In this study the representation/sensitivity of ISO and ENSO in the GFDL FMS GCM, is investigated. Sensitivity of the ISO to modifications in the atmosphere only GCM's (AGCM) convective parameterization has been seen. In particular, this study focuses on the sensitivity of ISO to a simple diffusive cumulus momentum transport (CMT) and setting a minimum bound on lateral entrainment rates for deep convection (Tokioka modification). It is found that adding the Tokioka modification enhances the ISO while the CMT tends to weaken it. In addition to amplitude, the representation of the ISO in the GCM is assessed by validating its propagation characteristics and the geographic location of centers of activity. The representation of ENSO in a fully coupled GCM version (CGCM) also appears to be sensitive to changes to the convective parameterization. To investigate the predictive impact, ensembles of 1 year predictions are performed with the CGCM, for a control and each modification to the GCM convective parameterization. The ocean state is initialized with an ocean data assimilation system (ODA) and the atmospheric states are taken from an ensemble of AGCM integrations. Periods of strong MJO activity in both the GCM and observations, are identified using extended EOF (EEOF) analyses of band passed (i.e., 30 - 90 days) precipitation pentads (5 day means). GCM predictive skill is assessed on subseasonal to ENSO time scales, for a variety of space domains and for periods of active versus inactive ISO.