Mechanism of the Wet Season Onset over Tropical South America

Previous studies have shown that the change of timing of the rainy season contributes more to the interannual variation of precipitation than the rainrate (e.g. Liebmann and Marengo 2001). However, the specific mechanism that controls the onset is still unclear. Using the fifteen-year instantaneous European Center for Medium-range Weather Forecasts Reanalysis data (ERA), we have examined the large-scale atmospheric conditions and the local surface fluxes through the transition period from the dry to wet seasons. Our composite results suggest that the transition is initiated by the increase of local land-surface sensible and latent fluxes, which elevate moist static energy in the lower troposphere. The transition is accelerated by dynamic feedbacks with the stretching of the atmospheric column, resulting from a rising potential energy and probability of convection. This stage can be characterized by the reversal of the low-level, cross-equatorial flow, which consequently increases moisture transport from the tropical Atlantic and the low-level moisture convergence. In the middle and upper troposphere, the divergent kinetic energy begins to convert into rotational kinetic energy. These processes lead to the onset of the wet season. After this onset, the increase of lower tropospheric potential energy slows, but the conversion from divergent to rotational kinetic energy continues to spin up the upper troposphere anticyclonic circulation associated with the Bolivian High in subtropical South America to its full strength. The implications of the results to the impact of land use on the wet season onset will also be discussed.