The Western part of the tropical African continent gets most of its annual rainfall during the boreal summer months from June to September.

This rainy season is associated with the seasonal reversal of the winds in the lowest level of the atmosphere which is called the monsoon.

Monsoon comes from the Arabic and means “season”. The wind reversal being known to the earlier trade sailors travelling from Mozambic to India.

In winter, the wind indeed blows from the cool continent to the warm ocean. Following the Sun apparent movement in the course of the year, the continent warms faster than the ocean. This thermal contrast drives the surface pressure contrast between the ocean (high pressure) and the continent (low pressure) and the set up of the monsoon circulation. Similarly to a giant see-breeze, at the beginning of the summer, the wind changes and eventually blows from the ocean to the continent.

Over the ocean, the air parcels are moist and transported over the continent where this moisture is released, trough deep vertical movements within convective storms, in the form of precipitation.

This simple picture of the monsoon should be complicated a bit to fit to reality and in particular, the earth rotation effects and the moist convective processes need to be accounted for. The role of the Sahara region is also important in the monsoon and it provides a low pressure center that reinforces the Inter Tropical Convergence Zone depression hence favouring the surface pressure gradient.

The West African Monsoon differs in many aspects from the Asian Monsoon. Over West Africa, the large scale structure is very symmetric in the zonal direction while over the Indian subcontinent the flow is more complex. Another important difference, among many, lies in the fact the Indian monsoon seems more resilient that the African one in terms of rainfall. Over the 20th century, India never experienced more than two consecutives years of droughts while the Sahelian region suffered from a long lasting drought for the last twenty years.
It is expected that the science from AMMA will contribute to a better understanding of this strong decennial variability that characterizes the West African Monsoon.