6.6: Regional Scale Winds - The Monsoon
Monsoon means wind that changes direction with season. Monsoon conditions are found in a variety of regions around the world, the most noted is the monsoon of Asia. During the summer, the continent of Asia heats up more than the surrounding ocean due to the differences in the way land and water heat. The warm surface creates a large area of low pressure over north-central Asia and a smaller one over India. This creates an onshore wind bringing the moisture laden maritime air from the Pacific and Indian oceans onto land. As the air streams across the land, convection induced by the warm surface, convergence into the areas of low pressure, and uplift along major mountain systems like the Himalayas force the air to rise creating the abundant rainfall of the wet monsoon season . During the winter, the flow of air reverses. The continent cools rapidly forming a large area of high pressure over north central Asia, known as the Siberian High, and a smaller area over India. Now the drier, colder air of the continent blows offshore creating the dry monsoon season .
Monsoon conditions are also found in east-central Africa. This region experiences a seasonality of precipitation induced by alternating influence of global pressure belts. During the high sun season, the ITCZ moves in bringing warm, moist unstable air that induces precipitation. During the low sun season the ITCZ shifts out and the influence of the descending motions of the subtropical high dominate the region. During this period the subtropical high inhibits the development of precipitation.
The southwest desert of the United States experiences a pseudo-monsoon during the summer. The intense heating of the surface creates a thermal low over the desert at this time of year. The low induces moist Gulf air to move into the region where it is uplift by convergence or orographic uplift to produce precipitation. During the winter the desert surface cools, the low dissipates, and dry conditions prevail.