Energetics of Madden Julian Oscillations in the NCAR CAM3: A Composite View

摘要

This study further examines the simulation of the tropical Madden-Julian oscillation (MJO) using the modified Zhang-McFarlane convection scheme in the National Center for Atmospheric Research Community Atmospheric Model version 3 (CAM3). The results demonstrate that modifications to the Zhang-McFarlane scheme lead to significantly enhanced MJO and much improvement of the MJO structures. However, the propagation speed is too fast compared to observations. The westward tilting structures of moisture flux convergence over the western Pacific warm pool are simulated very well by the modified CAM3 (CAM3m). The modified Zhang-McFarlane convection scheme also improves the energetic structures of the MJOs. With the climatological mean of the energetics from the composite MJO removed, the energy budget terms show clear eastward propagation following MJO movement in the CAM3m and ECMWF reanalysis (ERA40). The results further support that the interaction between convection and large-scale circulation is important in the maintenance and growth of the MJO through perturbation kinetic energy (PKE) conversion from perturbation available potential energy (PAPE) generated by the correlation of specific volume (α) and large-scale heating (Q 1). Too weak Q 1 and PAPE generated through (α, Q 1) is responsible for the weak MJO and intraseasonal variability simulated by the standard CAM3. Too strong climatology mean of the energetics in the CAM3m may be caused by too strong stationary intraseasonal variability. Moisture flux convergence is responsible for the changes of specific humidity in the CAM3m and CAM3 during the cycle of the MJO over the Indian and western Pacific Oceans. These results indicate that interaction between convection, moisture and convergence in the lower troposphere may be responsible for the MJO development over the Indian and western Pacific Ocean in both observations and simulations. The weak intraseasonal variability and MJO simulated by the original Zhang-McFarlane deep convection scheme is attributed to the lack of coherent shallow convection ahead of deep convection in the MJO cycle.