Dynamics and thermodynamics of the fast and slow Madden–Julian Oscillation

Abstract The structure and governing processes associated with fast and slow Madden–Julian Oscillation (MJO) propagation speed are examined. On the basis of spectral analysis and three‐dimensional normal‐mode decomposition, we find that slow MJOs exhibit a shorter zonal scale relative to faster MJOs, in agreement with previous work. Thermodynamic distinctions between the two MJO types are then considered by evaluating four criteria that so‐called moisture modes should satisfy, and by examining their column‐integrated moist static energy (MSE) budget. Slow MJO events align more closely with the moisture‐mode criteria than fast events, satisfying all four criteria throughout the broad Indo‐Pacific warm pool, whereas fast MJOs only meet these conditions over the Indian Ocean. These results imply that moisture governs the thermodynamics of slow MJOs, while temperature and moisture play comparable roles in fast MJO thermodynamics. When examining the column MSE budget, we find that the slow MJO MSE anomalies propagate eastward primarily via horizontal MSE advection, with zonal moisture advection accounting for up to 50% of the total MSE tendency over the Indo‐Pacific warm pool. In contrast, vertical MSE advection plays a larger role in fast MJOs, especially east of the Maritime Continent. Lastly, we compare the vorticity budget of fast and slow MJOs. The vorticity anomalies in both fast and slow MJOs propagate eastward via vortex stretching. However, advection of planetary vorticity opposes this vortex stretching and this process is stronger in slow MJOs. Collectively, these findings indicate that MJO governing processes vary with propagation speed and region.