Mechanisms limiting the southward extent of the South American summer monsoon

An intermediate atmospheric model coupled with a simple land-surface model and a mixed-layer ocean model is used to examine effects that determine the southward extension of summer precipitation over South America. The extent of the continental convergence zone is mainly determined by two mechanisms, which we term ventilation and the "interactive Rodwell-Hoskins mechanism". Ventilation refers to the import into South America of low moist static energy air from the cooler ocean, primarily the Pacific. In the interactive Rodwell-Hoskins mechanism, Rossby-wave-induced subsidence to the west of the diabatic heating interacts with the convection zone. Because of the shape of South America, the interactive Rodwell-Hoskins mechanism is of comparable importance to ventilation. Soil moisture feedback also helps limit poleward movement of the continental convergence zone, but its effect is relatively weak compared to the above two effects. The characteristic northwest-southeast tilt of the continental convergence zone appears to be due to a combination of ventilation and the interactive Rodwell-Hoskins mechanism.