SIMULATION AND PREDICTABILITY IN A COUPLED TOGA MODEL

A model of the tropical ocean and global atmosphere is described. It c onsists of an aqua-planet form of version one of the NCAR Community Cl imate Model coupled to a primitive equation model for the upper tropic al ocean in a rectangular basin. A 24-year simulation is described tha t has almost no climate drift, a good simulation of the mean temperatu re gradient across the ocean, but smaller than observed annual and int erannual variability. The coupled model is analyzed to see where it oc curs on the schematic bifurcation diagram of Neelin. In years 9-16 of the simulation there is a dominant oscillation with a period of two ye ars. The spatial pattern of this oscillation shows up clearly in the f irst empirical orthogonal function calculated from monthly averages of sea surface temperature anomalies. A series of 19 model-twin predicta bility experiments were carried out with the initial perturbation bein g a very small change in the ocean temperature field. The correlation coefficient of monthly sea surface temperature anomalies from these mo del-twin experiments decreases rapidly over the first 6 months and aft er that, more slowly, showing that there is some predictability out to a year. The predictability times are marginally increased if only the coefficient of the first empirical orthogonal function of monthly ave raged sea surface temperature anomalies or NINO3 sea surface temperatu re is predicted. There is some evidence to indicate that it is easier to predict the onset of a model warm event than to predict the onset o f a model cold event. More detailed analysis of the first model-twin e xperiment shows that the initial divergence in the integrations is a c hange at day 6 in the incoming solar radiation due to a change in the atmospheric model clouds. The dominant early change in sea surface tem perature occurs by this change in radiative heat flux. If the cloud fe edback is set to zero, then the first changes are delayed to day 12 an d occur in the evaporative and sensible heat fluxes and in the atmosph eric wind stress. In this case the dominant early change to sea surfac e temperature is by advection due to the changed wind stress.