Central And Southern Africa

1 The African monsoon

The annual climatic regime over West Africa has many similarities to that over South Asia, the surface airflow being determined by the position of the leading edge of a monsoon trough (see Figure 11.2). This airflow is southwesterly to the south of the trough and easterly to northeasterly to its north (Figure 11.38). The major difference between the circulations of the two regions is due largely to the differing geography of the land to sea distribution and to the lack of a large mountain range to the north of West Africa. This allows the monsoon trough to migrate regularly with the seasons. In general, the West African monsoon trough oscillates between annual extreme locations of about 2°N and 25°N (Figure 11.39). In 1956, for example, these extreme positions were 5°N on 1 January and 23°N in August. The leading edge of the monsoon trough is complex in structure (see Figure 11.40B) and its position may oscillate greatly from day to day through several degrees of latitude. The classical model of a steady northward advance of the monsoon has recently been called into question. The rainy season onset in February at the coast does propagate northward to 13°N in May, but then in mid-June there is a sudden synchronous onset of rains between about 9°N and 13°N. The mechanism is not yet firmly established, but it involves a shift of the lower tropospheric African Easterly Jet (AEJ) (see Figure 11.40B).

In winter, the southwesterly monsoon airflow over the coasts of West Africa is very shallow (i.e. 1000 m) with 3000 m of overriding easterly winds, which are themselves overlain by strong (>20 m s-1) winds (see Figure 11.41). North of the monsoon trough, the surface

Equatorial Westerlies Rain Summer

Figure 11.38 The major circulation in Africa in (A) June to August and (B) December to February. H: subtropical high-pressure cells; EW: equatorial westerlies (moist, unstable but containing the Congo high-pressure ridge); NW: the northwesterlies (summer extension of EW in the southern hemisphere); TE: tropical easterlies (trades); SW: southwesterly monsoonal flow in the northern hemisphere; W: extratropical westerlies; J: subtropical westerly jet stream; JA and JE: the African easterly and tropical easterly jet streams; and MT: monsoon trough.

Figure 11.38 The major circulation in Africa in (A) June to August and (B) December to February. H: subtropical high-pressure cells; EW: equatorial westerlies (moist, unstable but containing the Congo high-pressure ridge); NW: the northwesterlies (summer extension of EW in the southern hemisphere); TE: tropical easterlies (trades); SW: southwesterly monsoonal flow in the northern hemisphere; W: extratropical westerlies; J: subtropical westerly jet stream; JA and JE: the African easterly and tropical easterly jet streams; and MT: monsoon trough.

Source: From Rossignol-Strick (1985), by permission Elsevier Science Publishers BV, Amsterdam.

Source: From Rossignol-Strick (1985), by permission Elsevier Science Publishers BV, Amsterdam.

MONTH

Figure 11.39 The daily position of the monsoon trough at longitude 3°E during 1956. This year experienced an exceptionally wide swing over West Africa, with the trough reaching 2°N in January and 25°N on 1 August. Within a few days after the latter date, the strongly oscillating trough had swung southward through 8° of latitude.

MONTH

Figure 11.39 The daily position of the monsoon trough at longitude 3°E during 1956. This year experienced an exceptionally wide swing over West Africa, with the trough reaching 2°N in January and 25°N on 1 August. Within a few days after the latter date, the strongly oscillating trough had swung southward through 8° of latitude.

Source: After Clackson (1957), from Hayward and Oguntoyinbo (1987).

northeasterlies (i.e. the 2000-m deep Harmattan flow) blow clockwise outward from the subtropical high-pressure centre. They are compensated above 5000 m by an anticlockwise westerly airflow that, at about 12,000 m and 20 to 30°N, is concentrated into a subtropical westerly jet stream of average speed 45 m s-1. Mean January surface temperatures decrease from about 26°C along the southern coast to 14°C in southern Algeria.

With the approach of the northern summer, the strengthening of the South Atlantic subtropical high-pressure cell, combined with the increased continental temperatures, establishes a strong southwesterly airflow at the surface that spreads northward behind the monsoon trough, lagging about six weeks behind the progress of the overhead sun. The northward migration of the trough oscillates diurnally with a northward progress of up to 200 km in the afternoons following a smaller southward retreat in the mornings. The northward spread of moist, unstable and relatively cool southwesterly airflow from the Gulf of Guinea brings rain in differing amounts to extensive areas of West Africa. Aloft, easterly winds spiral clockwise outward

Figure 11.40 The structure of the circulation over North Africa in August. (A) Surface airflow and easterly tropical jet. (B) Vertical structure and resulting precipitation zones over West Africa. Note the high-level tropospheric easterly jet and the lower African easterly jet.

Notes: r? = thunderstorm activity; MT = monsoon trough. Sources: (A) Reproduction from the Geographical Magazine, London; (B) From Maley (1982), copyright © Elsevier Science; reproduced by permission; and Musk (1983).

Figure 11.40 The structure of the circulation over North Africa in August. (A) Surface airflow and easterly tropical jet. (B) Vertical structure and resulting precipitation zones over West Africa. Note the high-level tropospheric easterly jet and the lower African easterly jet.

Notes: r? = thunderstorm activity; MT = monsoon trough. Sources: (A) Reproduction from the Geographical Magazine, London; (B) From Maley (1982), copyright © Elsevier Science; reproduced by permission; and Musk (1983).

from the subtropical high-pressure centre (see Figure 11.41) and are concentrated between June and August into two tropical easterly jet streams; the stronger TEJ (>20 m -1) at about 15,000 to 20,000 m and the weaker AEJ(>10 m s-1) at about 4000 to 5000 m (see Figure 11.40B). The lower jet occupies a broad band from 13°N to 20°N, on the underside of which oscillations produce easterly waves which may develop into squall lines. By July, the southwesterly monsoon airflow has spread far to the north and westward-moving convective systems now determine much of the rainfall. The leading trough reaches its extreme northern location, about 20°N, in August. At this time, four major climatic belts can be identified over West Africa (see Figure 11.40A):

1 A coastal belt of cloud and light rain related to frictional convergence within the monsoon flow, overlain by subsiding easterlies.

2 A quasi-stationary zone of disturbances associated with deep stratiform cloud yielding prolonged light rains. Low-level convergence south of the easterly jet axes, apparently associated with easterly wave disturbances from east central Africa, causes instability in the monsoon air.

3 A broad zone underlying the easterly jet streams, which help to activate disturbance lines and thunderstorms. North-south lines of deep cumulonimbus cells may move westward steered by the jets. The southern, wetter part of this zone is termed the Soudan, the northern part the Sahel, but popular usage assigns the name Sahel to the whole belt.

4 Just south of the monsoon trough, the shallow tongue of humid air is overlain by drier subsiding air. Here there are only isolated storms, scattered showers and occasional thunderstorms.

In contrast to winter conditions, August temperatures are lowest (i.e. 24 to 25°C) along the cloudy southern coasts and increase towards the north, where they average 30°C in southern Algeria.

Both the summer airflows, the southwesterlies below and the easterlies aloft, are subject to perturbations,

Figure 11.41 Mean wind speeds (m s-1) and directions in January and July over West Africa up to about 15,000 m. Ocean water temperatures and the positions of the monsoon trough are also shown, as are the area affected by the August little dry season and the location of the anomalous Togo Gap. The locations of Abidjan (Ab), Atar (At), Bamako (B) and Conakry (C) are given (see precipitation graphs in Figure 11.42).

Source: From Hayward and Oguntoyinbo (1987).

Figure 11.41 Mean wind speeds (m s-1) and directions in January and July over West Africa up to about 15,000 m. Ocean water temperatures and the positions of the monsoon trough are also shown, as are the area affected by the August little dry season and the location of the anomalous Togo Gap. The locations of Abidjan (Ab), Atar (At), Bamako (B) and Conakry (C) are given (see precipitation graphs in Figure 11.42).

Source: From Hayward and Oguntoyinbo (1987).

which contribute significantly to the rainfall during this season. Three types of perturbation are particularly prevalent:

1 Waves in the southwesterlies. These are northward surges of the humid airflow with periodicities of four to six days. They produce bands of summer monsoon rain some 160 km broad and 50 to 80 km in north-south extent, which have the most marked effect 1100 to 1400 km south of the surface monsoon trough, the position of which oscillates with the surges.

2 Waves in the easterlies. These develop on the interface between the lower southwesterly and the upper easterly airflows. These waves are from 1500 to 4000 km long from north to south. They move westward across West Africa between mid-June and October with a periodicity of three to five days and sometimes developing closed cyclonic circulations. Their speed is about 5 to 10° of longitude per day (i.e. 18 to 35 km hr-1). At the height of the summer monsoon, they produce most rainfall at around latitude 14°N, between 300 and 1100 km south of the monsoon trough. On average, some fifty easterly waves per year cross Dakar. Some of these carry on in the general circulation across the Atlantic, and it has been estimated that 60 per cent of West Indian hurricanes originate in West Africa as easterly waves.

Figure 11.42 Mean number of hours of rain per month for four West African stations. Also shown are types of rainfall, mean annual totals (mm) and, in parentheses, maximum recorded daily rainfalls (m) for Conakry (August) and Abidjan (June). Dots show the mean monthly rainfall intensities (mm hr-1). Note the pronounced little dry season at Abidjan. Station locations are marked on Figure 11.41.

Source: From Hayward and Oguntoyinbo (1987).

Figure 11.42 Mean number of hours of rain per month for four West African stations. Also shown are types of rainfall, mean annual totals (mm) and, in parentheses, maximum recorded daily rainfalls (m) for Conakry (August) and Abidjan (June). Dots show the mean monthly rainfall intensities (mm hr-1). Note the pronounced little dry season at Abidjan. Station locations are marked on Figure 11.41.

Source: From Hayward and Oguntoyinbo (1987).

3 Squall lines. Easterly waves vary greatly in intensity. Some give rise to little cloud and rain, whereas others have embedded squall lines when the wave extends down to the surface, producing updrafts, heavy rain and thunder. Squall line formation is assisted where surface topographic convergence of the easterly flow occurs (e.g. the Air Mountains, the Fouta-Jallon Plateau). These disturbance lines travel at up to 60 km hr-1 from east to west across southern West Africa for distances of up to 3000 km (but averaging 600 km) between June and September, yielding 40 to 90 mm of rain per day. Some coastal locations suffer about forty squall lines per year, which account for more than 50 per cent of the annual rainfall (see Plate 29).

Annual rainfall decreases from 2000 to 3000 mm in the coastal belt (e.g. Conakry, Guinea) to about 1000 mm at latitude 20°N (Figure 11.42). Near the coast, more than 300 mm per day of rain may fall during the rainy season but further north the variability increases due to the irregular extension and movement of the monsoon trough. Squall lines and other disturbances give a zone of maximum rainfall located 800 to 1000 km south of the surface position of the monsoon trough (see Figure 11.40B). Monsoon rains in the coastal zone of

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Figure 11.43 The contributions of disturbance lines and thunderstorms to the average monthly precipitation at Minna, Nigeria (9.5°N).

Source: After Omotosho (1985), by permission of the Royal Meteorological Society.

Nigeria (4°N) contribute 28 per cent of the annual total (about 2000 mm), thunderstorms 51 per cent and disturbance lines 21 per cent. At 10°N, 52 per cent of the total (about 1000 mm) is due to disturbance lines, 40 per cent to thunderstorms and only 9 per cent to the monsoon. Over most of the country, rainfall from disturbance lines has a double frequency maximum, thunderstorms a single one in summer (see Figure 11.43 for Minna, 9.5°N). In the northern parts ofNigeria and Ghana, rain falls in the summer months, mostly from isolated storms or disturbance lines. The high variability of these rains from year to year characterizes the drought-prone Sahel environment.

The summer rainfall in the northern Soudana to Sahelian belts is determined partly by the northward penetration of the monsoon trough, which may range up to 500 to 800 km beyond its average position (Figure 11.44), and by the strength of the easterly jet streams. The latter affects the frequency of disturbance lines.

Anomalous climatic effects occur in a number of distinct West African localities at different times of the year. Although the temperatures of coastal waters always exceed 26°C and may reach 29°C in January, there are two areas of locally upwelling cold waters (see Figure 11.41). One lies north of Conakry along the coasts of Senegal and Mauretania, where dominant offshore northeasterly winds in January to April skim off the surface waters, causing cooler (20°C) water to rise, dramatically lowering the temperature of the afternoon onshore breezes. The second area of cool ocean (19 to 22°C) is located along the central southern coast west of Lagos during the period July to October, for a reason that is as yet unclear. From July to September, an anomalously dry land area is located along the southern coastal belt (see Figure 11.41) during what is termed the little dry season. The reason is that at this time of year the monsoon trough is in its most northerly position. The coastal zone, lying 1200 to 1500 km to the south of it and, more important, 400 to 500 km to the south of its major rain belt, has relatively stable air (see Figure 11.40B), a condition assisted by the relatively cool offshore coastal waters. Embedded within this relatively cloudy but dry belt is the smaller Togo Gap, between 0° and 3°E and having during the summer above-average sunshine, subdued convection, relatively low rainfall (i.e. less than 1000 mm) and low thunderstorm activity. The trend of the coast here parallels the dominant low-level southwesterly winds, so limiting surface frictionally induced convergence in an area where temperatures and convection are in any case inhibited by low coastal water temperatures.

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  • Caden
    What is the climate of central and southern africa?
    8 years ago
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    What is the source of southwesterly winds in nigeria?
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