Forecasts of Tropical Pacific SST Using a Dynamical Ocean Model Coupled to a Statistical Atmosphere

An intermediate dynamical ocean-empirical 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 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 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 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 regions of the central equatorial Pacific--in particular, in the regions Nino 3 and Eq2 (130-170oW, 5oN-5oS). Figures 7-1a 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 Nino 3 and Eq2 regions. Correlation skill at 6 months lead is about 0.59 for Nino 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 0, 3, 6, and 9 months lead. 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 all lead times show some recovery of the SSTs with respect to the strongest negative anomalies. The observations have recently recovered slightly from their recent lows. The recovery in the SST forecasts follows minimum values that are considerably lower than present anomalies. In the short lead forecasts these cold SSTs are already supposed to be occurring, while in the longer lead forecasts they are called for toward summer of 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, 1190-1207.

Figures

Figure 1. Oxford coupled model forecasts of the SST anomalies in the Nino 3 (left column) and Eq2 (130-170W;right column) regions for 0, 3, 6 and 9 month leads. The latest forecast was initialized in January 1996. The vertical bars represent the RMSE-based 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.