ENSO SIMULATION AND PREDICTION WITH A HYBRID COUPLED MODEL

A hybrid coupled model (HCM) consisting of a tropical Pacific Ocean an d global atmosphere is presented. The ocean component is a linear redu ced gravity model of the upper ocean in the tropical Pacific. The atmo spheric component is a triangular 30 horizontal resolution global spec tral general circulation model with 18 unevenly spaced levels in the v ertical. In coupling these component models, an anomaly coupling strat egy is employed. A 40-yr simulation was made with HCM and the variabil ity in the tropical Pacific was compared to the observed variability. The HCM produces irregular ENSO events with a broad spectrum of period s between 12 and 48 months. On longer timescales, approximately 48 mon ths, the simulated variability was weaker than the observed and on sho rter timescales (approximately 24 months) the simulated variability wa s too strong. The simulated variability is asymmetric in the sense tha t the amplitude of the warm events is realistic, but there are no sign ificant cold events. An ensemble of 60 hindcast predictions was made w ith the HCM and the skill was compared to other prediction systems. In forecasting sea surface temperature anomalies in the eastern Pacific, the HCM is comparable to the other prediction systems for lead times up to 10 months. The anomaly correlation coefficient for the eastern P acific SSTA remains above 0.6 for lead times of up to 11 months. Consi stent with the 10-yr simulation, hindcasts of cold events have little skill, particularly when compared to hindcasts of warm events. Specifi c hindcasts also demonstrate that the HCM also has difficulty predicti ng the transition from warm conditions to normal or cold conditions.