Land-sea geometry and its effect on monsoon circulations

The role of land-sea geometry in determining the existence and character of monsoon circulations is examined using an atmospheric general circulation model with land surface properties represented by the simplified simple biosphere model. Idealized land-sea distributions are used to examine the effects of the latitude of continental coastlines, the meridional and zonal extent of continents, the shape of continents, and the presence of orography on circulation in the tropics and subtropics. It is found that the latitude of the continent is critical to the establishment of a distinct seasonality of precipitation and a summer monsoon circulation. Low-latitude positions of the continent lead to landlocked precipitation maxima, and poleward positions lead to a Mediterranean (winter monsoon) climate. In every case, the subsiding branch of the Hadley cell at continental longitudes locks over some part of the land. Variations in the meridional extent of land affect the strength of the Hadley cell and the degree of seasonality in the climate over land. The extension of a subtropical continent into the tropics is critical for establishing heavy convective rainfall over land in the summer. When tropical peninsulas are added to a subtropical continent, the distribution of moisture and rain over semi-arid and arid regions of the original continent are affected. When a broad mountain of 1200 m maximum elevation is present on the idealized continent, there is a band of increased precipitation to the south and east of the mountain with a systematic decrease to the south, north, and west of the band.