Three-Dimensional Structure and Evolution of the Moisture Field in the MJO

The large-scale circulation features that determine the structure and evolution of MJO-related moisture and precipitation fields are examined using a linear analysis protocol based on daily 850- minus 150-hPa global velocity potential data. The analysis is augmented by a compositing procedure that emphasizes the structural features over the Indo-Pacific warm pool sector (60 degrees E-180 degrees) that give rise to the eastward propagation of the enhanced moisture and precipitation.It is found that boundary layer (BL) convergence in the low-level easterlies to the east of the region of maximum ascent produces a deep but narrow plume of equatorial ascent that moistens the midtroposphere, while weakly diffluent flow above the BL spreads moisture away from the equator. Vertical advection of moisture from this plume of ascent accounts for the eastward propagation of the positive moisture anomalies across the Maritime Continent into the western Pacific. When the convection is first developing over the Indian Ocean, horizontal moisture advection contributes to both the eastward propagation and the amplification of the positive moisture anomalies along the equator to the east of the region of enhanced convection. Neither horizontal advection nor the net moistening from vertical advection and the apparent moisture sink exhibit significant westward tilt with height in the equatorial plane, but when they are superposed they explain the westward tilt of the moisture field. The strong spatial correlation between relative humidity and vertical velocity underscores the important role of equatorial wave dynamics in shaping the structure and evolution of the MJO.