Mechanisms limiting the poleward extent of the summer monsoon: South America, North America and Africa

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, North America and Africa. The extent of the continental convergence zone is mainly determined by three mechanisms, which we term soil moisture, ventilation and the ``interactive Rodwell-Hoskins mechanism". Ventilation refers to the import into continents of low moist static energy air from the cooler ocean. In the interactive Rodwell-Hoskins mechanism, Rossby-wave-induced subsidence to the west of the diabatic heating interacts with the convection zone. Soil moisture feedback also helps limit poleward movement of the continental convergence zone. Shapes of continents and surface albedo determine which mechanism is more important. It is conjectured that variations in ventilation associated with the subtropical jet might contribute to interannual variability of the poleward extension of the continental convergence zone. 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.