Previous studies have shown that both land surface fluxes and moisture transport can be important to the onset of the wet season. However, the relative importance between them in driving the transition from dry to wet season, and the specific role each of them play to initiate the transition have not been addressed. These questions are critical to determine the influence of land surface changes on wet season onset and consequently the precipitation climatology. To address this question, we have applied a composite analysis of fifteen-years of instantaneous ECMWF reanalyses, we found that the transition between dry and wet seasons is initiated by the increase of land surface sensible and latent fluxes, which elevate the low-level tropospheric equivalent potential temperature. The contribution from the large-scale moisture transport at this initial stage of the transition is secondary. The increase of lower-troposphere buoyancy increases the probability of convection and presumably induces northerly winds in the western Amazon, which increases moisture convergence in the Amazon basin and further enhances the stretching of the air column. From the initiating phase to the onset of the wet season, the large-scale moisture transport contributes as much moisture as that of the surface latent flux. After the onset, the contribution of moisture transport increases with time and consequently becomes a more important source of water vapor for rainfall. The increased rainfall further enhances the atmospheric column stretching. The latter transforms available potential energy into divergent kinetic energy, resulting in divergence in the upper troposphere, consequently spins up the upper-level anticyclonic flow until the surface fluxes no longer increase.