niño 3 and Eq2 regions made with the Oxford coupled model. The initial conditions are for July (dotted line), August (dashed line), Septembeer (dash-dot), and October (dash-dot-dot-dot) of 1997. Each prediction has a duration of 12 months. The ocean initial conditions (thin solid line) are produced by forcing the ocean model with observed FSU winds and also by sub-surface assimilated data. The thick solid line shows the observed SST anomalies.

Fig. 3. As in Fig. 1 (coupled model forecasts), except using the new version of the model that predicts changes.

Fig. 4. As in Fig. 2 (individual initial condition predictions), except using the new version of the model that predicts changes.



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