Monsoon mecanisms and its components:
Atmospheric dynamics, Continental water cycle, Atmospheric chemistry and Surface conditions
Rainfall over West-Africa is notoriously unreliable, especially in its northern part.
The famines that struck the Sahel in the 70’s (1972-1974) and in the 80’s (1983-1985) have prompted a number of
authors ( to investigate possible mechanisms responsible for these dramatic events. In fact, these two sequences of a few extremely dry years were part of a longer drought that lasted continuously from the end of the 60’s to the mid 90’s, as illustrated in this Figure:
Déficits pluviométriques (mm) entre les périodes humides (1950-1969) et sèches (1970-1989)
Les zones en pointillés correspondent à des déficits allant 150 à 230 mm.
This unusual dry spell was not limited to the Sahel but was felt down to the Guinea Coast as well (Lebel et al 2000), the
average rainfall deficit of the 70’s and 80’s with respect to the 50’s and 60’s being 180 mm in average
over the area shown in Figure 2. The consequences of the related abrupt decrease of water resources
were devastating to both populations and economies. In the Sahel, the cattle population was divided
by two, some exportation cultures disappeared. In the more humid regions to the South, a severe
shortage of electricity was caused by hydropower plants being shut down, during the summers of
1984 and 1998, following the weakness of the preceding rainy seasons. It was estimated that the solid
economic growth (at a rate of over 5% a year) of a country like Ghana was stopped in 1999, due to
this water resources related problem.
This drought is only one manifestation of the climate variability of West-Africa, spanning a large
range of scales from intraseasonal to decadal.
Given its strong impact on human activities at the
regional scale and its climatic effects at larger scale - discussed below - there is an imperious need
for a better understanding of this variability and for improved seasonal forecasting skills. As a matter
of fact, we currently have considerable difficulties in analyzing, simulating and forecasting that
variability, even though we know that it is under the control of a dominant atmospheric feature: the
West African Monsoon (WAM). The central reason of these difficulties lies in the complex
interactions between the atmosphere, the biosphere and the hydrosphere that are controlling the
dynamics of the WAM and the life cycle of the associated rain producing systems. Recent modelling
work in this area have shown that the oceans, the vegetation and the topography very likely play an
important role in the establishment of a monsoonal circulation over West Africa.
Beyond the regional motivations exposed above, one has also to consider the role of Africa - and
singularly West Africa -in a more global context.
This role has generally received little attention.
However Africa is one of the major heat source of the earth climate, characterized by a strong
meridional migration, which impacts on the annual cycle of other tropical and mid-latitude regions.
The significant correlation existing between Atlantic hurricanes and West Sahelian rainfall is but one example of the links between the WAM and the climate of other
regions of the world. Similarly, from a global atmospheric chemistry point of view, there are strong
evidences that the African monsoon region is critical in the emission of ozone precursors and of
aerosols and their redistribution over the whole troposphere. Trace gas and aerosol emissions in the
African Tropics are strongly affected by human activities. Biomass burning in savanna and forest
ecosystems is the most important source of atmospheric pollution, emitting huge
amounts of reactive gases and particles (including soot and organic carbon) that have a direct impact
on regional radiative budget. The savanna burning over Africa is estimated to correspond to
0.7 GT Carbon by year (i.e. around 20 % of the global biomass burning). Atmosphere dynamics and
chemistry, vegetation dynamics and the continental water cycle are thus closely interrelated and have
to be studied in conjunction if one is to better understand and predict the West African rainfall
From a purely scientific viewpoint it is worth noting that, due to unique geographical and vegetation
features leading to a first-order zonally symmetry, the West African Monsoon (WAM)
represents probably the less complicated monsoon
system to study on both observational and modeling aspects. In this perspective, it is challenging that General Circulation Models poorly
simulate the WAM climate and its variability. This is
indisputably linked to the numerous interactions that have to be taken into account if one is to better
represent the WAM in atmospheric or coupled atmospheric-hydrologic models. The problem,
however, is that many processes or state variables controlling the interactions between the land and
ocean surfaces and the atmosphere are ill-documented in this part of the world.
Time has come for an integrated research project on the West African Monsoon and its related
atmospheric, biospheric, hydrologic, and oceanic components.
Such a project will be interdisciplinary
by nature, given the motivations and context detailed above. It will have to link observations, data
analysis and modeling. It will also span a broad range of scales, especially since one important
component of the WAM variability is linked to its forcing by fluctuating surface conditions. This
component is dominant at the seasonal to decadal scales (and beyond) in a regional context. On the
other hand, there is an internal component which is fundamental at the intraseasonal scale, involving
numerous interactions. This component is mostly observed at the meso and synoptic scales.
major field of studies will consequently have been considered,
in order to get a full and
integrated picture of the climate and water cycle variabilities in West Africa:
(internal variability of the WAM), Continental water cycle, Atmospheric chemistry, Surface
conditions (forced variability of the WAM both by the continent and by the ocean).
- Dynamism of the Monsoon©AMMA