APERIODIC VARIABILITY IN THE CANE-ZEBIAK MODEL - A DIAGNOSTIC STUDY

The finite predictability of the coupled ocean-atmosphere system is de termined by its aperiodic variability. To gain insight regarding the p redictability of such a system, a series of diagnostic studies has bee n carried out to investigate the role of convergence feedback in produ cing the aperiodic behavior of the standard version of the Cane-Zebiak model. In this model, an increase in sea surface temperature (SST) in creases atmospheric heating by enhancing local evaporation (SST anomal y feedback) and low-level convergence (convergence feedback). The conv ergence feedback is a nonlinear function of the background mean conver gence field. For the set of standard parameters used in the model, it is shown that the convergence feedback contributes importantly to the aperiodic behavior of the model. As the strength of the convergence fe edback is increased from zero to its standard value, the model variabi lity goes from a periodic regime to an aperiodic regime through a broa dening of the frequency spectrum around the basic periodicity of about 4 years. Examination of the forcing associated with the convergence f eedback reveals that it is intermittent, with relatively large amplitu de only during 2 or 3 months in the early part of the calendar year. T his seasonality in the efficiency of the convergence feedback is relat ed to the strong seasonality of the mean convergence over the eastern Pacific. It is shown that if the mean convergence field is fixed at it s March value, aperiodic behavior is produced even in the absence of a nnual cycles in the other mean fields. On the other hand, if the mean convergence field is fixed at its September value, the coupled model e volution remains close to periodic, even in the presence of the annual cycle in the other fields. The role of convergence feedback on the ap eriodic variability of the model for other parameter regimes is also e xamined. It is shown that a range exists in the strength of the SST an omaly feedback for which the model variability is aperiodic even witho ut the convergence feedback. It appears that in the absence of converg ence feedback, enhancement of the strength of the air-sea coupling in the model through other physical processes also results in aperiodicit y in the model.