The Madden-Julian Oscillation (MJO) [1,2] is the most promi-nent intraseasonal mode in the tropics and is characterized by a slow eastward phase speed,a planetary zonal scale and a period of 30-60 d.Although the main convection of the MJO is confined to the tropics,it can exert a large remote impact on weather and climate around the globe [3].The MJO is the major source of pre-dictability for sub-seasonal forecasts.It is therefore important to understand the fundamental dynamics of the MJO.rnVarious theories have been developed to understand the mech-anisms of the eastward propagation of the MJO (see[3-5]).So far the most accepted theory is the moisture mode theory,which emphasizes the effect of moisture perturbation[6-8].The moisture mode theory may,in general,be separated into two types[4,9].The first type emphasizes the zonal asymmetry of the moisture pertur-bation in the planetary boundary layer (PBL)[6].Moisture leading in the PBL is primarily attributed to the advection of mean mois-ture by the anomalous ascending velocity associated with bound-ary layer convergence.The PBL convergence leading results from a free atmospheric Kelvin wave response to MJO heating and a warm sea surface temperature (SST) anomaly east of the MJO cen-ter[6].Moistening of the boundary layer in front of the convection helps to trigger local shallow convection[9],promoting the east-ward propagation of the MJO.