TABLE OF CONTENT
Title page…………………………………………………………………………………………….i
Certification ……………………………………………………………………………………….ii
Dedication…………………………………………………………………………………………iii
Acknowledgement………………………………………………………………………………iv
Table of content…………………………………………………………………………………..v
List of figures…………………………………………………………………………………..viii
Abstract……………………………………………………………………………………………..x
CHAPTER ONE:
INTRODUCTION……………………………………………………………………………..1
- Background of the study……………………………………………………………..1
1.2 Statement of the research problem………………………………………………..4
1.3 Research questions……………………………………………………………………..6
1.4 Aim and objectives……………………………………………………………………..7
1.5 Hypothesis…………………………………………………………………………………7
1.6 Significance of study…………………………………………………………………..7
1.7 Scope of the study………………………………………………………………………8
1.8 Study area………………………………………………………………………………….8
CHAPTER TWO:
LITERATURE REVIEW AND CONCEPTUL FRAMEWORK……….14
2.0 Introduction……………………………………………………………………………..14
2.1 Conceptual framework………………………………………………………………15
2.1.1 Mechanisms of the Hadley cell model………………………………15
2.1.2 Zonally symmetric steady model of the Hadley cell………..……….16
2.1.3 The ITD model…………………………………………….…………18
2.1.3.1 ITD motions…………………………………………………..21
2.1.4 The Subtropical highs……………………………………………….23
2.2 Literature review…………………………………………………….26
2.2.1 General atmospheric circulation…………………………………….26
2.2.2 The trade winds………………………………………………..…….33
2.2.3 The Harmattan……………………………………………………….35
2.2.4 Drought………………………………………………………………37
2.2.4.1 Meteorological drought……………………………………….39
2.2.5 The climate of West Africa………………………………………….40
2.2.5.1 Position of the intertropical discontinuity (ITD………………..41
2.2.6 Rainfall onset and cessation in Nigeria………………………………43
2.2.6 Trends in Nigerian rainfall…………………………………………..47
2.2.7 Literature gap……………………………………………………….48
CHAPTER THREE:
RESEARCH METHODS…………………………………………………………………52
3.0 Introduction……………………………………………………………………………..52
3.1 Data source andtype…………………………………………………52
3.2 Data need…………………………………………………………….52
3.3 Method of data collection……………………………………………53
3.4 Data standardization and Arrangement………………………………53
3.5 Qualitative technique………………………………………………..54
CHAPTER FOUR:
DATA ANALYSIS, PRESENTATION AND DISCUSSION………………56
4.1 Mean monthly rainfall and mean ITD positions……………………………56
4.2 Mean annual rainfall and mean annual ITD position……………………..61
4.3 Analysis of other climatic factors associated with rainfall variability……64
4.4 Testing of hypothesis………………………………………………………………..65
4.5 Discussion of findings………………………………………………………………66
CHAPTER FIVE:
SUMMARY, CONCLUSION AND RECOMMENDATION……………..69
5.1 Summary of findings…………………………………………………………………69
5.2 Recommendation……………………………………………………………………..70
5.3 Conclusion………………………………………………………………………………71
5.4 Future research…………………………………………………………………………73
References……………………………………………………………………………….74
Appendices………………………………………………………………………………86
LIST OF FIGURES
Fig. 1.1: Nigeria showing the location of synoptic stations………………….9
Fig. 2.1: Schematic diagram showing the three cells…………………………16
Fig. 2.2: A simple model of the Hadley cell…………………………………….18
Fig. 2.3: Diagram showing the subtropical high pressure systems………24
Fig. 2.4: Simple Hadley cell circulation………………………………………….29
Fig. 2.5: Modified vertical circulation pattern………………………………….30
Fig. 2.6: Idealized global circulation of surface winds……………………..32
Fig. 2.7: A schematic of the near-surface winds………………………………43
Fig. 4.1: Mean monthly rainfall for 2010……………………………………….57
Fig. 4.2: Mean monthly rainfall 2011……………………………………………..57
Fig. 4.3: Mean monthly rainfall 2012……………………………………………..58
Fig. 4.4: Mean position of the ITD 2010…………………………………………58
Fig. 4.5: Mean position of the ITD 2011…………………………………………59
Fig. 4.6: Mean position of the ITD 2012…………………………………………59
Fig. 4.7: Mean annual rainfall (2010 – 2012)…………………………………..61
Fig. 4.8: Mean annual ITD position (2010 – 2012)…………………………..62
Fig. 4.9: Mean annual rainfall and Mean ITD position (2010 – 2012)…63
ABSTRACT
An attempt is made in this research to examine the distribution of rainfall in Nigeria within the framework of the dominant influence of the Intertropical Discontinuity (ITD) which is a boundary separating the dry northern air from the moist southern air. The aim of the study is to know the influence of the ITD in the distribution of rainfall in Nigeria. The Student ‘t’ test was employed to test the hypothesis as well as the significant difference, while the correlation analysis was used to explain the strength of the relationship between the two variables (rainfall and ITD position). The result shows that there is a significant difference between the distribution of rainfall in Nigeria and the position of the ITD and there was a weak relationship between the two variables. There should be easy accessibility to data which will aid in the proper and productive research on subsequent areas of study; decision-makers should also key into this research, falling back to it as a guide or reference and with the support of the government, more researches in climate-related studies should be encouraged.
CHAPTER ONE
INTRODUCTION
The forecasting of surface rainfall in the tropics is a difficult proposition. The problem is compounded by insufficient data, and inadequate knowledge of the synoptic systems which gives rise to the space-time variations of observable rainfall. Moreover, the physical processes which produce rainfall are not only very complex, but the main cause of rainfall, vertical motion, is not directly measurable. The existence of large-scale vertical motions in the atmosphere is, therefore, usually inferred from the flow patterns of major air streams, and convective motion from stability considerations. As a consequence, the evaluation of the casual factors of rainfall in the tropics has to be considered within the framework of the circulation patterns and the modification introduced by surface topography (Lamb, 1972).
The generally accepted idea of the structure of the lower troposphere over the tropic is that there are two main air streams; a moist but rather cool southerly air with a west-southwesterly component which forms a wedge under, and a dry and relatively warm northerly air with an east-northeasterly component. The obvious consequence of this arrangement is the creation of a more or less pronounced “humidity discontinuity.”
For many years, the appropriate term for this boundary was plagued by semantic difficulties. Tropical weather analyst have consequently identified the boundary by several names: the Intertropical Convergence Zone ( ITCZ), the Intertropical Front (ITF), the West African Monsoon (WAM), and the Intertropical Discontinuity (ITD). The terms used reflects the great diversity of views which have been advanced to account for the structure and behaviour of the boundary on both land and water (Sawyer, 1952; Thompson, 1957; Soliman, 1958).
It has established, however that the region where the hot and dry northern air is separated from the cool and moist southern air is neither frontal nor invariably convergent (Forksdyke, 1949; Bleeker, 1958a, b; Kirk, 1946). It is primarily a region of “maximum surface moisture gradient” i.e., a humidity discontinuity (Ilesanmi, 1969). It is a region where the hot and dry northern air, sometimes marked by negative dew points, is separated from the cool and moist southern air often characterized by dew points ˃70F.
Similarly, theIntertropical Convergence Zone (ITCZ) is a zone of low-pressure near the equator where two easterly trade winds originating from the Northern and Southern hemispheres converge. This zone of enhanced convection, cloudiness, and rainfall constitutes the rising branch of the meridional Hadley circulation. The non-uniform distribution of land and sea introduces zonal asymmetries in heating, which drive an east-west overturning, known as the Walker circulation, wherein air rises at longitudes of the heating and sinks at the other longitudes. The three-quasi permanent centers of the rising branch of the Walker circulation are located over Indonesia, central Africa, and the Amazon basin. From satellite data, the ITCZ can be identified as a meandering band of cold infrared effective temperature and high albedo located over the warmest equatorial region. It plays an important role on the atmospheric energy balance (Waliser and Gautier 1993) and the earth’s climate (Zhang 1993). The excessive heat absorbed at the surface over the tropical oceans is transferred to the lower troposphere through evaporation then transported to higher altitudes through convection and latent heat release and to higher latitudes through the Hadley circulation. The convective latent heat release plays a vital role in driving low-latitude circulations and in supplying energy to balance the radiative heat losses and ’fuel’ the wind systems of middle and high latitudes.
Furthermore, the enhanced cloudiness associated with convective cloud systems contributes significantly to the planetary albedo, absorptivity, and transmisivity of the two incident solar radiation. Therefore, the structure, position, and migration of the ITCZ are important in defining and analyzing the earth’s climate on a global scale (Nicholson, 2009). Accordingly, the strength and character of the air-sea coupling are important in determining the earth’s climate on a local scale.
1.2 Statement of the Research Problem
Different from the large-scale southward displacement of the ITD, drought occurrence can also be attributed to factors which inhibit rainfall activity behind (i.e. to the south of) the ITD. This may take the form of a weakening of the rainy reason “intensity”.
According to Palmer (1986), warming of the tropical Atlantic ocean reduces the meridional gradient of sea surface temperature (SST) south of the ITD and this results in a weakening of the Hadley meridional circulation (i.e. the pattern of circulation of the atmosphere over the tropics). The weakened circulation reduces the intensity of the southwest monsoon flow into West and Central Africa and consequently rainfall over southern Nigeria.The major drought in the Sahel since 1970s has been attributed to an eastward and southward extension of the Azores high (Barry et al., 1976). Knowledge of this flow if not properly detected would mean that drought conditions might take the populace unaware.
Accordingly, droughts in Nigeria, and indeed over West Africa, are associated with a restricted northward advance of the ITD. On the other hand, wet years result from a considerable northward advance of the ITD.
If the onset of rainfall is getting persistently delayed over the Coastal belt of the country, will the Guinea Savanna belt that lies immediately north of it, also experience delayed start of the rains? Also, if the rains retreat earlier than usual over the Sahelian Zone will the other regions to its south similarly experience this? The answers to these questions lie in a study of the spatial coherence of rainfall variations between the four regions of the country that was carried out byOlaniran (1990) and Olaniran and Sumner (1989, 1990a, b). They studied the degree of spatial coherence of rainfall variation between the four regions of Nigeria. A high degree of association was found in the date of retreat of rainfall and in the annual rainfall frequency i.e. rain days over the whole country. The implication is that the other three regions to the south will similarly experience a later or earlier retreat of rainfall that begins in the Sahel.
Ayo J.O., ObidiJ.A., RekwotP.I. (2010) defines desertification as a condition of human-induced land degradation that occurs in arid, semiarid and dry sub-humid regions and leads to a persistent decline in economic productivity of useful biota related to a land use or a production system. Climatic variations intensify the decline in productivity, restorative management mitigates it. Drylands or territories susceptible to desertification occupy 39.7% of the global terrestrial area. The highest concentration of drylands occurs in Africa, Asia and Australia.
Desertification which is the absence of rainfall or other form of precipitation over a long period of time is closely attached to this. If the onset of rain is delayed for a long time, there will be food scarcity as well as issues of desertification. These are all chains of activities that are closely related to the position of the ITD.
According to Ayoade (2008), the ITD is at its northernmost position at around latitude 20-22°N in August while it reaches the southernmost position around latitude 6-7°N in January/February. With this, one can predict the date for the onset and cessation of rainfall in Nigeria. It would also go a long way to assist farmers make wise choice in regards to early planting periods and use of irrigation agriculture when the need arise. The discontinuity therefore provides a framework for following the south-north motions of the rain-producing southerly air whose depth and motion not only influence the rainfall rate but the duration and spread of rain as well.
1.3 Research Questions
The research will seek to answer a number of issues, including the following:
- Does the annual movement of the ITD north and south of Nigeria influence the distribution of rainfall in the country?
- What other climatic variability contributes to the impact of ITD on rainfall distribution?
- What other climate agent contributes to the impact of ITD on rainfall distribution?
1.4 Aim and Objectives
The principal aim of this research is to determine the extent to which the Intertropical Discontinuity (ITD) has influenced the distribution of rainfall in Nigeria.
This aim will be pursued by carrying out a number of tasks with the following objectives, to:
- Investigate the mean monthly rainfall distribution of Nigeria in relation to the position of the ITD for the years under study.
- Analyze the significance of the Intertropical Discontinuity as a climatological entity.
- To analyze and describe other climatic factors associated with rainfall variability in Nigeria.
1.5 Hypothesis
Ho: There is no significant difference between the distribution of rainfall in Nigeria and the position of the ITD.
1.6 Significance of Study
In view of the above considerations, the result of this study may have some practical implications. The findings from this research might be helpful to agrometerologistand agriculturist as well as to decision makers and environmentalist on issues concerning food security in the country, because the knowledge on the position of the ITD and its influence on rainfall could help predict and solve issues like drought to a large extent and keep the populace abreast of harmattan severity. The date of rainfall onset and cessation is also of great significance as well in this regard.
1.7 Scope of the Study
The scope of this research shall cover areas relating to the influence of ITD in the rainfall distribution over Nigeria. It goes further to highlight those factors which encourages the formulation of a discontinuity zone. The subtropical north and south will be exemplified in this study from Azores archipelagos to St. Helena as well as using the dew point temperature to denote the position and contributions to the movement of the Intertropical Discontinuity (ITD). The season of the year (harmattan and rainy season) is a baseline for analyzing these factors.
1.8 Study Area
Geographically, Nigeria (Figure 1.1) is situated in the West African region with the coordinates longitude 3°and 14° East and latitude 4° and 14° North. The longest distance from East to West is about 767 kilometers, and North to South 1,605 kilometers. Its area covers about 923,769 square kilometers (made up of 909,890 square kilometers of land area and 13,879 square kilometers of water area) (national bureau of statistics, 2010). It is bordered to the north by the Republics of Niger and Chad; it shares to the West with the Republic of Benin, while the Republic of Cameroun shares the eastern borders right down to the Atlantic Ocean which forms the southern limits of Nigerian Territory.
Fig 1.1: Nigeria showing the location of synoptic stations
1.8.1 Topography
The geology of Nigeria is dominated by igneous structures that form most of the highlands and hills. Therocks of the Basement Complex, mainly of igneous origin, are encountered in over 60% of the surface area.
Younger Granites are intruded into these rocks in Jos Plateau and environs. Volcanic rocks are also extrudedon to the surface in places such as Jos Plateau and Adamawa Highlands. Areas of sedimentary formationsare restricted to the coastal belt; the Niger-Benue Trough, including the southeastern scarps land and theSokoto-Rima basin; and the Chad Basin.
The landforms can simply be classified into highlands, plateaus, hills, plains and river valley systems.Suffice it to state that the landforms are more deeply dissected in the southern parts than in the northernparts. Indeed, except for the Eastern Highlands in Adamawa area and the Jos Plateau, basins characterizedby broad gently sloping plains dominate the northern half of Nigeria. An extensive section of this area isidentified as the High Plains of Hausa land (Udo, 1970).
The geology and the geomorphological processes that shaped the landforms have greatly influenced thesoils. The major soil types in Nigeria, according to the FAO soil taxonomy legends are fluvisols, regosols,gleysols, acrisols, ferrasols, alisols, lixisols, cambisols, luvisols, nitosols, arenosols, and vertisols . These soiltypes vary in their potential for agricultural use.
1.8.2 Vegetation
In line with the rainfall distribution, a wetter south and a drier northern half, there are two broad vegetation types: Forests and Savanna. There are three variants of each, running as near parallel bands east to west across country.Guinea savanna, Sudan savanna, Tropical (high) evergreen; and Sahel savanna.The southern forest that is, both the swamps and the rain forest constitutes the country’s main source ofwood. The derived savanna zone, about 250km wide, was once the northern part of the forest zone, buttransformed by such activities into a vegetation type consisting largely of deciduous trees and grasses. Thevegetation still supplies some wood. Most of the remaining part of the country is the Sudan Savannaaccounting for more than 25% of the surface area, and expanding at the expense of the Guinea Savanna. Atthe northeastern and northwestern corners of the country is the Sahel that ordinarily does not account formore than 5 – 10% of the surface area, but is now growing larger at the expense of the Sudan zone. Indeed,it is now more meaningful to take the two driest zones together as the Sudano-Sahelian zone.
1.8.3 CLIMATE
Nigeria has a tropical Climate which is made up of rainy and dry seasons. This depends essentially on the location. It is mostly hot and wet during the year in the southeast but dry in the southwest and farther inland. In the northern part of the country, the climate is characterized by wet and dry seasons as well as in the west while a steppe climate with little precipitation is usually the case in the far north. The boundary surface area betweenthe two air masses is known as the Inter-tropical Discontinuity (ITD). The ITD migrates north and south of the country bringing rainfall or dryness to different areas of thecountry at different times of the year. Roughly, its northward movement brings the wet season to all areasof its location, while its southward migration brings the dry season to areas north of its location.
The length of the rainy season decreases from the south to the north. In the south the rainy season lasts from March to November, whereas in the far north it lasts only from mid-May to September. A marked interruption during the rainy season occurs in August in the southern part of the country, resulting in a short dry season often referred to as the “August break” (national bureau of statistics, 2012).
Precipitation is heavier in the south, especially in the southeast which receives more than 120 inches (3,000mm) of rain a year, compared with about 70 inches (1,800mm) in the southwest. Rainfall decreases progressively away from the coast, with the north receiving not more than 20inch (500mm) a year. Temperature and humidity remain relatively constant all through the year in the south while the seasons vary considerably in the north, during the dry season.
The daily temperature range during this time increases as well. On the coast, the mean monthly maximum temperatures are steady throughout the year remaining at about 90°f (32°C) in Lagos and about 91°f (33°C) in Port Harcourt, while the mean monthly minimum temperatures are approximately 72°f (22°C) for Lagos and 68°f (20°C) for Port Harcourt. The mean maximum temperature is usually higher in the north, with a lower mean minimum temperature. In the North East (Maiduguri), the mean monthly maximum temperature may exceed 100°f (38°C) during the hot months of April and May, while in the same season there is the likelihood of frost at night. Humidity is generally high in the north, but it falls during harmattan (the hot dry northeast trade wind) which blows for more than 3 months in the north but rarely for more than 2 weeks along the coast.
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