Introduction

SEASONS IN INDIA

• • Winter season, mid-December to mid-March

• • Hot weather season, mid-March to May.

• • The rainy season, June to September

• • Season of retreating monsoon, October to mid-December.

There are three seasons in India: summer, winter, and monsoon. The climate of India is essentially a subtropical monsoon. The word ‘monsoon’ has been derived from the Arabic word “Mausim” which means ‘season’. Originally, the word ‘monsoon’ was, used by Arab navigators several centuries ago, to describe a system of seasonal reversal of winds along the shores of the Indian Ocean, especially over the Arabian Sea, in which the winds blow from the south-west to north-east during the summer season and from the north-east to south-west during the winter season.

The monsoon is a unique weather phenomenon due to the seasonal reversal of winds.

Features of the Indian Monsoon

Monsoon has the following features:

1.  Sudden onset
2.  Gradual advance
3.  Gradual retreat
4.  Variation – temporal and regional

INDIAN MONSOON

• • Monsoons are a complex meteorological phenomenon. Experts in meteorology have developed a number of concepts about the origin of monsoons. Some of the important concepts about the origin of monsoons have been given as under

• • • The Thermal Concept of Halley :

• • • • The primary cause of the annual cycle of the Indian monsoon circulation was the differential heating effects of the land: and the sea.

• • • • Monsoons are the extended land breeze and sea breezes on a large scale, produced by the differential heating of continents and ocean basins. During the summer season in the Northern Hemisphere, when the Sun’s rays are vertical over the Tropic of Cancer, the huge landmass of Asia heats quickly and develops a strong low-pressure center near Lake Baikal (Siberia) and Peshawar (Pakistan). This thermal low extends up to 700 MB. Moreover, the poleward shift of the Inter-Tropical Convergence Zone (ITCZ) to a position over southern Asia reinforces the thermally induced low-pressure center. In comparison to this, the pressure over the adjacent water of the Indian and the Pacific Oceans is relatively high. Under these conditions, a sea-to-land pressure gradient develops. Consequently, the surface airflow is from the high pressure over the oceans towards the low-pressure areas over the heated landmass. Under the extremely low-pressure condition on land, the wind from the southern part of the Indian Ocean (south of the Equator) is attracted towards the subcontinent of India.

• • • • The air coming from oceans toward land is warm and moist. When land barriers like mountain ranges and plateaus come in the way of the moisture-laden winds, they ascend and result in saturation, condensation, and precipitation

• • • • In the Northern Hemisphere during the winter season, there develops high-pressure areas near Baikal Lake (Siberia), and Peshawar {Pakistan}. As compared to these high pressures, the Indian Ocean and the Pacific Ocean (south of Japan) remain relatively warm, having low-pressure areas.

• • • • Consequently, there is an outflow of air from the high pressure of the land to the low-pressure areas of the oceans. The air blowing from high-pressure areas of land towards the sea is cold and dry. This cold and dry air is incapable of giving precipitation unless it comes into contact with some water body (ocean/sea)

• • • Criticism of The Thermal Concept of Halley

• • • • The low-pressure areas that develop over the continents during the summer season in the Northern Hemisphere are not stationary. These low-pressure areas change their position (location) suddenly, This sudden change in the low-pressure areas is not exclusively related to low thermal conditions.

• • • • Had the monsoon been thermally induced, there would be anti-monsoon circulation ill the upper air of the troposphere, which is lacking.

• • • • Halley did not take into consideration the Coriolis effect of rotation of the earth on its axis. On rotating earth, the wind has a tendency to move towards its right in the Northern Hemisphere and towards its left in the Southern Hemisphere.

• • • • The role of latent beat passing into the atmosphere through water vapor was also not considered by Halley; water vapor also plays an important role in the origin and development of monsoons.

• • • • Modern research in meteorology has shown that monsoon rainfall is not wholly orographic. They are an amalgamation of convectional, orographic, and cyclonic rainfall.

Indian Monsoon  

Stage 1:

The apparent movement of the sun in the northern hemisphere in summer is seen. This heats the Tibetan plateau and the air rises upwards. This creates a Tropical Eastern Jetstream which descends on Madagascar and creates a high-pressure zone there. An intertropical convergence zone [ITCZ] formed at the equator moves to the Tibetan plateau.

Stage 2:

The Somali jet stream intensifies the Somalian ocean current during summer. Stronger the Somalian current better the monsoon.

Stage 3:

Subtropical westerly jet maintains high pressure over North India. The STWJ is bifurcated by the Himalayas into two branches.

The summer sees the apparent movement of the sun and the heating of the Tibetan plateau. This creates a low-pressure belt over Tibet.

The STWJ still continues to be over South Himalayas and it resists the monsoon winds.

At this time there is local convectional rainfall in Bengal, Bihar called Kalbaisakhi, mango showers. These are pre-monsoon thundershowers.

Then the STWJ southern branch moves over north and STWJ withdraws from the Indian subcontinent. If this branch reestablishes then monsoon break is seen.

Stage 4: Indian Ocean Dipole

Low-pressure conditions over Australia and the Tibetan plateau and high pressure over Madagascar create an Indian ocean dipole. A strong dipole pushes monsoon winds towards the Indian subcontinent.

Stage 5: onset of monsoon

The monsoon winds have two branches Arabian and Bay of Bengal. The Arabian branch moves over western ghats and is blocked. This brings heavy rainfall over the western region.

The SW Arabian branch isn’t blocked by Aravallis so less monsoon over Rajasthan. The gap between Vindhyas and Aravallis means SW winds reach over to Himachal Pradesh.

The Eastern branch of the SW monsoon goes over the Bay of Bengal and isn’t blocked by the eastern ghats. It passes through their gaps and reaches Vindhya, Satpuda range, and Chota Nagpur plateau. This gives rainfall to Eastern India and its interiors.

The second part of the eastern branch of the SW monsoon passes over the northeast and brings rain there.

Stage 6: retreat from monsoon

In the month of September, the ITCZ retreats towards the equator from the Tibetan plateau. As it passes over India the eastern branch of the Southwest monsoon and northeast winds collide.

The eastern branch of Southwest monsoon withdraws first. Then North East winds are off the land and hence dry but they pick moisture over the Bay of Bengal and bring rain to the southern tip of India i.e. Coast of Andhra Pradesh, Tamil Nadu.

The retreating eastern branch of the Southwest monsoon brings cyclones to the land.

Fluctuations in the Indian monsoon:

1.  If the Indian ocean dipole is weak.
2.  Southern oscillation is reversed or not strong.
3. SWTJ doesn’t retreat from India or come back once again.
4. Inadequate heating of Tibetan plateau.