An intermediate dynamical ocean­empirical atmosphere coupled model is currently being used in the Atmospheric, Oceanic and Planetary Physics Department at the University of Oxford to predict sea surface temperature (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 1961­91. 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 has been tested using a set of 252 hindcast experiments, each of 24 months duration, with initial conditions taken from the simulation run at one month intervals during the period 1970­91. 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 regions of the central equatorial Pacific­­in particular, in the regions NiZo 3 and Eq2 (130­170^oW, 5^oN­5^oS). Figures 7­1a and 7-1b in the December, 1994 issue of this Bulletin show the correlation and RMSE skills, respectively, for model hindcasts of the SST anomalies in the NiZo 3 and Eq2 regions. Correlation skill at 6 months lead is about 0.59 for NiZo 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 1970­91 period. The forecasts for NiZo 3 (left column) and Eq2 (right column) at both lead times show considerable recovery of the SSTs relative to the strongest negative anomalies. The observations have also recovered somewhat from their early winter 1995-96 lows. The recovery in the SST forecasts follows minimum values that are lower than the minima in the observed anomalies, especially for NiZo 3. At both lead times the forecast anomalies have had the correct sign and have lagged the observed anomalies.

The last four individual forecasts, with initial conditions from January to April 1996, are shown in Fig. 2 for NiZo 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 good agreement for both NiZo 3 and Eq2, and indicate gradual warming from the cool model values of the first quarter of the 1996.

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, 1190­1207.

Figures