Precipitation Budget of the Madden–Julian Oscillation

Column moisture and moist static energy (MSE) budgets have become common tools in the study of the processes responsible for the maintenance and evolution of the MJO. While many studies have shown that precipitation is spatially correlated with column moisture, these budgets do not directly describe the MJO-related precipitation anomalies. Other spatially varying fields may also play a role in determining the horizontal distribution of anomalous precipitation. In this study, an empirical precipitation anomaly field is derived that depends on three variables in addition to column moisture. These are the low-frequency distribution of precipitation, the low-frequency column saturation water vapor, and the sensitivity of precipitation to changes in column relative humidity. The addition of these fields improves upon moisture/MSE budgets by confining these anomalies to the climatologically rainy areas of the tropics, where MJO activity is strongest. The derived field adequately describes the MJO-related precipitation anomalies, comparing favorably with TRMM precipitation data. Furthermore, a “precipitation budget” is presented that emphasizes moist processes over the regions where precipitation is most sensitive to free-tropospheric moisture. It is found that moistening from vertical moisture advection in association with regions of shallow ascent plays a central role in the propagation of the MJO. The overall contribution from this process is comparable to the contribution from horizontal moisture advection to propagation. Consistent with previous studies, it is found that vertical advection arising from longwave radiative heating maintains the intraseasonal precipitation anomalies against drying by horizontal moisture advection.