Unified Forecast System Prediction of the Madden-Julian Oscillation and Its Influence on East Pacific During Boreal Summer

This study evaluates the subseasonal-to-seasonal (S2S) prediction skill of the Madden-Julian Oscillation (MJO) and its impacts on the east Pacific (EP) tropical cyclogenesis in the Unified Forecast System (UFS) during boreal summer (May–October). Utilizing four experimental versions, Prototypes 5–8, the study finds that although the UFS generally captures the MJO's propagation characteristics near the initialization time, it encounters difficulty in accurately predicting the propagation speed and decay rate of the MJO beyond 15 days. Specifically, the phase transition rate of the MJO in the UFS is slower than observed, although this behavior is improved in Prototype 8. The UFS overestimates intraseasonal east Pacific genesis potential index anomalies as a function of the MJO phase. Analysis of the vertically integrated moist static energy (MSE) budget reveals that all four UFS prototypes underestimate the damping effect of vertical MSE advection and the amplifying effect of longwave radiative heating, indicating weaknesses in tropical convective parameterization and cloud radiative feedbacks, although these biases are somewhat improved in Prototype 8. These deficiencies result in less efficient vertical MSE export, weaker damping of MJO convection, slow MJO propagation, and delayed MJO's impacts on the EP. Thus, improving the UFS's ability to simulate MJO propagation and maintenance processes is crucial for better predicting the MJO's effects on EP TC genesis and enhancing S2S forecast capability for MJO-modulated TC activity.