RECTIFICATION OF THE ENSO VARIABILITY BY INTERDECADAL CHANGES IN THE EQUATORIAL BACKGROUND MEAN STATE IN A CGCM SIMULATION

A 200-yr long coupled general circulation model (CGCM) simulation is used to investigate the characteristics and impact of the low frequency variability of the equatorial background stratification on the ENSO modulation. A vertical mode decomposition of the model mean vertical structure is performed and the interdecadal variability of the baroclinic mode parameters is analyzed. The low frequency variations of the baroclinic mode parameters associated to the decadal mode in the model are then used to force an intermediate coupled anomaly model "tuned" with the CGCM derived climatologies and wave parameters. Results of sensitivity experiments indicate that a rectification of the interannual variability (ENSO timescales) by the interdecadal variability associated to changes in the oceanic mean states does take place. However, the magnitude and characteristics of this rectification highly depends on the strength of the interaction between the ENSO mode and the seasonal cycle. Implications for ENSO decadal predictability is then discussed