The impact of the mean moisture bias on the key physics of MJO propagation in the ECMWF reforecast

The impact of the seasonal mean background state on the physical processes responsible for better Madden-Julian Oscillation (MJO) propagation and prediction was investigated using 20 year European Centre for Medium-Range Weather Forecasts (ECMWF) reforecast data sets. While the reforecast captures the MJO propagating signal in the circulation field, the outgoing longwave radiation and column-integrated moist static energy (MSE) anomalies show a weaker signal than the reanalysis when the MJO detours and propagates in the southern Maritime Continent. By comparing individual MSE budget terms, it is found that the predicted positive MSE tendency to the east of the MJO convection center is weaker than the reanalysis due to the weaker horizontal MSE advection. The dry bias in the seasonal mean lower tropospheric moisture is a key factor that weakens the horizontal MSE advection, and thus deteriorates the MJO propagation in the reforecasts. Therefore, improvements in the mean state could further extend the MJO prediction and associated subseasonal weather phenomena over the globe.