A Study of Moisture Budget of Intraseasonal Oscillations over the Indo-Pacific Warm Pool

The evolution processes for propagating intraseasonal oscillations (ISO) [the MJO in winter and BSISO in summer] with strong magnitude over Indian Ocean (IO) and Maritime Continent (MC) are investigated through a diagnosis of ECMWF Re-analysis in November-April, 1982-2011. A scale-separated lower-tropospheric (1000-700 hPa) moisture budget is analyzed in four stages of composite life cycle: suppressed, cloud developing, convective, and decaying. Overall, the budget evolution of the eastward propagation in IO and MC are all dominated by wave-induced boundary-layer convergence in the anomalous easterlies (WC) and advection. For the MJO, starting from the suppressed stage in central IO, moistening by WC and advection by easterly anomalies west of central IO contributes to an initiation of the MJO convection in western IO while surface evaporation/shallow convection moistens CIO. In the following cloud developing and convective stage in central IO, moistening by WC and advection by downstream Kelvin/Rossby wave east of central IO lead to eastward propagation of deep convection. In MC, the suppressed stage coincides with the convective stage in central IO that renders anomalous easterlies, subsidence and enhanced diurnal convection in MC. Unlike WC and advective moistening in IO that both occur in the equatorial zone, advective moistening in MC occurs primarily south of equator causing a southward detour of the eastward-propagating MJO. This is due to islands in MC causing higher and inhomogeneous moisture and weaker intraseasonal winds in lower troposphere by more active diurnal mixing. As a result, moisture advection in MC is disorganized relative to the more organized moisture advection over Arafura Sea south of MC. For BSISO, similar mechanisms discussed above keep the oscillation moving eastward. In addition, prominent northward propagation is induced by moisture advection that is dominated by westerly monsoon flow over IO and southwesterly monsoon flow in MC and SCS, consistent with the additional barotropic response to the ISO heating in the presence of monsoon mean flow.