Role of the Atmospheric Mean-state on the Initiation of the Madden-Julian Oscillation in a Tropical Channel Model

Tropical channel models have recently been used as promising tools for simulating the Madden-Julian Oscillation (MJO) and understanding its dynamics. Influences from the extratropics through the meridional boundaries have been found to be essential fo the initiation of certain MJO events. This led to the hypothesis that multi-year simulations using a tropical channel model would reproduce reasonable MJO statistics under the influence of prescribed lateral boundary conditions derived from global reanalyses. Interestingly, the MJO statistics in such a multi-year simulation by a nested tropical channel model are not an improvement over those from global climate models. The error in the atmospheric mean state was found to be the reason. Nevertheless, even with a large error in the mean state, the multi-year simulation captures two MJO events previously shown to be initiated by extratropical influences. Meanwhile, the model does not reproduce a third event, whose initiation is not directly influenced by the extratropics. This indicates that in the absence of dynamical connections between the MJO and the lateral boundary conditions, the error in the mean state could be sufficient to prevent the MJO initiation. Sensitivity simulations show that the simulation of this third event is not critically influenced by the cumulus schemes, nor the initial conditions when the model is integrated for two weeks prior to the MJO initiation. However, the model does capture this event when the MJO signal is present in the initial conditions. The use of high resolution SST does not improve the simulation of this event. The use of high-resolution nested domains, including a cloud-system resolving domain over the Maritime Continent, has little effect on MJO initiation over the Indian Ocean. It takes less than two weeks for the error in the simulated mean state to attain the climate error. The role of the simulated MJO on the mean state is explored. The implications of these results will be summarized. Utilization of a tropical channel model in the WCRP-WWRP/THORPEX Year of Tropical Convection (YOTC) project will be summarized, using the ECMWF T799 (25 km) global analysis to provide high-resolution meridional boundary conditions.