Transcript Topic 3: Africa: Climate and Vegetation

Topic 3: Africa: Climate and Vegetation
 Character of African
Climate
- Temperature
- Precipitation & Evaporation
- Implications
 Accounting
for Climatic Differences
- Atmospheric Pressure and Wind
Systems


in January (winter)
in July (Summer)
Topic 3: Africa: Climate and Vegetation
 Climatic
-
Regions of Africa
Tropical Rainforest Climate (Af)
Savanna Climate (Aw)
Hot Steppe Climate (Bsh)
Cool Steppe Climate (Bsk)
Hot Desert Climate (Bwh)
Cool Desert Climate (Bwk)
Mediterranean Climate
Topic 3: Africa: Climate & Vegetation
 Drought
in Africa
- Types of drought:
meteorological drought
 agricultural drought
 hydrological drought

- Patterns of Drought Distribution
- Causes and Effects of Drought
Topic 3: Africa: Climate & Vegetation

Vegetation and Soils
- Types and Patterns
- Deforestation of Tropical Rainforest:
 Patterns
 Causes and Effects
African Climate: Temperature

most tropical continent by location, hence
much of Africa has tropical climate

however, the extreme northern and southern
portions of the continent have subtropical
climate

temperature is high everywhere (>10oC)

spatial variation of temperature is gradual due
to the absence of major relief features
African Climate: Temperature (July) Map
African Climate: Temperature (January) Map
African Climate: Temperature

however, prevailing high temperatures
moderated in areas of high elevation

pattern of temperature distribution is
determined by:
- local topography or elevation
- offshore ocean currents
- pattern of atmospheric circulation
- land area
African Climate: Temperature

local topography or elevation:
- hence much of East Africa is cooler

offshore ocean currents:
- cool currents depress local temperature,
hence:
Morocco
Cape Verde
Cape Town
Cape Lopez, have cooler temperatures
African Climate: Temperature

pattern of atmospheric circulation:
- lower winter temperature for northernmost
and southernmost parts of Africa due to
mid-latitude depression or westerly winds
- example:
 Cape Town (13oC in July (winter season)
 Algiers (12oC in Jan (winter season)
 Land area
- Southern Africa has smaller land area,
hence comes under stronger oceanic
influence and records lower temperature
African Climate: Temperature
 distance
from the coast:
- temperature range increases away
from the coast
African Climate: Precipitation

rainfall varies much more widely in amount
and seasonal incidence than temperature

hence, rainfall is a more critical variable for
climate differentiation over much of Africa

Generally, rainfall is highest within latitude
15o north and south of the equator

and decreases both northward and
southward of the equator
African Climate: Precipitation Map
African Climate: Precipitation
 Rainfall
-
is generally high:
along the coast of West Africa
stretching
into the Congo basin
- northern Malagasy Republic (300 mm)
- northern coast of Mozambique across
to Angola
African Climate: Precipitation

However, anomalously low rainfall amount is
observed along the coast of Ghana particularly
east of Cape Three Points because of:
-
-
the tendency for winds to blow parallel to
the shoreline so that frictional divergence
occurs
and the relative coolness of its ocean water
due to upwelling of cold water which has the
effect of stabilizing the air and causing fog
off the coast rather than precipitation
African Climate: Precipitation

rainfall amount is elevated in areas of high
relief:
- East African Highlands
- mountains of southern Africa
- hilly areas of Futa Djallon Mt., Cameroon
Mt. and Jos Plateau

Most parts of Africa receive their rainfall
during the summer season, exception the
coast lands of North Africa and Cape Town with
winter rainfall
African Climate: Precipitation

Length of the rainy season increases
towards the Equator

rainfall regime is generally double-peak
around the equatorial belt and becomes
single peak regime the farther away from
equator

Rainfall amount, duration and regime are
primarily controlled by the location and
movement of the ITCZ
Double-Peak Rainfall Regime
Rainfall Regime and Movement of ITCZ
A:
C:
D:
Dry, Haze
B: Moist Air Little Rain
Deeper, Moist Air Convection, Line Squall
Less intense more continuous rain
Single-Peak Rainfall Regime
African Climate: Precipitation

Perpetually dry areas:
- Sahara desert
- Namibian desert and horn of Africa

Sahara aridity caused by the permanent
existence of high pressure (air subsidence)
in the region

Namibian coast land aridity caused by the
effect of the cold Benguela current and the
southeasterly that arrive as dry winds
African Climate: Evaporation

In general, evaporation increases away from
the wet equatorial region to the dry savanna

annual evaporation rate:
- In humid equatorial regions is <750 mm
- in hot and dry deserts is >2000 mm

mean annual values of potential
evapotranspiration (PE):
-
>114 cm in Africa north of the equator,
except north Africa cool coast lands
African Climate: Evaporation
- >175cm recorded in the hinterland of
West Africa, Sudan and Somali coast
lands
- <114 cm in Africa south of the
equator
African Climate: Implications
-
For Health
For Agriculture
Population and Language Diversity
etc
Accounting for Climatic Differences

climatic differences and patterns explained
in terms of:
- variations of rainfall amount and
- seasonal distribution of rainfall

Whereas, variations in these rainfall
characteristics depend on:
- atmospheric pressure and wind systems
- distance from the coast
- local relief and topography
Atmospheric Pressure and Wind
Systems

Africa has distinct areas of high and low
pressure:
- two sub-tropical high pressure belts at lat.
30o N&S
- one major equatorial trough of low pressure

the location of the belts vary from day to day
and from season to season

the belts follow the overhead sun
Atmospheric Pressure and Wind
Systems

atmospheric pressure determines:
- general direction of air flow
- occurrence or absence of precipitation

Whereas, converging air at centers of low
pressure is uplifted to produce precipitation

air subsidence around high pressure belts
produces dry and cloudless weather
Pressure and Wind Systems in
January (Winter Season)

a high pressure belt extends from Morocco
through Sinai peninsular in the north

another high pressure belt is centered on the
south Atlantic Ocean

And the equatorial trough of low pressure

the equatorial trough is most intense along
the west African coast and across southern
Africa centered on central Angola, Zambia,
and Mozambique
Map of Pressure and Wind Systems in
January (Winter)
Pressure and Wind Systems in
January (Winter)

NE trade winds flow from the high pressure
belt in Morocco southward toward the
equatorial trough

NE winds are hot, dry and dusty and are
known as the Harmattan in West Africa

SE trade winds originates from the high
pressure belt in the south Atlantic and move
towards the equatorial trough
Pressure and Wind Systems in
January (Winter)

SE trade winds are deflected to the right at
crossing the equator to become the SW
monsoon winds at the Guinea Coast

SE and SW winds are maritime and
therefore moist

the boundary zone separating the dry, hot NE
and the moist SE or SW winds and passing
through the center of low pressure is generally
known as the Inter-Tropical Convergence
Zone (ITCZ)
Pressure and Wind Systems in
January (Winter)

NE monsoon reaches East Africa as warm,
dry air bringing clear sunny weather

NE becomes NW over south of Zanzibar
and picks up moisture over the Indian
Ocean to bring precipitation to Malagasy
Pressure and Wind Systems in
July (Summer)
 High
pressure belt is more intensely
developed over southern Africa
 High
pressure belt has moved further
north and outside Africa
 hence,
north equatorial low pressure
trough is displaced to around latitude
15-20o north of the equator
Map of Pressure and Wind Systems in July
(Summer
Pressure and Wind Systems in
July (Summer)

the south equatorial low pressure trough
over central Angola disappears

one strong ITCZ develops

SE and SW air flows are very strong

since SW and SE winds bring clouds and
precipitation to most of Africa south of
the surface position of the ITCZ
Pressure and Wind Systems in
July (Summer)
 midlatitude
westerly develops over
the southern extremities of Africa
Intra-Tropical Convergence Zone (ITCZ)

movement of ITCZ determines seasonal
changes in Africa

ITCZ moves north-south with the
overhead sun with about 3-4 weeks lag

movement of the ITCZ is controlled by the
location and intensity of the subtropical
anticyclones

ITCZ is at its northernmost position
around latitude 20oN in August
Intra-Tropical Convergence Zone
(ITCZ)

ITCZ is at its southernmost position around
latitude 6oN in January

northward movement of the ITCZ is gentle
and gradual (160 km/month)

southward movement of the ITCZ is abrupt
and twice as fast (320 km/month)
Climatologic Importance of ITCZ:
-
provides a framework for monitoring
the north-south movement of the rainproducing SW monsoons
-
its movement and depth influence rainfall
amount, duration and distribution
-
most heavy rainfall activities occur about
300 km south of the surface location of the
ITCZ because of its great depth and
convective activity at such location
CLIMATIC REGIONS OF AFRICA:
Major climatic belts of Africa:
-
Tropical Rainforest Climate (Af)
Savanna Climate (Aw)
Hot Steppe Climate (BSh)
Cool Steppe Climate (BSk)
Hot Desert Climate (BWh)
Cool Desert Climate (BWk)
Mediterranean Climate:
CLIMATIC REGIONS OF AFRICA
CLIMATIC REGIONS OF AFRICA

Tropical Rainforest Climate (Af):
-
mean temperature of coldest month is at
least 18oC
-
no distinct dry season (wet all year
round)
-
hot and moist climate
-
found along Guinea coast and part of
Congo basin around the equator and east
Malagasy
CLIMATIC REGIONS OF AFRICA

Savanna Climate (Aw):
- mean temperature of coldest month is at
least 18oC
-
distinct dry season with rainfall occurring
in summer
-
prevails over a belt stretching from West
Africa to Central Africa, east coast of East
Africa (from Tanzania to Mozambique)
and northwest Malagasy
CLIMATIC REGIONS OF AFRICA

Hot Steppe Climate (BSh):
-
very little rainfall (semi-arid)
-
mean annual temperature above 18oC
-
prevails in West African Sahel zone,
southern margin of northwest Africa, much
of southern Africa and western Malagasy
-
also occurs from eastern Ethiopia through
Kenya into northern Tanzania
CLIMATIC REGIONS OF AFRICA

Cool Steppe Climate (BSk):
-
similar to BSh in terms of rainfall
characteristics but cooler (mean annual
temperature <18oC)
-
occurs in northwest and southwest
Africa
-
coolness in NW Africa due to effect of
high altitude while in SW Africa due to
the effect of cold Benguela current
CLIMATIC REGIONS OF AFRICA

Hot Desert Climate (BWh):
-
arid climate of little or no rain
-
hot with mean annual temperature
>18oC
-
occurs over Sahara desert and coast of
Angola through Namibia into South
Africa, most of Somali and extreme
southwestern Malagasy
CLIMATIC REGIONS OF AFRICA

Cool Desert Climate (BWk):
-
arid climate with little or no rain
-
cool with mean annual temperature
<18oC
-
occurs in southwestern Africa
CLIMATIC REGIONS OF AFRICA
 Mediterranean
Climate:
- occurs in the extreme southwest of
South Africa and coast lands of
northwest Africa
- rains occur in winter and summer is
hot and dry in north Africa but
warm and dry in the south due to
the cold Benguela current
DROUGHT IN AFRICA:

long record of dry or drought years:
-

1913, 1972, 1973, 1983, and 1984 are very
dry years across Sahelian Africa
dry years tend to follow dry years or wet
years to follow another wet years:
- 1940s - dry years
- 1950s - wet years
- 1968 - dry year
- 1970-74 - dry years
- 1979-84 - dry years
Drought in Africa

1988 was a very wet year across Africa:
- >50 mm of rain recorded in Khartoun
- >400 mm of rain recorded in August alone
in Kano City resulting in large
dam
(Bakolori dam disaster) failure

Sahelian drought of 1970-1974 first drew
world attention to Africa drought problems

since then, drought of greater intensity
occurred in West Africa Sahel, Ethiopia, the
Horn of Africa, East Africa and southern
Africa
Drought Affected Countries in Africa
Drought in Africa
MAIN TYPES OF DROUGHT:
 meteorological drought

agricultural drought

hydrological drought
Drought in Africa
Meteorological Drought:
 occurs
when the amount of rainfall is less
than the expected long-term average
annual rainfall in the location
 or when
percentage reduction or negative
departure from the long-term average
rainfall occurs
Drought in Africa

Meteorological drought definition has a
number of problems:
-
paucity of data
-
does not account for soil moisture,
ambient temperature, rates of
evaporation, etc, important to policy
makers and farmers
-
difficult to identify with any degree of
reliability
Drought in Africa
Agricultural Drought:
 occurs when there is not enough moisture
available at the right time for the growth
and maturation of crops

timing of precipitation throughout the
growing season is as important as the
absolute amount per month or season
because crops have varying needs for
moisture as they develop
Drought in Africa
 occurs
even when rainfall amount
seems more than adequate and
 soil
in valley bottoms and flood
plains become waterlogged leading
to diminished aeration and crop
failure
Drought in Africa
Hydrological Drought:

occurs when stream flow falls below a predetermined level significant to hinder certain
human activities like:
-

shipping
HEP generation
irrigation water distribution
periods of low level of stream flow and
extended period of soil dryness
Drought in Africa
Effects of Drought:
 scarcity of water for human and animal
consumption

widespread crop failure as soil moisture
dries up

widespread famine, hunger, malnutrition

over 100,000 lives lost to famine in 1973
alone in the sahelian drought of 1973
Drought in Africa

thousands of lives lost in 1983/84 in the
sahel and in 1987 in the horn of Africa

large-scale out-migrations of people from
the sahel belt

more than 10% of the population of Chad,
Mauritania, Ethiopia, Niger became
drought refugees

slump in agricultural export and food crops
Drought in Africa

weak economies of drought affected
countries

intensification of desertification and
increased dust storms and shifting sand
dunes

drastic reduction in lake levels and areal
extent due to excessive loss of water:
- in 1966, Lake Chad was 22,000 km2
but dropped to <2000km2 by the end of
the 1979-1984 drought
Drought in Africa
 drop
in water table due to reduced
recharge of aquifers causing dry
valleys
Causes of Drought

El Nino Southern Oscillation (ENSO) or
Teleconnection:
- argued that El Nino years tend to
correspond to drought years in the Sahel
Causes of Drought

Sea-Surface Temperature Changes:
- results in the reduction of the northward
penetration of the southwest monsoon
winds

Prevalence of dust storms caused by wind
erosion:
- it argues that dust storms warm up the
atmosphere thereby inhibiting the upward
flow of air needed for rainfall
Causes of Drought

The albedo effects (Charney hypothesis):
- dry and bare surfaces have higher albedo
than moist covered surfaces
-
hence desert surfaces are cooler because
of the higher reflection of solar energy
-
cooler air has less tendency to rise thereby
limiting the probability of rainfall
-
this explanation has not been duplicated
with much certainty in other environment
Causes of Drought

Degradation-induced Changes or LandSurface Feedback Mechanism:
-
it is an extension of the arguments of
Charney Hypothesis
Sequence of Climate/Land-cover Feedback
Systems in Semi-arid Sahel
Vegetation of Africa
Main vegetation belts:

Tropical Rainforest
 Guinea Savanna
 Sudan Savanna
 Semi-desert or Sahel Savanna
 Desert
 Mediterranean Woodland
 Highveldt Grassland and Shrub
Vegetation of Africa
Africa’s Major Biomes
Vegetation Types
Related Climates Related Soil Types
Tropical Rainforest Equatorial
Oxisols
Guinea Savanna
Oxisols, Alfisols,
Ultisols
Alfisols (Ustalfs)
Humid tropical
Sudan Savanna
Tropical wet &
dry
Semi-Desert (Sahel) Semiarid
Alfisols, Aridisols
Desert
Desert
Aridisols
Mediterranean
Mediterranean
Alfisols (Xeralfs)
African Soils
Tropical Rainforest

occurs in close association with the humid
equatorial climates

Tropical rainforest areas:
- coastal areas of West Africa and Central
Africa (Congo basin, Gabon, southern
Cameroon, southern Nigeria, Benin
Republic)
-
part of southern Ghana, Cote d'Ivoire and
Liberia
Tropical Rainforest

rainfall in excess of 1500 mm and at least 8
months of rainy season

bulk of the biomass is tree or woody species
which are closely packed

trees occur in three layers of canopies at
different heights:
The emergent layer (uppermost tree layer):
- consists of isolated tall trees (40-50m tall)
called emergents because they tower
above the general level of the forest
Tropical Rainforest
The middle tree layer:
- consists of large trees with a dense and
continuous canopy
-
the trees are 20-30 meters tall
The lower tree layer:
- consists of trees with canopy barely
touching each other
-
trees have narrower profile and 10-15m
tall
Tropical Rainforest

trees include timber producing species:
- Chlorophora excelsa (locally called Iroko)
- Triplochiton scleroxylon (African maple
tree called locally as Obeche)
- Entrandrophragma cylindricum (locally
called African mahogany),etc
The Shrub Layer:
- Found below the shrub layer
- consists of dwarf trees and tree saplings
- generally less than 5 meters tall
Tropical Rainforest
The Undergrowth Layer:
- covers the forest floor
- consists of herbaceous plants
- a sparse layer of fungi, ferns, and
geophytes (plants with underground stems)

stratification of the rainforest into layers is a
response to competition for sunlight

sun-loving species grow very tall and shade
tolerant species occupy the lower strata
Tropical Rainforest

stratification of the rainforest has created
numerous micro-climatic zones in the forest

hence, the rainforest is rich in plant and
animal species (it has the largest biodiversity
of species)

over 100 species of woody flowering plants
per hectare

African rainforest is not as rich in species as
the South American and Asian rainforests
Tropical Rainforest

African rainforests are mainly mixed forests
(i.e. a wide range of species growing together
in mixtures with no single species
dominating)

mixed forests occur in well drained soils with
medium to good fertility status

single-dominant rainforests are common in
Congo basin, Cameroon, some part of
Nigeria and Gabon (i.e. upper tree layer is
dominated by one or a few species)
Tropical Rainforest

single-dominant forests occur in nutrientdeficient soils or swamps

major soil under tropical rainforest is the
oxisol

African rainforests are evergreen, especially
where annual rainfall is up to 2000 mm

drier rainforests (1300-1600 mm) are semideciduous
Tropical Rainforest

Woody climbers known as lianas are common

epiphytes (plants attached to tree trunks):
ferns, orchids, and mosses

parasites, lichens, and stranglers like fig tree
(ficus sp.) are other flora

African rainforest fauna include:
- Mammals: squirrels, monkeys, chimpanzee
and African elephant
- Reptiles: crocodiles, alligators, snakes, etc
Tropical Forest Area and Deforestation Rate, 1981-1990
Regions
Africa
West Sahel
East Sahel
West Africa
Central Africa
Tropical
Southern Africa
Forest
Forest
Deforested Rate of
Area, ’80 Area, ’90 Annually, change
(‘000ha) (‘000ha) 1981-1990 (1981-1990)
650,000 600,100
5,000
-0.8
41,900
92,300
55,200
38,000
85,300
43,400
400
700
1,200
-0.9
-o.8
-2.1
230,100
215,400
1,500
-0.6
217,700
206,300
1,100
-0.5
Insular Africa
Latin America
13,200
923,000
11,700
839,800
200
8,300
-1.2
-0.9
Asia
310,800
274,900
3,600
-1.2
Deforestation of African Rainforest

deforestation rates are very high in African
rainforest

highest rate of deforestation in the world is
recorded in West Africa with about 2.1% per
year between 1981 and 1990

Cote d'Ivoire has the highest rate of 5.2% per
year in Africa

except for Central Africa and Madagascar,
<15% of the original rainforest is left
Deforestation of African Rainforest
 most
of the remaining forest are in
protected reservations
 rainforest
over much of Zaire, Gabon,
Congo, and Cameroon have the lowest
rate with >40% of its original forests
due to very low population density
Causes of Deforestation
Agriculture:
- land clearance for cultivation is still a
major cause of deforestation
-
natural forest increasingly replaced by
plantations of tree crops like rubber, oil
palm, coffee and cocoa
-
in East Africa and Cameroon, forests
are replaced by plantations of tea,
coffee and pyrethrum
Causes of Deforestation
-
several centuries of cultivation has
transformed rainforest ecosystems
into some kind of anthropogenic
savanna called "derived savanna“
logging:
- major source of hardwoods in high
demand worldwide
Causes of Deforestation
- production of hardwood rose from 4.7
million cubic meters in 1950 to an
estimated 25 million cubic meters in 1990
- volume of hardwood exports rose from 1.5
million cubic meters to an estimated 12
million cubic meters fuel-wood
consumption
- the bulk of the rainforest is secondary
forest