This study analyzes the tropical intraseasonal variability and associated Madden-Julian oscillations (MJO) simulated in the National Center for Atmospheric Research Climate Model CAM3 using two versions of the Zhang-McFarlane convection scheme. The results are compared with those from the Xie-Arkin observations and European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is shown that modifications to the convection scheme in the model lead to substantial enhancement of the intraseasonal variability and MJO. The spatial scale of the precipitation anomalies associated with intraseasonal variability and MJO is also in better agreement with the Xie-Arkin observations and ECMWF reanalysis. By contrast the intraseasonal variability and MJO in the CAM3 are weak. The analysis of the energetics of intraseasonal variability shows that perturbation kinetic energy (PKE) and its sources and sinks through conversion from potential energy and mean flow and generation from wave energy flux are too strong in the modified CAM3 and too weak in the CAM3 when compared with the reanalysis. It also shows that different mechanisms are responsible for the PKE production in different locations. In convectively active regions, conversion from potential energy and vertical transport are important to the maintenance of the upper troposphere PKE; in convectively suppressed regions, horizontal wave energy flux convergence and barotropic conversion are important. The interaction between convection and large-scale circulation plays an important role in the maintenance of intraseasonal variability and MJO through PKE conversion from potential energy generated by convective heating.