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1. Summer monsoon, winter monsoon
The term “monsoon” comes from an Arabic word “mauism” which means season. The phenomenon is expressed by a great
contrast between seasons, between summer and winter, which affects the climate of a large part of Asia. During summer (May
to October), the South-West winds carrying water drawn from the Indian Ocean are sucked along by the low pressure of the high
plateauxof the Himalaya.As theyblow theybring rain to the West coasts ofIndia and to Bangladesh.
In winter (November to April), high pressures build up over the Asian continent and the
Himalayan chain. The winds then change direction, blowing from the North to the East of
the continent towards the ocean. A sunny dry period then sets in.
The summer monsoon in Asia brings brief but intense episodes of rain. India receives three quarters of
its annual rainfall during this period. Cherrapundji in the North-East of the country is the world’s rainiest
town. An average of five metres of water comes down in July and August alone and the annual amount
reaches 12 m of rain.
The monsoon rains are vital for
populations of the Sahel.
2. In Africa too...
The monsoon is not only an Asian experience. The system also prevails
in Africa, especially in the West, in the Sahel countries. In summer, when
the African continent heats up, it attracts cool, humidity-laden air coming
from the Gulf of Guinea. Over the Sahel, this flow of oceanic air changes
into storm systems moving from East to West. They bring with them rains
that are long awaited in the whole region but which provoke cyclones in
the tropical Atlantic.
The rains the African monsoon generate are very irregular. Continual severe drought has
been affecting the Sahel regions since the 1970s. This climate variability remains an enigma
for scientists. An international research programme involving several French teams has
therefore been launched to identify the driving forces behind the African monsoon system,
assess its impact better and improve prediction techniques. One group of these researchers
is focusing especially on the interactions between the sea surface temperatures in the
Gulf of Guinea and the monsoon rains, by means of an extensive oceanographic research
campaign.
200 °
190 °
180 °
The fall in water resource
s owing to the irregularit
y of monsoon rains has
some tragic consequenc
es in West Africa. In the
Sahel for example, livest
has halved in 30 years. Fu
ock
rther South, power statio
ns
ha
ve
ha
d to shut down
operations.