Some mechanisms limiting the poleward extent of the summer monsoon convection zones (Invited)

Summertime solar heating of continents extends much further poleward than the region of most intense deep convective rainfall. If thermodynamic factors suficiently favor maintenance of a deep convection zone in a region, moisture convergence often tends to supply moisture in a positive feedback. From a coupled ocean-atmosphere point of view, it is thus of interest to ask what dynamical mechanisms limit the poleward extension of deep convective zones in summer monsoon systems. An intermediate atmospheric model coupled with a simple land-surface model and a mixed-layer ocean model is used to examine large-scale dynamical and thermodynamical factors. Cases for an idealized continent, South America, North America and Africa are contrasted. The extent of the continental convection zone is affected by surface effects, such as (i) soil moisture feedback and (ii) albedo. We find it useful to define two other mechanisms: (iii) ventilation and (iv) 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. Numerical experiments are conducted in which mechanisms are suppressed to attempt to test their importance. The continents differ in which mechanisms have most impact, but ventilation and the interactive Rodwell-Hoskins mechanism both appear to be important. 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.