Monsoon systems
The word monsoon derives from the Arabic word for season. Monsoons are features of the tropical circulation which show strong dependence between the seasons - in particular, this applies to tropical climates with a very distinct shift from wet to dry seasons. Essentially, the monsoon can be thought of as a giant sea breeze, with moist oceanic air flowing over land, where it ascends, producing convection and rainfall over a large area. This is a result of the seasonal movement of the intertropical convergence zone, which typically moves north and south with the positioning of maximum solar insolation. However, its movement is influenced by the presence of large landmasses (such as the Indian sub-continent), displacing it from the equator. 

The best known monsoon is certainly the Indian summer monsoon, which occurs from late May to early July each year. 80% of India's rainfall occurs during the monsoon, meaning that agriculture and the Indian economy are strongly tied to its timing, intensity and duration. In general, there are three major monsoon systems in the world:
  • ​the south Asian monsoon, centred over India
  • the west African monsoon, centred over the Gulf of Guinea
  • the Australasian monsoon, centred over north-western Australia. 
Sea Breeze
A sea breeze is a good example of an atmospheric circulation that arises at the boundary between the land and ocean. On a hot summer's day, many people flock to the beach to get away from the inland heat and experience a refreshing sea breeze. Sea breezes help to moderate the climate of coasts, and temperatures can often be 10˚C cooler along the coast than inland as a result. The sea breeze involves air blowing from the relatively cool sea towards the warm land surface, preventing temperatures rising significantly near the coast. At night, the opposing land breeze blows from the cool land surface out towards the warmer sea. 
Illustration of the average daily precipitation for December, January, February (top) and June, July, August (bottom). Units are in mm/day. 
Credit: University of Reading.
The Indian monsoon is the most consistent, in terms of its rainfall and date of arrival, of all the monsoon systems. The annual rains usually arrive in south-eastern India during the very end of May, travelling north-west and reaching Pakistan by the middle of July. As a result, Indian agriculture has become heavily dependent on the monsoon rains, and when they are reduced, this can have catastrophic consequences. 

Monsoons are essentially thermally driven circulations that are driven by contrasts in temperature between land and ocean. 
  • Land has a lower heat capacity than water, meaning that it heats up and cools down more during the year.
  • During the spring and early summer, the Indian landmass warms up much faster than the surrounding Indian Ocean. 
  • This generates a meridional temperature and pressure gradient that is dynamically similar to that in the                        circuation (although on a much larger scale). 
  • The key to the heavy rains is that the moisture collected from a large area of tropical ocean is concentrated and then precipitated over a much smaller area of land. 
sea breeze
Mean surface wind vectors for July over the northern Indian Ocean and south Asia. The winds turn to the north and west over the Bay of Bengal as a result of the developing surface heat low. 
The monsoon circulation can be explained in the same way as the sea breeze. During the spring, a heat low develops over India and Pakistan as the land heats up quickly. This drives low level winds from the south that feed deep moist convection. 
​Credit: MetEd/COMET.
The monsoon is a summer phenomenon that provides rainfall and relief from the intense heat that would otherwise grip those locations affected, and it sustains the lives of millions of people. 

What makes the Indian monsoon so special? 
A unique set of geological features enhance the precipitation of the Indian monsoon, and mean that it is much less likely to fail. 
  • Firstly, the Ethopian Highlands constrain the low-level trade winds and focus the flow across the equator, forming the Somali Jet. This essentially provides further moisture transport from the southern hemisphere for the Indian monsoon, as well as a greater low-level flow rate that can feed developing storms. 
  • Secondly, the Himalayas and Tibetan Plateau constrain this low level flow to the north, focusing and converging the winds over India, leading to enhanced ascent. These mountains also act as a barrier to prevent influxes of of cold and dry air from the mid-latitudes that could prevent the development of deep moist convection. 
  • Finally, the high Tibetan Plateau, at around 4000 m, heats up during the spring and summer, and puts a strong heat flux into the mid and upper troposphere directly above. This heating in the upper troposphere at around 30 ˚N leads to a strong reversal in the meridional temperature gradient (with temperature increasing northwards from the equator). This leads to the formation of the Tropical Easterly Jet at 200 hPa, which extends from India eastwards to Africa. India happens to be located close to the right jet entrance of this jet streak, and the associated ageostropic circulation promotes ascent over India that leads to enhanced convection. 

​Indian monsoon and ENSO

Schematic showing the typical Walker circulation pattern during the warm (El Niño) and neutral phases of ENSO. El Niño conditions tend to lead to a drier monsoon, due to large-scale descent over the Indian Ocean. Normal conditions lead to neaturally favourable conditions, while La Niña conditions tend to lead to a wetter monsoon, due to large-scale ascent over the Indian Ocean. 
Credit: Pennsylvania State University.
Although we can expect a drier-than-average monsoon during an El Niño and a wetter-than-average monsoon during a La Niña, short-term variability in the monsoon is driven by 'active' and 'break' phases in rainfall. This variability is dominated by northwards propagating bursts of convection that may be associated with the Madden Julian Oscillation, although this is still an active area of research. The relative dominance of active or break periods is the major factor in determining a weak or strong monsoon. For example, in 2002, an extended break period in July led to a significant drought. 
Daily all-India rainfall for 2002. Only 50% of the average rainfall was recieived for July, with just 81% of the average rainfall for the entire monsoon season.