In this article we will discuss about:- 1. Location of Monsoon Climate 2. Temperature of Monsoon Climate 3. Air Pressure and Winds 4. Precipitation 5. Natural Vegetation.
Location of Monsoon Climate:
Monsoon climate is generally related to those areas which register complete seasonal reversal of wind direction and are associated with tropical deciduous forests but there are some departures from this close relationship and near correspondence between the regions of monsoon climate and tropical deciduous forests. Monsoon climate is found in the zone extending between 5° and 30° latitudes on either side of the equator (fig. 39.2).
In fact, this zone comes under the domain of trade wind belt which experiences seasonal shifting due to northward and southward migration of the sun. Onshore winds blow for six months from warm tropical oceans towards the continents and offshore winds blow for another six months from land to the sea.
The areas of monsoon climate are divided in the following categories:
(1) True monsoon areas include India, Burma (Myanmar), Pakistan, Bangladesh, Thai land, Combodia, Laos, North and South Vietnam, southern China, Philippines, and northern coastal area of Australia.
(2) Areas of monsoonal tendencies or pseudo- monsoons are found along the south-west coast of Africa including the coasts of Guinea, Sierra Leone, Liberia and Ivory Coast; eastern Africa and western Madagascar (Malagasy).
(3) Areas of monsoon effects include north-east coast of Latin America e.g., east Venezuela, Guyana, Surinam, French Guyana, and north-east Brazil. Besides, Puerto Rico and Dominican Republic in the Caribbean Island also enjoy mild monsoonal effect.
(4) Areas of modified monsoon are found in parts of Central America and south-east USA.
Temperature of Monsoon Climate:
Though mean annual temperature is fairly high but summer and winter seasons are sharply differentiated due to northward (summer solstice) and southward movement of the sun (winter solstice).
There are three main seasons in a year in Indian Subcontinent and surrounding monsoonal areas e.g.:
(1) Dry warm summer season (March to June),
(2) Humid warm summer season (July to October), and
(3) Dry winter season (November to February).
Average temperature of warm dry summer months ranges between 27°C and 32°C but maximum temperature ranges between 38°C and 48°C during May and June. Warm humid summer months record average temperatures ranging between 20°C and 30°C. The mean temperature during day in winter months varies from 10°C to27°C.
Annual range of temperature ranges between 2°C and 11°C and is controlled by nearness or remoteness of the sea (i.e. distance from the sea), continental, latitudinal and altitudinal influences. For example, annual range of temperature increases inland (5.3°C at Rangoon, 11°C at Mumbai, 18.4°C at Allahabad, 20.2°C at Agra).
Similarly, diurnal range of temperature is low in the coastal areas but it increases inland. Diurnal range of temperature is much higher in dry summer season than in other seasons.
For example, in the Ganga plains of India maximum temperature during day time may go as high as 44°C to 48°C and the minimum temperature during nights may come down as low as 20°C to 25°C thus registering diurnal range of 23°C to 24°C. The temperature during May and June becomes exceptionally high due to prevalence of hot winds locally known as loo.
Air Pressure and Winds of Monsoon Climate:
Monsoon areas are affected by high and low pressure systems due to winter and summer seasons respectively. In fact, there is complete reversal of pressure gradients over Asiatic landmass because of northward and southward migration of the sun and consequent differential heating of the continent and adjoining oceanic areas.
Due to southward migration of the sun after 23 September (autumnal equinox) high pressure centres are developed on the landmass of Asia during winter season while low pressure is developed in the southern Indian and Pacific Oceans, with the result pressure gradient is developed from land areas to the oceanic areas resulting into the outflow of surface winds from high pressure centres of the land areas towards oceanic low pressure areas.
This wind system having north-east direction is called winter monsoon which is nothing more than the reestablished northeast trade winds which are displaced during summer season due to northward shifting of inter-tropical convergence (ITC) because of northward migration of the sun.
These offshore winds are generally dry because they come from over the land areas but wherever they pass over the oceanic areas; they pick up moisture and yield rainfall when effectively obstructed. For example, north-east monsoon winds while passing over Bay of Bangal pick up moisture and give rainfall in the coastal regions of Tamil Nadu during winter season.
The pressure system is completely reversed during summer season when the sun registers northward migration after 21 March and becomes almost vertical over tropic of Cancer on June 21. Thus, thermally induced low pressure develops due to very high temperature over huge landmass of Asia.
These low pressure centres are further intensified due to northward movement of inter-tropical convergence (ITC) up to 20° to 35°N latitudes while high pressure centres are developed over southern Indian and Pacific Oceans, with the result sea to land pressure gradient is steepened and onshore winds are generated. These onshore winds are the south-west or summer monsoon winds which are moist as they pass over the sea surfaces.
According to the advocates of the concept of thermal origin of monsoon the north-east or winter monsoons and south-west or summer monsoons are originated due to differential heating of Landmasses and oceanic areas during summer and winter seasons and resultant thermally induced high and low pressure areas.
According to them the summer and winter monsoons are south-east and north-east trade winds. In fact, the south-east trades while crossing over the equator during summer season due to northward shifting of pressure belts caused by northward migration of the sun becomes south-westerly in direction according to Ferrel’s law.
Since these winds come from over the ocean, they become moist and give rainfall. According to the advocates of the dynamic origin of monsoon the belt of doldrums and North Intertropical Convergence (NITC) are drawn over south and south-east Asia during summer season due to northward migration of the sun and thus equatorial westerlies of the belt of doldrum are also established over south and south-east Asia and thus they become south-west monsoons. The tropical disturbances (cyclones) associated with the intertorpical convergence (ITC) yield copious rainfall.
Precipitation of Monsoon Climate:
Monsoon regions receive most of their annual rainfall through cyclonic and orographic types of rains though convective mechanism also yields some rainfall. On an average, the average annual rainfall is around 1500mm but there are much variations in the temporal and spatial distribution.
Sometimes, a few areas receive less than 500mm of mean annual rainfall. Even the temporal distribution of rainfall within a single year is highly variable because more than 80 per cent of mean annual rainfall is received within 3 wet months of summer season (July, August, and September). Thus, the rainy season records much surplus water whereas dry winter and summer seasons have marked deficit water because dry seasons receive less than 25 mm of rainfall per month.
There is maximum evaporation during warm dry summer months which results in desiccation of soils and marked reduction in soil water. This seasonal regime of annual rainfall gives deciduous character to the vegetations which shed their leaves during the transitional period between winter and summer seasons.
Most of the annual monsoonal rainfall is received through moisture laden south-west monsoon winds which come from over the ocean surface. The outbreak of monsoon generally occurs in India around mid-June. When these moisture laden monsoon winds strike the mountain barriers they give copious rains.
This is why the western coastal plains of India receive more than 2500mm of annual rainfall because the Arabian Sea Branch of monsoon winds are obstructed by the Western Ghats and hence they are forced to ascend and soon become unstable and saturated. The leeward sides of the mountains fall in rain-shadow region because descending winds are adiabatically warmed and thus become stable and dry. This is why Mangalore, located on the windward side of the Western Ghats receives 2000mm of annual rainfall whereas Bangalore, located on the leeward side receives only 500mm of annual rainfall.
The eastern coast of Tamil Nadu and Andhra Pradesh receives much rainfall during winter season through north-east monsoons as they while passing over the Bay of Bengal, pick up sufficient moisture and yield rainfall. January and February are generally driest months in India but the Ganga plains receive some rains from westerly disturbances or temperate cyclones coming from Mediterranean Sea.
Variability of rainfall in terms of both amount and duration is the characteristic feature of monsoon climate. Secondly, monsoonal rainfall is basically cyclonic in character.
Natural Vegetation of Monsoon Climate:
The number of plant species is far less in the monsoon climatic regions than the equatorial climatic regions. The height of most of the trees ranges between 12m and 30m. There are four strata or layers in vertical structure of the tropical deciduous forests. The uppermost and second strata consist of trees; the third stratum is formed by shrubs whereas the ground stratum represents herbaceous plants. Most of the trees are deciduous but the shrubs of the third stratum are evergreen.
The trees are characterized by thick girth of stems, thick, rough and coarse bark and large hydromorphic leaves or small, hard xeromorphic leaves. Deciduous trees of monsoon climate denote complete adaptability to wet-dry climate. The large hydromorphic leaves enable the trees to trap more and more rainfall during wet season but these are shed in dry periods to conserve moisture while small and hard xeromorphic leaves enable the trees to withstand dry weather and water deficiency. Important species of trees include sal, teak, bamboo, mango tree, mahua, jamun, neem, shisham etc.
The tropical and subtropical monsoon deciduous forest biome is one of the most disturbed ecosystems of the world. The forests have been so rapidly destroyed through both natural (forest fires) and anthropogenic processes due to rapacious utilization of forest resources for commercial and industrial purposes and large-scale clearance through mass felling of trees for agricultural land that the vegetation cover has shrunk to a very critical size.
More than 80 per cent of annual rainfall occurring in only 3 wet months of July, August and September through heavy rainstorms generates maximum surface runoff which is causing enormous loss of rich fertile soils through accelerated rate of soil erosion.