Ocean-atmosphere-land feedbacks in an idealized monsoon

An intermediate-complexity atmospheric model coupled with a simple land-surface model and a mixed-layer ocean model is used to investigate the processes involved in an idealized monsoon occurring on a single rectangular continent. Idealized divergences of ocean heat transports are specified as an annual average 'Q-flux'. In this simple coupled configuration, the mechanisms that affect land-ocean contrast and, in turn, the seasonal movement of the continental convergence zones are examined. These include soil-moisture feedbacks; cooling of tropical oceans by ocean transport; ventilation, defined as the import into continental regions of low moist static-energy air from ocean regions where heat storage opposes summer warming; and the 'interactive Rodwell-Hoskins mechanism', in which Rossby-wave-induced subsidence to the west of monsoon heating interacts with the convection zone. The fixed ocean transports have a substantial impact on the continental convection. If Q-flux is set to zero, subtropical subsidence and ventilation tend to substantially limit the poleward movement of summer monsoon rainfall. When land hydrology feedbacks are active. the drying of subtropical continents disfavours continental convection even in the tropics. When ocean transports are included, tropical oceans are slightly disfavoured as regions for producing convection which, by contrast, favours continental convection. The monsoon circulation then produces moisture transport from the ocean regions that allows substantial progression of convection into the subtropics over the eastern portion of the continent. The western portion of the continent tends to have a dry region of characteristic shape. This east-west asymmetry is partly due to the interactive Rodwell-Hoskins mechanism. The ventilation is of at least equal importance in producing cast-west asymmetry and is the single most important process in limiting the poleward extent of the continental convection zone.