Role of the Boundary Layer Moisture Asymmetry in Causing the Eastward
Propagation of the Madden-Julian Oscillation


Previous studies have pointed out a marked zonal asymmetry of the moisture relative to the MJO convection in the planetary boundary layer (PBL), but what causes such a moisture asymmetry is not clear. In this study we address this issue through the diagnosis of the PBL moisture budget using 1979-2001 ERA-40 reanalysis data. It is shown that the PBL moisture asymmetry leads to a convectively unstable stratification to the east of the MJO convection and favors the eastward propagation of the MJO. The PBL moisture budget analysis indicates that the PBL moisture convergence is a major term that contributes the moisture asymmetry.

The cause of the zonal asymmetry in the PBL divergence is further examined. It is found that heating induced free-atmospheric wave dynamics account for 75-90% of the total PBL convergence ahead of the convection, while the warm SST anomaly induced by air-sea interaction contributes 10-25% of the PBL convergence. The horizontal advection also plays a role in contributing to the PBL moisture asymmetry. The leading term in the moisture advection is the advection of the background moisture by the MJO flow. Whereas meridional moisture advection by the MJO northerly flow dominates in the western Indian Ocean, Maritime Continent and western Pacific, the zonal advection plays a greater role in the eastern Indian Ocean.

The contribution of the horizontal moisture advection by synoptic-scale (3-10-day) eddies is in general small. They have a negative impact on the moisture advection over the Indian Ocean and western Pacific but a positive impact in the Maritime Continent region.