How Do Atmosphere and Land Surface Influence Seasonal Changes of Convection in the Tropical Amazon?

Although the wet season in the tropical Amazon (10N-20S) at any one place and in any one year is initiated rapidly by synoptic systems, large-scale thermodynamic conditions modulate the frequency and intensity of these synoptic systems and hence control the climatology of the wet season. In this study, the satellite radiances, radiosondes, and assimilation data of the atmosphere are analyzed to show that the conditioning of the largescale thermodynamics for the onset of the wet season is controlled by a moistening of the planetary boundary layer (PBL) and a lowering of temperature at its top, hence reducing convective inhibition energy (CINE). These changes occur either in phase with or lagging by one month the enhancement of low-level moisture convergence. Integration of a slab mixed-layer model shows how a higher humidity can reduce the drying effect of the entrainment and increase the humidity of the daytime PBL. Hence, the increase of low-level moisture convergence may provide enough moisture to initiate the wet seasons.