Moistening Processes for Madden-Julian Oscillation over Indian Ocean and Maritime Continent

Abstract:

The moistening processes for MJO over Indian Ocean (IO) and Maritime Continent (MC) are investigated through a diagnosis of ECMWF Re-analysis (ERA-Interim) data in Nov.-April, 1982-2011. During this period, 27 MJO events with strong magnitude and clear propagation are identified and further classified as either primary, or successive, according to the existence of preceding event. While the successive events are analyzed in composite fields, the primary events will be explored individually.

A composite of scale-separated lower-tropospheric (1000-700 hPa) moisture (q_L) budget is analyzed in four stages: suppressed, cloud developing, convective and decaying, each corresponding to the RMM index phase 567(781), 81(23), 2(4), and 34(56), respectively, for IO (MC). In the suppressed stage, the dominant moisture source over both region is surface evaporation/shallow convection (-Q₂). Nonlinear zonal (meridional) advection by synoptic disturbances also has non-negligible contribution over IO (MC). In the cloud developing stage, q_Lapproaches maximum with moistening tendency to its east. This moistening is contributed by the advection of mean moisture by anomalous easterlies associated with downstream Rossby wave response of the dry anomaly and boundary layer moisture convergence. In the convective stage, while the zonal advection of anomalous westerlies and intense precipitation dries the atmosphere, the moistening of meridional advection by downstream Rossby anti-cyclonic gyres leads to the eastward propagation of deep convection. In the decaying stage, the strong westerlies bring in dry air from the west causing widespread drying. Overall, the moisture evolution of MC is consistent with IO expect meridional component is more essential in suppressed stage.

A column-integrated moist static energy (MSE) budget is also analyzed to further identify the role of radiation and surface flux. The result shows that longwave heating is the dominant energy source in convective stage and latent heat flux is more prominent in decaying stage when the westerly is strong. The in-phase relation of longwave heating with column-integrated MSE suggests that longwave heating acts to maintain MSE and retard the propagation. Latent heat flux also slows down the propagation due to the phase lag.