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The Madden-Julian oscillation is recognized as a fundamental mode of tropical low-frequency variability. Because of its importance for tropical weather prediction, especially its modulation of the precipitation in the Indian and Australian monsoon regions and its possible role in ENSO events, it is crucial to obtain a better understanding of the mechanisms that drive the Madden-Julian Oscillation (MJO). Tropical cloud regimes are defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud top pressure (CTP) optical thickness (TAU) joint distributions from the ISCCP D1 dataset (Fig 1) and are used to characterize organized and disorganized convection as a function of MJO phase. The clusters analysis results for the whole tropics covering 21.5 years (1983 2004) show six cloud regimes. Three correspond to regimes dominated by deep convective clouds, cirrostratus (mesoscale) anvils and smaller (unorganized) deep convection with mid-level cumulus congestus. Three others represent suppressed cloud regimes: isolated cirrus, shallow trade cumulus and marine stratocumulus. In the context of the MJO, the tropical cloud regime classification reveals systematic variations of distribution of the weather states. Composite Relative Frequency of Occurrence (RFO) bar-charts of the regimes are formed at seven lag times (in pentads) with respect to an MJO phase index (Fig 2) in the period 1983-2004. The RFOs of the deep convective and anvil regimes increase significantly to the peak phase of MJO (Fig 3). After the peak, the occurrence of deep convection decreases immediately while the anvil regime lingers for another pentad or so, then decreases gradually. The small-scale convection with mid-congestus, and shallow cumulus regimes dominate several weeks before the MJO peak, decreasing (increasing) before (after) the peak. The RFO of the cirrus regime is almost constant with MJO phase, suggesting that the presence of isolated cirrus is ubiquitous and not associated with convective activity. The relative behavior of the third convectively active regime is different in the long-term climatology, being more nearly constant over the MJO cycle, than during this particular period, which may indicate an interaction between the MJO and strong ENSO event. In other words, the MJO wave is associated with a change of organization of deep convection with more mesoscale systems at MJO peak than otherwise. These results suggest a strong interaction between MJO and deep convection. To further understand this interaction, the analysis is extended to the complete atmospheric diabatic heating by radiation, precipitation and surface fluxes to examine how the changing distribution of tropical weather states alters the atmosphere-surface interactions.
Keywords : Madden-Julian Oscillation, Tropical Deep Convection,
ISCCP, Weather State, Cloud Regime.
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