scholarly journals The Intraseasonal Variability of African Easterly Wave Energetics

2014 ◽  
Vol 27 (17) ◽  
pp. 6559-6580 ◽  
Author(s):  
Ghassan J. Alaka ◽  
Eric D. Maloney
Keyword(s):  
West Africa ◽  
Energy Conversion ◽  
East Africa ◽  
Diabatic Heating ◽  
Heating Temperature ◽  
Intraseasonal Variability ◽  
West African ◽  
Boreal Summer ◽  
Lake Chad ◽  
Baroclinic Energy Conversion

Abstract African easterly waves (AEWs) and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer. Consistent with East Africa (e.g., east of Lake Chad) being an initiation region for AEWs, previous studies have shown that increased East African PKE precedes and leads to increased West African AEW activity on intraseasonal time scales. In this study, reanalysis budgets of PKE and perturbation available potential energy (PAPE) are used to understand this behavior. The variability of PKE and PAPE sources is analyzed as a function of Madden–Julian oscillation (MJO) phase and a local 30–90-day West African PKE index to diagnose when and where eddy energy conversions terms are important to periods of increased or decreased intraseasonal AEW activity. In East Africa, an increased meridional temperature gradient locally enhances baroclinic energy conversion anomalies to initiate periods of increased intraseasonal AEW activity. Downstream barotropic and baroclinic energy conversions associated with strong AEWs are important for the maintenance of intraseasonal AEW activity in West Africa. Barotropic energy conversions dominate south of the African easterly jet (AEJ), while baroclinic energy conversions are most important north of the AEJ. In both East and West Africa, diabatic heating does not appear to aid intraseasonal PKE creation. Instead, negative PAPE tendency anomalies due to the diabatic heating–temperature covariance act as a negative feedback to increased baroclinic energy conversion downstream in the AEJ.

scholarly journals Internal Intraseasonal Variability of the West African Monsoon in WRF

2017 ◽  
Vol 30 (15) ◽  
pp. 5815-5833 ◽  
Author(s):  
Ghassan J. Alaka ◽  
Eric D. Maloney
Keyword(s):  
East Africa ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
West African Monsoon ◽  
West African ◽  
Boreal Summer ◽  
Kelvin Wave ◽  
The West ◽  
African Monsoon ◽  
External Influences

The West African monsoon (WAM) and its landmark features, which include African easterly waves (AEWs) and the African easterly jet (AEJ), exhibit significant intraseasonal variability in boreal summer. However, the degree to which this variability is modulated by external large-scale phenomena, such as the Madden–Julian oscillation (MJO), remains unclear. The Weather Research and Forecasting (WRF) Model is employed to diagnose the importance of the MJO and other external influences for the intraseasonal variability of the WAM and associated AEW energetics by removing 30–90-day signals from initial and lateral boundary conditions in sensitivity tests. The WAM produces similar intraseasonal variability in the absence of external influences, indicating that the MJO is not critical to produce WAM variability. In control and sensitivity experiments, AEW precursor signals are similar near the AEJ entrance in East Africa. For example, an eastward extension of the AEJ increases barotropic and baroclinic energy conversions in East Africa prior to a 30–90-day maximum of perturbation kinetic energy in West Africa. The WAM appears to prefer a faster oscillation when MJO forcing is removed, suggesting that the MJO may serve as a pacemaker for intraseasonal oscillations in the WAM. WRF results show that eastward propagating intraseasonal signals (e.g., Kelvin wave fronts) are responsible for this pacing, while the role of westward propagating intraseasonal signals (e.g., MJO-induced Rossby waves) appears to be limited. Mean state biases across the simulations complicate the interpretation of results.

The Influence of the MJO on Upstream Precursors to African Easterly Waves

2012 ◽  
Vol 25 (9) ◽  
pp. 3219-3236 ◽  
Author(s):  
Ghassan J. Alaka ◽  
Eric D. Maloney
Keyword(s):  
West Africa ◽  
West African Monsoon ◽  
West African ◽  
Temperature Fields ◽  
Boreal Summer ◽  
Tropospheric Temperature ◽  
The West ◽  
Lake Chad ◽  
African Monsoon ◽  
African Easterly Jet

The Madden–Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the “trigger region” in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region. In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa.

Muslim Brotherhoods in West African History

2021 ◽  
Author(s):  
Mauro Nobili
Keyword(s):  
West Africa ◽  
18Th Century ◽  
West African ◽  
The Novel ◽  
African History ◽  
Lake Chad ◽  
History Of ◽  
Independent States ◽  
European Colonialism ◽  
Colonial Government

Muslim Sufi brotherhoods (ṭuruq, sing. ṭarīqa) are ubiquitous in contemporary Islamic West Africa. However, they are relative latecomers in the history of the region, making their appearance in the mid-18th century. Yet, Sufism has a longer presence in West Africa that predates the consolidation of ṭuruq. Early evidence of Sufi practices dates to the period between the 11th and the 17th centuries. By that time traces of the Shādhiliyya and the lesser-known Maḥmūdiyya are available between the Atlantic Ocean and Lake Chad, but it was the activities of the Kunta of the Qādiriyya and of al-ḥājj ‘Umar of the Tijāniyya that led to the massive spread of Sufi brotherhoods in the region. The authority of leaders of ṭuruq did not disappear with the imposition of European colonialism. In fact, the power of those leaders who adjusted to the novel political situation further consolidated thanks to their role as mediators between their constituencies and the colonial government. Eventually, the end of the colonial period did not signal the decline of ṭuruq in West Africa. Conversely, during the postcolonial years, Sufi brotherhoods continued flourishing despite the secular nature of West African independent states and the increasing tension with a plethora of equally rising Salafi movements.

Some Problems in the Development of Air Transport in West Africa

1953 ◽  
Vol 57 (512) ◽  
pp. 477-490
Author(s):  
Hubert Walker
Keyword(s):  
West Africa ◽  
East Africa ◽  
Sierra Leone ◽  
Ivory Coast ◽  
West African ◽  
Early History ◽  
Gold Coast ◽  
Air Transport ◽  
Financial Difficulties ◽  
History Of

West Africa, particularly British West Africa, has been one of the last areas to be opened up to Air Transport and because of physical and financial difficulties, progress has been slower than in most other parts of the Empire.As West Africa, even today, is not very well known in other parts of the Empire, it will be useful to give a brief description of the territory and the early history of aviation there before dealing with the special problems encountered in the development of air transport. While the particular territories dealt with in this lecture are the four British West African Colonies and Protectorates of the Gambia, Sierra Leone, the Gold Coast and Nigeria, it will be necessary, from time to time, to make passing reference to the adjacent French territories and even to the Anglo–Egyptian Sudan. The four British territories, unlike those in East Africa, are not contiguous but each is surrounded on the land side by the intervening French territories of Senegal, Guinea, Ivory Coast, Dahomey, Volta, Niger, Chad and the Cameroons.

On the recent Swarming of Locusta migratorioides, R. & F.

1931 ◽  
Vol 22 (3) ◽  
pp. 365-378 ◽  
Author(s):  
Owen B. Lean
Keyword(s):  
West Africa ◽  
East Africa ◽  
Water Regime ◽  
Breeding Ground ◽  
Lake District ◽  
Lake Chad ◽  
Breeding Grounds ◽  
Imperial Institute ◽  
Ultimate Control ◽  
River Niger

The present paper attempts to summarise the information collected by the Imperial Institute of Entomology on the swarming of migratorioides in Africa.It is concluded that the outbreaks in West, Central and East Africa are all interrelated and probably originated in one or at most two permanent breeding-grounds.The area most suspected is the lake district of the middle Niger, where conditions seem suitable for the formation of the swarming phase. Lake Chad is probably a secondary rather than a permanent breeding-ground.It is indicated that the key to the problem of the cause of the original outbreak may be found in the study of recent variations in the water régime of the River Niger.Should these suppositions be substantiated, we are well on the way towards a solution of the problem. The ultimate control of the locusts depends not on control during the swarming periods, but upon preventing the swarming phase from developing in the permanent breeding-grounds.It is therefore important to check and augment the records of the early swarms in West Africa and more important still, to study the behaviour of the locust, its ecology, and the water régime in the suspected areas.In conclusion the author wishes to acknowledge the constant help and advice of Mr. B. P. Uvarov. His thanks are also due to Mr. H. B. Johnston for his permission to make use of his paper in manuscript.

Turkish Archival Sources for West African History

1967 ◽  
Vol 10 (03) ◽  
pp. 59-65 ◽  
Author(s):  
B. G. Martin
Keyword(s):  
West Africa ◽  
East Africa ◽  
Political System ◽  
Central Africa ◽  
West African ◽  
Early History ◽  
Northern Nigeria ◽  
African History ◽  
History Of ◽  
Islamic Society

About 1960, the study of West African history took a new turn as historians became aware of the interest and value of Islamic sources for their work, particularly manuscript materials in Arabic. To be sure, the use of Arabic sources for the history of West Africa is nothing new: in 1841, W. Des-borough Cooley published his The Negroland of the Arabs Examined and Explained; or, an Inquiry into the Early History and Geography of Central Africa. But Cooley's pioneering book was discounted by later British and American writers on Africa as the work of an eccentric. In the 1880's and 1890's, many of these writers were spellbound by their vision of what Christianity might do for the African, while others were preoccupied by what they deemed to be the morally indefensible activities of the Muslims as slave-raiders and traders in West and East Africa. As late as the 1930's, the well-known British anthropologist C. K. Meek indicted Islam in northern Nigeria when he wrote: “The institution of slavery is a pivotal feature of Islamic society, and we are justified with charging Muhammadanism with the devastation and desolation in which Northern Nigeria was found at the beginning of this century.” Other writers, like Sir A.C. Burns for Nigeria, and A. W. Cardinall and W. E. F. Ward for Ghana, dismissed the Islamic side of West African history in few words, or gave it no mention at all. There were other reasons for this lack of emphasis. In northern Nigeria, for example, many British officials were apprehensive of an outbreak of “Mahdism” among the Muslims; and very frequently, French officials looked on Islam as a rival political system, dangerous and potentially subversive.

A Comparison of African Kingdoms

Africa ◽  
1962 ◽  
Vol 32 (4) ◽  
pp. 324-335 ◽  
Author(s):  
J. Vansina
Keyword(s):  
West Africa ◽  
East Africa ◽  
Preliminary Analysis ◽  
West African ◽  
Political Systems ◽  
Few Data ◽  
Opening Paragraph ◽  
Central South

Opening ParagraphIt has long been recognized that there is a distinctive category of African political systems, which is characterized by centralized authority. The preliminary analysis and classification of African kingdoms proposed in this article are based on comparison of most of the States in Central, South, and East Africa. Only a few data on West African kingdoms have been incorporated, but we feel that the classification will prove generally valid for West Africa as well.

Development of Plant Food Production in the West African Savannas: Archaeobotanical Perspectives

2018 ◽  
Author(s):  
Katharina Neumann
Keyword(s):  
West Africa ◽  
Pearl Millet ◽  
West African ◽  
Shea Butter ◽  
Plant Domestication ◽  
Small Scale ◽  
Vegetable Crops ◽  
Plant Food ◽  
The West ◽  
Lake Chad

The West African savannas are a major area of independent plant domestication, with pearl millet, African rice, fonio, several legumes, and vegetable crops originating there. For understanding the origins of West African plant-food-producing traditions, it is useful to have a look at their precursors in the Sahara during the “African humid period” between 10,500 and 4,500 years ago. The Early and Middle Holocene Saharan foragers and pastoralists intensively used wild grasses for food but did not intentionally cultivate. Due to increasing aridity in the late 3rd millennium bce, the pastoralists migrated southward into the savanna zone. In this context pearl millet was domesticated and spread rapidly in West Africa during the 2nd millennium bce. It was first cultivated by agro-pastoral communities, predominantly on a small scale. The 1st millennium bce was a transitional phase: most of the early agricultural societies disappeared, but it was also a time of numerous economic and social innovations. Due to increasing aridity, the floodplains around Lake Chad and the valleys of the rivers Senegal and Niger became accessible to farming populations after 1000 bce. In the 1st millennium ce, agriculture intensified, with mixed cultivation of cereals and legumes and the integration of new African domesticates, such as sorghum, fonio, roselle, and okra. Pearl millet remained the major crop in most areas, while sorghum dominated in northern Cameroon. Imported wheat, date palm, and cotton appeared in the first half of the 2nd millennium ce. The combined exploitation of cultivated cereals, legumes, and wild fruit trees (e.g., shea butter tree) in agroforesty systems eventually resulted in a cultural landscape as it is still visible in West Africa today.

The Coin of the African Realm

2017 ◽  
Author(s):  
Jeffrey Herbst
Keyword(s):  
Twentieth Century ◽  
West Africa ◽  
West African ◽  
Regional Economies ◽  
African Countries ◽  
New Strategies

This chapter examines the politics of the currency in West Africa from the beginning of the twentieth century. A public series of debates over the nature of the currency occurred in West Africa during both the colonial and independence periods. Since 1983, West African countries have been pioneers in Africa in developing new strategies to combat overvaluation of the currency and reduce the control of government over the currency supply. The chapter charts the evolution of West African currencies as boundaries and explores their relationship to state consolidation. It shows that leaders in African capitals managed to make the units they ruled increasingly distinct from the international and regional economies, but the greater salience of the currency did not end up promoting state consolidation. Rather, winning the ability to determine the value of the currency led to a series of disastrous decisions that severely weakened the states themselves.

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