The tropical monsoon climate is found in those regions where there is a complete seasonal reversal of winds. On-shore summer winds blowing from over tropical warm oceans are generators of abundant precipitation, while the off-shore winds from over land make the weather dry during winter.
The most clearly defined monsoon climates are located in the coastal areas of the eastern and southern Asia. The following Asian countries have monsoon type of climate: India, Burma, Bangladesh, Indochina, Southern China and the Philippines.
Besides, the monsoon circulation also affects Taiwan, Japan and Korea where, because of their middle latitude location between the continent and the ocean, seasonal contrast of temperature results in seasonal reversal of winds.
It may be noted that surface relief, direction of the coast line and the extension of the monsoon into middle-latitudes cause a number of sub-types of the monsoon climate found over this most extensive continent.
For example, because of the direction of its coastline, the winter monsoon which strikes the coastal ranges after crossing the South China Sea brings the rainy season.
In the coastal regions of the tropical monsoon climate the temperatures are uniformly warm. Compared with the tropical rainforest climate air temperatures show a marked annual cycle.
The highest temperatures in this climate are recorded during summer just before the advent of rains when the skies are clear. During the rainy season, the temperatures are reduced because of the heavy cloud cover and the falling rains. Average temperatures for the summer months vary from 27° to 32°C.
It is noteworthy that certain inland stations in the northern plains of the Indian sub-continent record very high maximum temperatures ranging from 37.8° to 48.9°C, or even more during the months of May or early June.
On occasions, these plains experience a very hot and dry wind locally known as ‘too’. During winter, however, the average temperature at inland stations may vary from 10° to 26.7°C. In higher latitudes the drop in the minimum temperatures is considerable.
The occurrence of the freezing or sub-freezing minimum temperatures in certain parts is not uncommon.
The annual range of temperature in the tropical monsoon climate is much .larger than that in the equatorial climate. The annual range registers a wide variation ranging from 2°C to 11°C.
The variation in the degree of the annual range of temperature is controlled by continentally, vapour pressure, cloudiness and latitudinal as well as altitudinal effects. At Allahabad, the annual range of temperature is 18.4°C, while at Agra it is 20.2°C.
The annual range at Rangoon is only 5.3°C, while at Bombay it is 11°C. The diurnal range of temperature is still greater: Madras-less than 11°C, Bombay 8.3°C, while in Sind, it may exceed 16.7°C.
The diurnal range in the coastal regions is less than what it is in the continental interiors. Another characteristic feature of the diurnal range of temperature is the fact that it is considerably higher in the dry summer months than in any other part of the year.
Pressure and winds:
The monsoon circulation in Asia is characteristically governed by the migration of ITC. Because of differential heating of the continent and the adjoining oceans, there is a complete reversal of pressure gradient over the huge landmass of Asia.
During winter there are centers of high pressure over the continent so that there is an outflow of air towards the oceanic low pressure centers. These winds are termed ‘the winter monsoon’ or the ‘dry monsoon’ in the eastern and southern Asia.
In summer the huge landmass of Asia develops low pressure centers. These centers are reinforced by the ITC which moves suddenly to the north into the Indian subcontinent to 20° or 25° N latitude.
Under these conditions the sea-to-land pressure gradients are established resulting in on-shore winds in eastern and southern Asia. These winds pick up huge quantities of moisture from the warm tropical oceans.
Thus, the summer monsoons blowing from southwest in the southern Asia and southeast in eastern Asia are capable of giving heavy rains wherever conditions are favourable. As the winter approaches, the low-pressure centers are gradually replaced by the high pressure systems.
In fact, the so-called winter monsoons are nothing but the re-establishment of the northeast trades which are dry except in those areas where they reach after passing over the sea.
However, the mechanism of the monsoon circulation is very complex. It involves the surface as well as upper air flows.
Rains are abundant and intense in tropical monsoon areas. But there is a distinct dry season, though very short. However, the amount of precipitation during the rainy season is so heavy that it more than compensates the absence of rainfall for a few months. Soils retain moisture to support the plant cover.
In the tropical monsoon climate summer is generally the rainy season. During the high-sun period the on-shore winds bring a lot of moisture from over the tropical warm oceans to the land.
Wherever these moisture-laden winds are forced to rise, abundant precipitation results. However, the coastal regions, if backed by highlands, receive the maximum amount of precipitation.
The western coastal plains of Peninsular India get more than 250 cm. of rainfall on the windward slopes of the Western Ghats.
Similarly, the western coast of Burma receives heavy rainfall on the windward slopes of Arakan and Tenasserim Mountains. It is noteworthy that the leeward sides of these coastal ranges suffer from the rainshadow effects.
During the winter monsoon period, January and February are the driest months in India, but the northwestern region and the northern plains in the Ganges valley do receive moderate to scanty rains from the westerly disturbances of temperate cyclones. In the Peninsular India, the winter rainfall is more pronounced in the eastern coastal regions.
The distribution of rainfall in our subcontinent is more uneven than elsewhere. The rainfall decreases from east to west and from north to south in northern India, so much so that the Punjab and other western and north-western regions have almost semi-arid climate.
In the tropical monsoon climate, the amount and distribution of precipitation determine, to a large extent, the type of natural vegetation. Towards the equator-ward margins, where the precipitation is heavy, the tropical monsoon forest resembles the tropical rainforest.
However, because of the seasonality of rainfall, species are limited in the monsoon forests. Towards the drier margins, rainforests are replaced by more sparse jungles, thorn forests and savanna grasslands.
In India, for example, there are different types of natural vegetation ranging from the tropical rain-forests of the Malabar and Assam to the deciduous forests of the areas with moderate rainfall to thorny bushes of the more arid regions with scanty rainfall.
Teak is the most valuable timber, and it is found in Burma and certain other parts of India. Besides, shisham, sal, mahua, mango tree, jamun, neem, and many more species of trees are found in the deciduous forests which shed their leaves before the commencement of the long dry season in order to conserve moisture.
In fact, the deciduous trees of the monsoon forest are a fine illustration of their adaptability to a wet-dry climate in which rainy season with adequate water surplus alternates with a dry season with soil-water deficit.
These forests are generally more open than the tropical rainforests. The trees are less tall and the branching starts at a lower level. Tree species may vary from 30 to 40 in a small area. Trees have thick and rough barks as moisture conserving device. Tree tops are fairly large and round.
Outside India, the typical monsoon forest is found in Burma, Thailand and Cambodia. In West Africa and in Central and South America there are large areas of deciduous monsoon forest bordering the equatorial rain forests?
In more humid areas where the forest is dense, the flying and climbing species of the animal life of the equatorial forest are dominant.
But in drier regions where the trees are widely spaced with grasslands, large animals constitute the animal kingdom.
Some of these animals are carnivores, while others are herbivores, such as, tigers, lions, leopards, wolves and jackals, elephants, wild buffalo, rhinoceros, deer, and antelopes.