In this article we will discuss about:- 1. Location of Savanna Climate 2. Temperature of Savanna Climate 3. Air Pressure and Winds 4. Rainfall 5. Vegetation.
Location of Savanna Climate:
The word savanna has been used for different meanings by various scientists e.g., the word savanna region has been used by the climatologists to indicate a particular type of climate i.e., tropical wet-dry climate (Aw climate of Koeppen) as savanna climate, while the botanists have used the word savanna for a typical type of vegetation community of tropical regions characterized by the dominance of grasses.
This climate is also called as Sudan type of climate. Savanna type of climate is located between 5°-20° latitudes on either side of the equator (fig. 39.3). Thus, savanna climate is located between equatorial type of climate (Af) and semi-arid and subtropical humid climate. In other words, this climate is located between equatorial low pressure belt or rain producing inter-tropical convergence and subtropical high pressure belt.
The most characteristic areas of savanna climate include the Llanos of Orinico Valley including Columbia and Venezuela, the Guiana Highlands, the Campos of Brazil (south central parts), and Paraguay in South America; hilly areas of Central America; southern part of Zaire, Angola, Zambia, Mozambique, Tanjania, Uganda, and Central Rhodesia, all to the south of the Congo Basin, and central Nigeria, southern Kenya and Uganda, Central African Rupublic, Dahomey, Togo, Chad, Ghana, Ivory Coast and eastern Guinea in Africa; northern Australia and some areas of India (the savanna of India is not the original and natural vegetation cover rather it has developed due to human interference with the original forest cover resulting into the development of widespread man-induced grasslands).
Temperature of Savanna Climate:
The Savanna climate is characterized by distinct wet and dry seasons, mean high temperature throughout the year (ranging between 24°C and 27°C), and abundant insolation. Temperature does not fall below 20°C in any month of the year. Thus, Savanna climate is similar to equatorial climate as regards temperature but the annual range of temperature ranging between 3°C and 8°C is greater than in the equatorial climate.
There are three main seasons on the basis of the combination of temperature and humidity (though on an average there are only two seasons as referred to in the beginning but the dry season is further divided into warm dry season and cold dry season on the basis of temperature):
(1) Cold dry season is characterized by high day temperature ranging between 26°C and 32°C but relatively low temperature during nights, usually 21°C.
(2) Warm dry season is characterized by almost vertical sun’s rays, high temperature ranging between 32°C and 38°C due to abundant insolation.
(3) Warm wet season receives between 80 to 90 per cent of the total annual rainfall and thus records relatively lower temperature than warm dry season.
Air Pressure and Winds of Savanna Climate:
The regions of Savanna climate are affected by low and high pressure systems in a year. Due to northward migration of the sun during summer solstice (21 June) the equatorial low pressure belt and doldurm are shifted northward and thus Savanna climate comes under the influence of Inter Tropical Convergence (ITC) which is associated with atmospheric disturbances (cyclones) which yield rains.
Due to southward migration of the sun during winter solstice (23 December) Savanna climatic zone comes under the influence of subtropical high pressure belt and thus anticyclonic conditions dominate the weather and bring dry conditions. The descending stable winds under anticyclonic conditions cause dry conditions. Besides, the coastal areas are affected by local winds and sea breezes.
Eastern coasts are influenced by trade winds. Strong and high velocity tropical cyclones dominate the weather conditions during warm season. It is apparent that the Savanna type of climate is induced due to the introduction of wet summer and dry winter seasons because of northward and southward migration of the sun respectively. Since the Savanna climate is located between equatorial wet and tropical dry climates and hence there is gradual variation in weather conditions away from the equator as the aridity increases poleward.
Rainfall of Savanna Climate:
The average annual rainfall ranges between 100cm and 150cm but there is much pronounced variation in the spatial distribution of mean annual rainfall in different parts of Savanna climate mainly because of two factors viz.:
(1) Distance from the equator, and
(2) The nature of topographic features.
For example, the Savanna region of Brazil, locally called as Cerrado having the average absolute relief of 1300m AMSL, records mean annual temperature and mean annual rainfall of 20°C-26°C and 75cm-200cm respectively. The Llanos of Columbia is characterized by mean annual rainfall of 200cm-400cm (near Andes Mountain), mean annual temperature of 22°C, and maximum temperature of 32°C.
The Indian Savanna is characterized by the highest temperature (being 45°C- 48°C in May and June) and lowest temperature (being 5°C or even less during the month of January) of all the Savanna regions of the world and the mean annual rainfall well below 150cm, 80 to 90 per cent of which is received during a brief period of 3 months (July to September).
Since the Savanna climate is a transitional belt between humid equatorial and dry tropical climates and hence there is much variability in the amount and duration of rainfall at the wet (equatorward) and dry (poleward) boundaries. There is copious rainfall in the equatorward margins because of convergence of surface winds and convective mechanism of ascending unstable winds but at the poleward margins near dry climate (BWh) is found and rainfall significantly decreases due to descending air and anticyclonic conditions.
Here mean annual rainfall becomes as low as 25 cm. As the Intertropical Convergence (ITC) moves northward due to northward migration of the sun, thunderstorms begin to develop by March and the amount of rainfall increases in the following months. The Intertropical Convergence reaches its northernmost limit by August.
Thus, rainfall continues to increase, upto August and most of rains are received through tropical cyclones and thunderstorms. Rainfall decreases due to southward shifting of ITC and dry trade winds are re-established after November resulting into dry weather condition. It is apparent that the period of wet weather decreases while that of dry weather increases as the distance from the outer limit of the equatorial climate increases.
The amount of mean annual rainfall also decreases from east to west. Savanna climate is also characterized by high variability of rainfall as there may be so heavy and abundant rainfall in a particular year that floods are caused while the following year may receive so little rainfall that drought conditions prevail.
Vegetation of Savanna Climate:
Though general characteristics of typical Savanna vegetation are trees and grasses but the Savanna Biome is, no doubt, dominated by grasses.
The Savanna vegetation community has developed layered structure wherein three distinct layers have clearly developed:
(1) The ground layer is dominated by various types of grasses and herbaceous plants, The grasses are generally coarse, stiff and hard having the height ranging between 80cm to 350cm. The African elephant grass attains enormous height of 500cm. The grasses bear deserted look during dry summer season but they become lush green again during humid summer season.
(2) The middle layer consists of shrubs and stunted woody plants.
(3) The canopy layer is formed by trees of various types. The general characteristics of trees depend on the availability of water and moisture and therefore there is great taxonomic variety of Savanna trees which are usually 6-12m in height.
The Savanna trees have developed various unique characteristics to withstand dry conditions. For example, there are a few species of trees which have developed such mechanisms which help them to reduce evapotranspiration from their leaves during warm dry season and enable them to remain green even during dry season of deficient water supply.
On the other hand, there are such tree species which cannot withstand dry conditions and therefore they shed their leaves and bear the characteristics of deciduous trees. The roots of Savanna trees have also developed according to the environmental conditions as they are very large which can penetrate into the soils and ground upto the depths from 5m to 20m so that they can obtain water from groundwater even during dry season when the groundwater table falls considerably.
The small plants and many herbaceous plants have special kinds of root systems characterized by root tubers and swellings so that they may preserve water which may be used by plants during dry season.
On the basis of the proportion of trees and grassland and the structure of vegetation, the Savanna vegetation is divided into 4 types:
(1) Woodland savanna is dominated by trees and shrubs and absence of epiphytes but some climbers having their roots in the ground are found.
(2) Tree savanna represents relatively open vegetation cover wherein trees and shrubs are sparsely distributed.
(3) Shrub savanna is represented by treeless vegetation which is dominated by grasses at the ground layer and shrubs at the second layer,
(4) Grass savanna is characterized by general absence of trees and shrubs and over-dominance of dense grasses.
Frequent fires, both natural and anthropogenic (deliberate annual burning of grasses by man), are common features of all the savanna biomes. The rapidly increasing human population for the last 50 years or so has put enormous strain on the natural savanna grasslands because a vast area of the original grasslands has been converted into agricultural fields to grow more food crops to feed the teeming millions. Thus, the areas of natural savanna grasslands have shrunk considerably. The savanna ecosystem, thus, has been greatly degraded and destabilized.