Southwest Monsoon Season| Class 11 Geography Notes

The Southwest Monsoon Season or the Rainy season starts its journey in June and continues till September. This season is noticeable by the onset and advance of the monsoon where the winds blow from the Arabian Sea and the Bay of Bengal towards the land.

These winds carry moisture along with them. When these winds hit or strike the mountain barriers, rainfall starts. The onset of the southwest monsoon over Kerala, i.e. the first point of entry into India’s mainland, is usually on June 1st. The country receives nearly 75% of its annual rainfall.

In this article, we will explore its onsets, factors, branches, and characteristics.

Southwest Monsoon Season| Class 11 Geography Notes

Onset of Monsoon

• During April and May the sun shines vertically over the Tropic of Cancer, the large landmass in the north of the Indian Ocean.
• The differential heating of land and sea during the summer months is the mechanism that sets the stage for the monsoon winds to drift toward the subcontinent.
• The shift in the position of the ITCZ (Inter Tropical Convergence Zone) is also related to the phenomenon of the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas.
• The easterly jet stream sets in along 15°°N latitude only after the western jet stream has withdrawn itself from the region. This easterly jet stream is held responsible for the burst of the monsoon in India.
• The southwest monsoon sets in over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June.
• By mid-July, southwest monsoon engulfs the entire subcontinent

Factors affecting South West Monsoon

Differential heating and cooling of land and water

• Due to the differential heating and cooling of land and water, India’s landmass has low pressure, while the seas around it have relatively high pressure.
• This greatly impacts the formation of the South-West Monsoons.
• As as result of this, the direction of monsoon winds is reversed.

Pressure and Winds

• By the middle of July, the low pressure belt nearer to the the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, roughly parallel to the Himalayas between 20° N and 25° N. By this time, the westerly jet stream withdraws from the Indian region.
• The ITCZ being a zone of low pressure, attracts inflow of winds from different directions. The maritime tropical airmass (mT) from the southern hemisphere, after crossing the equator, rushes to the low pressure area in the general southwesterly direction.
• It is this moist air current which is popularly known as the southwest monsoon.

Easterly Jet Stream and the Tropical Cyclones

• The easterly jet stream steers the tropical depressions into India. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.
• The tracks of these depressions are the areas of highest rainfall in India.
• The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.

Southern Oscillation

• The Southern Oscillation occurs when the tropical eastern south Pacific Ocean has high pressure while the tropical eastern Indian Ocean has low pressure.
• However, there is a reversal in pressure conditions every few years, with the eastern Pacific having lower pressure than the eastern Indian Ocean.
• The periodic change in the pressure conditions is referred to as the SO.
• As these condition changes. Summer temperature in parts of India can reach 45 degrees Celsius, while the India Ocean is significantly cooler. Weaker monsoons and even droughts are also sometimes the result of the above conditions.

Rainfall During the South West Monsoon Season

• The rain in the southwest monsoon season begins rather abruptly. One result of the first rain is that it brings down the temperature substantially.
• This sudden onset of the moisture-laden winds associated with violent thunder and lightening, is often termed as the “break” or “burst” of the monsoons.
• The monsoon may burst in the first week of June in the coastal areas of Kerala, Karnataka, Goa and Maharashtra while in the interior parts of the country, it may be delayed to the first week of July.
• The day temperature registers a decline of 5°C to 8°C between mid June and mid-July.

Rain – Bearing Systems and Rainfall Distribution

The intensity of rainfall over the west coast of India is related to two factors:

1. The offshore meteorological conditions.
2. The position of the equatorial jet stream along the eastern coast of Africa.

There seem to be two rain-bearing systems in India:

1. First, that originate in the Bay of Bengal causing rainfall over the plains of north India.
2. Second is the Arabian Sea current of the southwest monsoon which brings rain to the west coast of India.

Much of the rainfall along the Western Ghats is orographic as the moist air is obstructed and forced to rise along the Ghats.

Monsoon Winds of the Arabian Sea

The Monsoon winds of Arabian Sea are divided into three branches:

Branches	Characteristics
First Branch	Its one branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900-1200 m. Soon, they become cool, and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area. Find out the rainfall at Kozhikode, Mangalore, Pune and Bengaluru and note the difference.
Second Branch	Another branch of the Arabian sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
Third Branch	A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only a scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.

Monsoon Winds of the Bay of Bengal

• The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent.
• The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction.
• From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India.
• Its one branch moves westward along the Ganga plains reaching as far as the Punjab plains.
• The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains.
• Its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.

We need to know why the Tamil Nadu coast remains dry during this season. There are two factors responsible for it:

• The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest monsoon.
• It lies in the rain shadow area of the Arabian Sea branch of the south-west monsoon.

Break in the Monsoon

During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon. These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:

• In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
• Over the west coast the dry spells are associated with days when winds blow parallel to the coast.

Characteristics of Monsoon Rainfall

• Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.
• Monsoonal rainfall is largely governed by relief or topography. For instance the windward side of the Western Ghats register a rainfall of over 250 cm.
• The monsoon rainfall has a declining trend with increasing distance from the sea.
• The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.
• Monsoons play a pivotal role in the agrarian economy of India because over three-fourths of the total rain in the country is received during the southwest monsoon season.
• Its spatial distribution is also uneven which ranges from 12 cm to more than 250 cm.
• The beginning of the rains sometimes is considerably delayed over the whole or a part of the country.
• The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

Conclusion

By the beginning of September, Southeast Monsoon stats retrieving from the far northeast and gradually moves southward. Heavy rains brought on by these monsoon winds can cause devastating floods in India’s Northern Plains, causing loss of life and property. Droughts and heavy floods are caused by dry and wet spells in one part of the country, while droughts are possible in the other.

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FAQs On Class 11 Geography Southwest Monsoon Season

What is Southwest Monsoon?

The Southwest Monsoon Season or the Rainy season starts its journey in the month of June and continues till September. This season is noticeable by the onset and advance of the monsoon where the winds blow from Arabian Sea and the Bay of Bengal towards the land.

What are the factors that affect Southwest Monsoon?

The factors that affect the Southwest Monsoon are : differential heating and cooling of lands, pressure and winds, easterly Jet streams and Tropical cyclones and Southern Oscillations.

Why Tamil Nadu remains dry during the Southwest Monsoon?

Tamil Nadu remains dry during the Southwest Monsoon season because :The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest monsoon. It lies in the rain shadow area of the Arabian Sea branch of the south-west monsoon.

What do you mean by “break in monsoon”?

During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon. These dry spells are quite common during the rainy season.

Mention any four characteristics of Southwest Monsoon.

Any four characteristics of Southwest Monsoon are : Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.

Monsoonal rainfall is largely governed by relief or topography. For instance the windward side of the Western Ghats register a rainfall of over 250 cm.

The monsoon rainfall has a declining trend with increasing distance from the sea.

The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.