Mechanisms Limiting the Northward Extent of the Northern Summer Monsoons over North America, Asia, and Africa*

Mechanisms determining the poleward extent of summer monsoon convergence zones for North America, Asia, and Africa are examined in an intermediate atmospheric model coupled with a simple land model and a mixed layer ocean. Observations show that thermodynamical factors associated with the net heat flux into the atmospheric column provide favorable conditions for the monsoon convergence zone to extend farther poleward than actually occurs. To understand the discrepancy, a series of experiments are designed to test the importance of mechanisms previously examined in the South American case by the authors, namely, soil moisture, ventilation, and the interactive Rodwell–Hoskins mechanism. The latter refers to the interaction between baroclinic Rossby wave dynamics and convective heating. In North America, experiments suggest that ventilation by both temperature and moisture advection is a leading effect. The interactive Rodwell–Hoskins mechanism tends to favor east coast rainfall and west coast dryness. In Asia, ventilation by moisture advection is particularly important and the interactive Rodwell–Hoskins mechanism tends to favor interior arid regions and east coast precipitation. Overall, these dynamical factors are crucial in setting the poleward extent of the convergence zone over North America and Asia. Africa differs from the other continents because of the high surface albedo over much of northern Africa. Because there is less positive net flux of energy into the atmospheric column, convection is less thermodynamically favored and the dynamical factors, ventilation and the interactive Rodwell–Hoskins mechanism, have a weaker impact on preventing poleward extent of the convergence zone.