The monsoon systems of the world oscillate annually between their summer and winter configurations. Usually, winter monsoons usher in dry, cool conditions, replacing the rain and heat of their summer counterparts. Monsoons affect south, southeast and east Asia, northern Australia, west-central Africa and some warmer regions of North and South America.
The Meteorology of Monsoons
Monsoons are primarily seasonal winds in the warmer regions of the world caused by large temperature differences between land masses and adjacent oceans. In the winter, the circulation is from the cool land to the warmer sea, while in the summer, the opposite is true. The air-flow pattern of the winter monsoons, on a continental scale, leads to dry, cool conditions on land. Because of the quirks of local geography, however, some regions may experience rain during the winter monsoon.
Asia and Northern Australia
The main driver of the winter monsoon in Asia is the high pressure zone that develops over Mongolia and northwestern china between about November and March and pushes out cool, dry northeasterly winds over most of the continent. But winter rains occur in certain regions, such as along the eastern coast of southern India, Sri Lanka, Indonesia and Malaysia, because they happen to be downwind from either the Bay of Bengal or the South China Sea. In the Australian winter (May to September), dry easterly winds predominate over northern Australia.
Severe Weather in Eastern Asia
Northeasterly winter monsoon winds cause surges of strong winds and abnormally low temperatures in eastern Asia. Statistics compiled by the Hong Kong Observatory indicate episodes of exceptionally high wind speeds in December and January. Moreover, when the cold continental air interacts with the warm, moist air over the East China Sea, it creates extreme atmospheric instability which spawns cyclones – as hurricanes are known in that part of the world – in a process similar to the formation of hurricanes in the Atlantic by the interaction of the cold continental air with the Gulf Stream.
The monsoons – both summer and winter – are most prominent in Asia because of the huge temperature differences between land and sea, the size of the continent and its geography. But monsoons also occur in other parts of the world. In North America, western Mexico and parts of Arizona and some neighboring states receive summer monsoon rains. Winter monsoon conditions – dry, cool continental air -- set in during September and October when the monsoon rain moves south into the deep tropics. Severe thunderstorms, however, often occur in the winter over Arizona and northern Sonora when the cold air advancing from the north interacts with lingering moisture from the summer monsoon. In South America, the winter monsoon brings dry weather: In central-west Brazil, rainfall in the winter is about a tenth of that in the summer.