The main cause of monsoon circulation is the thermally direct cell resulting from surface temperature differences. For example, in winter, the Siberian High accumulate sub-zero masses of air, while in summer, the extreme heat reduces the pressure to 950mb over NW India.
The formation of monsoon is a result of the positions of the continents on the Northern hemisphere. During the northern summer, surface heating og the land masses is intense. A shallow surface low pressure centre forms over India and over central asia. The intertropical convergence zone or equatorial tough moves northwards allowing an inblowing of moist south-west to south winds to give the wet season in west Africa, India, china and most of SE asia.
In winter, the land loses heat more rapidly than the sea. The land surface is colder than the sea surface. Cold air over the land sinks. As a result, air pressure over the land is higher than the sea. Wind blows from the land to the sea and therefore, offshore winter monsoon winds are dry. The areas influenced by the winter monsoon are dry in winter.
Low lying clouds before rainfall in Singaporean asia, the northeastern winter monsoons take place from December to early march. The temperature over central asia is lower, creating a zone of high pressure there. The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across south asia, creating dry air streams which produce clear skies over India from the months of November to may. Meanwhile, a low pressure system develops over northern Australia and winds are directed towards Australia, known as a monsoon trough. During the northeast winter monsoon, Australia and southeast asia receive large amounts of rainfall.
The Indian subcontinent and se asia have been regarded as the most typical examples of monsoon climate. Many parts receive 80% of their total precipitation within 3-4 summer months. Cherrapunji, assam, still holds the world recold for the wettest month and year in 1860-61.
At the surface in India circulation is dominated by subtropical anticyclones. Trade winds blow outward from the land, over which air is subsiding, the air dries out and precipitation is largely prevented. 3km above the land surface the circulation is different. A strong subtropical westerly jet flows across mountain at this season. The southern branch of the jet is much stronger because of the strong thermal gradient above northern India. This upper westerly has the effect of intensifying the surface anticyclone.