Atmospheric Circulation Climatic Mechanisms and African Climate

P. Giresse

1 Atmospheric Mass and Flows

The general atmospheric circulation of tropical West Africa is controlled by various “cellular” processes. The circulation of the Hadley cells combines air mass movements from Equator to pole at a high altitude and from pole to Equator at low altitude through eastern winds (trade winds). Thus, high- and low-pressure areas are induced, respectively, by downward and upward motions. Consequently, low pressures are produced at the upward convergence of the hemispheric cells (Azores and Saint-Helen). Other atmospheric cell movements are latitudinal. These zonal circulations occur in the intertropical belts and are called “Walker cell”. At altitudes between 5,000 and 12,000 m, very strong winds (called jets or current-jets) blow from the east, their dynamic fronts are frequently located above the continent and especially in the Intertropical Convergence Zone (ITCZ).

Rainfall distribution and seasonally through Africa are controlled mainly by the presence and the opposition of two main atmospheric mass, the so-called “action centres” (Suchel, 1987). These centres induce the development of powerful air flows that fluctuate through the year giving way to well characterised and well contrasted seasonal climates. Some of these action centres and flows are permanent motors of the tropical atmospheric circulation; some others are the result of a specific situation that prevails during a portion of the year (Fig. 1).

1.1 The Subtropical Anticyclones

In the two hemispheres, a subtropical high-pressure belt extends more or less continuously around the globe near the 30° latitude. These two high-pressure belts are neither homogeneous, nor continuous, especially in the Northern Hemisphere. They are divided to anticyclone cells preferentially located in the eastern part of the oceans in relation to cold sea surface temperature and by the path of cold air masses controlled by polar front circulation. The two cells acting on the African climates are the Azores anticyclone in the Northern Hemisphere and the Saint-Helen anticyclone in the Southern Hemisphere. The first one largely overruns the Sahara during the boreal winter. The second is frequently associated with a South African or Indian anticyclone during the austral winter. In the two Hemispheres, the cells tend to disappear above the continents during the summer, but remain in the high atmosphere: a high-pressure belt is especially observed between 500 and 700 mbar above the Sahara. Generally, the air mass of the cells is subsident and divergent and maintains a marked dryness.

1.2 The Trade Winds

The trade winds of both hemispheres met within the two pressure troughs and constitute the surface flow of the tropical Hadley circulation. The trade winds are well developed in the eastern Atlantic and the neighbouring land areas. The Northern Hemisphere trade winds flow generally from the NE whereas the Southern Hemisphere trade winds circulate from the SE, but both of them tend to curve to the West in response to the anticyclone cell curvature (Fig. 1). The northeastern trade is called harmattan in the whole Sudanian area and is characteristic of the continental air mass. Initially this is very dry air, which becomes dryer as it crosses the Sahara. This subsiding and stable pattern of the harmattan is maintained until meeting the equatorial boundary from which it takes up moisture. Above the Atlantic, the trade air mass is generally stratified with a lower wet and slightly cool layer, and an upper dry and relatively warm layer. This thermal inversion is generally well marked. Near the anticyclonic cells, this inversion is very low (< 500 m) and prevents atmospheric upwelling, thus prohibiting rain development. As trade winds spread over the ocean water, the wet layer becomes thicker, the stability of the atmospheric mass decreases, and the intensity and frequency of the rainfall increases. Finally, all the trade mass becomes a moist process and the upwards air movements take precedence over the horizontal movements. This trade mass merges into the equatorial atmospheric mass. This vertical rain-making structure near the Geographical Equator is called Meteorological Equator by Leroux (1983).

1.3 Intertropical Convergence Zone (ITCZ)

The ITCZ is characterized by low atmospheric pressures (low intertropical pressures), as a consequence of the upward movement induced by the trade winds convergence. It results in high atmospheric pressure in the upper troposphere. It seems that the role of this ITCZ is more-or-less passive in the general circulation unlike the anticyclonic cells (Suchel, 1987). Near the ground, the winds are low, unstable and often only a local breeze. Unless there is a specific disturbance, wind still (doldrums) is common, especially when the low pressures are near the Equator. Generally, at high altitudes over the ocean, the Meteorological Equator remains vertically structured in the vicinity of the Geographical Equator. But, overland, the Meteorological Equator forms an oblique, northward dipping plane in the lower troposphere, the so-called Intertropical Front (ITF) structure or Inclined Meteorological Equator (IME). This plane is the zone of contact of two conflicting atmospheric air-flows, the eastwards oceanic winds and the northeastern continental winds, and a zone of major disturbance. The IME ranges from the Gulf of Guinea to far over the continent. The SE trades change direction after passing the Equator (between 2° and 5° and become the SW monsoon that we will consider later. The eastern equatorial winds maintain prevailing flow of the low atmosphere layers all year round. During the boreal summer, this flow is stronger and thicker. But it is necessary to admit a relative autonomy of this flow because it is acting during boreal winter, at a period in which the monsoon is absent. This ITF migrates seasonally: it reaches its northernmost position around 4° and the NE-trade winds blow off the Liberian coasts onto the Gulf of Guinea as January trades and induce the winter dust plume (Péwé, 1981; Dupont et al., 2000). In accordance with this annual migration, the regions nearest the Equator experience rainfall all year round, while the semi-arid desert fringe receives rains only at the height of the summer. Throughout the year, the mid-troposphere wind systems (jet stream) overlaying the surface circulation generally flows from the east (Leroux, 1983). Another convergence zone the Equatorial African Front or Congo-Zaire Air Boundary separates the flows of the Atlantic and Indian Oceans.

2 Climatic Mechanisms and Seasonal Variations

The seasonal reversal of atmosphere circulation and the precipitation patterns associated with the African monsoon are dominant components of tropical climate variability. Monsoon circulation results from the differing heat capacities of land and ocean. Sensible heat warms land surfaces much more rapidly than the ocean mixed layer.

2.1 Low Thermal Pressures of the Sahara

Sensible and latent heat over the Sahara during boreal summer drives the inflow of moisture-laden air from the adjacent eastern equatorial Atlantic. Thus, the Azores anticyclone is moved northwest and the coastal areas of Maghreb are still the only areas where the pressure is slightly higher than the average. Over the Sahara, the barometric condition is largely opposite to that prevailing during the winter where a relative high-pressure situation develops. During the summer this low-pressure area adds to the latitudinal movement of the ITCZ. However, it is admitted that the most important driving force of the atmospheric circulation during this period of austral winter is the Saint-Helen anticyclone. The anticyclonic cell expands toward the Equator and becomes especially active as a consequence of powerful Antarctic outlets.

2.2 The Monsoon

This name refers to the southwestern oceanic inflows that cross the Equator and penetrate into northwestern Africa during the boreal summer. The summer monsoon winds deliver sporadic but intense precipitation, which nourishes the Sahel. Beyond the coastal region, this monsoon is a relatively well-defined flow coming from SSW, SW, or WSW. Then, during its progress over the continent, it bends to the east and slows down. Due to its direction and speed, this monsoon is not clearly distinguished from the general westerly circulation that prevails all year round in the equatorial region. The monsoon takes on individual characteristics since the time when thermal depressions begin to get wider above the Sahara, namely during May–June. The IME, yet in a northern position, takes its place rapidly on this low-pressure mass whereas the Saint-Helen anticyclonic cell is growing just at the south of the Equator. The monsoon extends beyond the boundary of the permanent westerly circulation. It follows the IME during its northward course from May to August bringing precipitations in the Southern Sahara. Because of its very high water content (near the saturation value) and of the great thickness of the wet layer (generally more than 3,000 m thick), the monsoon atmosphere is characterised by a very important potential instability. It is admitted that the active evaporation of the warm waters of the Bay of Biafra plays an important role...