ON THE DEPENDENCE OF HINDCAST SKILL ON OCEAN THERMODYNAMICS IN A COUPLED OCEAN-ATMOSPHERE MODEL

Three different mechanisms for the generation of ENSO SST anomalies wi thin a simplified tropical Pacific Ocean model are examined: thermocli ne depth changes, Ekman-induced upwelling anomalies, and zonal advecti on changes. The effect of varying the relative influence of these term s on the realism of tropical Pacific coupled models is analyzed. The p rincipal tool used to assess such realism is hindcast skill, with forc ed ocean and oscillatory behavior also being examined. Of the mechanis ms considered, thermocline perturbations are shown to be crucially imp ortant for high coupled-model hindcast skill. Furthermore, it is concl uded that the realism of the model (as measured by hindcast skill) det eriorates markedly when the influence on SST of Ekman upwelling become s greater than a small fraction of the thermocline influence. This pro vides strong evidence for the hypothesis that Ekman upwelling anomalie s (which are essentially a local response to wind stress anomalies) ha ve only a small influence on the creation of real world SST anomalies. The implications of this latter point for coupled models involving oc ean general circulation models is briefly discussed. It is also demons trated that western boundary reflections provide a vital role by means of a negative feedback in ensuring realistic performance. The hindcas t skill (as measured by NINO3 anomaly correlation) demonstrated by a m odel involving only the thermocline mechanism can be tuned to exceed t hat of the benchmark Cane and Zebiak model for hindcast lags up to 7 m onths (from 7 to 12 months the model skills are roughly equal).