The word monsoon comes from the Arabic word ‘mausim’ which translates as ‘season’, which is suggestive of the seasonal nature of the monsoon and its associated rains.
A monsoon climate is characterised by a dramatic seasonal change in direction of the prevailing winds of a region which brings a marked change in rainfall. The monsoon climate results in high annual rainfall totals exceeding 1.5 m (5 ft) in many places.
Monsoons lead to distinct wet and dry seasons in many areas throughout the tropics and are most often associated with the Indian Ocean.
Monsoon conditions are best developed in the subtropics, such as in east and south-east Asia. The rainy season associated with monsoon winds is the outstanding feature of the climate of these regions though the term ‘the monsoon’ is popularly used there to denote the rains, without reference to the winds.
During the winter monsoon, large areas of high pressure remain persistently over Asia pushing cool, dry air south to the tropics providing the region with its dry season.
Monsoons around the world
While the Asian monsoon is the most widely known, monsoon conditions also occur also (though to a lesser degree) in northern Australia, parts of western, southern and eastern Africa, and parts of North and South America.
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation,but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea.
Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is sometimes incorrectly used for locally heavy but short-term rains,although these rains meet the dictionary definition of monsoon.
The major monsoon systems of the world consist of the West African and Asia-Australian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal has been debated.
Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau after the collision of the Indian sub-continent and Asia around 50 million years ago.
Because of studies of records from the Arabian Sea and that of the wind-blown dust in the Loess Plateau of China, many geologists believe the monsoon first became strong around 8 million years ago. More recently, studies of plant fossils in China and new long-duration sediment records from the South China Sea led to a timing of the monsoon beginning 15–20 million years ago and linked to early Tibetan uplift.