Arabian Sea branch of the monsoon

Topic: Climate of India and the Monsoon · NCERT: Class 11, Ch 4 "Climate"

Meaning

The Arabian Sea branch of the monsoon is the part of the southwest monsoon that comes over the Arabian Sea and strikes the west coast of India first. It is the stronger of the two branches, and on reaching India it splits into three arms: the Western Ghats arm, the Narmada–Tapi arm and the Kachchh–Rajasthan arm.

This branch explains three big rain patterns in India. The west coast gets very heavy rain. The Deccan has rain-shadow belts. Rajasthan stays a dry desert.

Explanation

How the branch forms

Heat over the land pulls the wind in:
In May–June, north-west India heats up strongly. A deep thermal low (an area of low pressure caused by heat) forms there.
By early June, this low pulls the southern trade winds across the equator. Trade winds are steady winds that blow towards the equator; in the southern hemisphere they come from the south-east.
The wind turns and picks up moisture:
After crossing the equator, the Coriolis force turns the winds. This is the turning effect of the Earth's spin, which bends winds to the right in the northern hemisphere. The winds now blow from the south-west.
They cross the warm Arabian Sea and pick up a lot of moisture.
The Peninsula splits the flow:
The Indian Peninsula divides the incoming wind into two branches: the Arabian Sea branch on the west and the Bay of Bengal branch on the east.
Relief (the height and shape of the land) then decides where each arm goes and how much rain it gives.

The three arms

Arm	Where it strikes	Path	Rain result
(i) Western Ghats arm	Western Ghats (Sahyadri), from the Tapi mouth to Kanyakumari	Pushed up the slopes to 900–1,200 m	250–400 cm on the windward side; rain shadow to the east
(ii) Narmada–Tapi arm	Coast north of Mumbai	East along the Narmada and Tapi valleys into central India	Rain over central India; only 15 cm at Chotanagpur; joins the Bay branch over the Ganga plains
(iii) Kachchh–Rajasthan arm	Saurashtra and Kachchh (Gujarat)	Over west Rajasthan, along the Aravallis	Scanty rain; joins the Bay branch over Punjab and Haryana

Arm (i): Western Ghats arm and the rain shadow

Why the windward side gets heavy rain:
The Western Ghats run north to south, almost at a right angle to the south-west wind, so they block it.
The moist wind is pushed up the slopes to 900–1,200 m.
Rising air cools. Its water vapour condenses and falls as heavy rain on the windward side (the side facing the wind).
The Sahyadri and the west coast get 250–400 cm of rain.
Why the leeward side stays dry (rain shadow area):
The wind crosses the top of the range and goes down the eastern slopes. This is the leeward side (the side away from the wind).
Sinking air is squeezed and becomes warmer. Warm air can hold more water, so it gives little rain.
The result is a dry belt over the eastern slopes and the Deccan plateau, such as Pune and Bengaluru.

Arms (ii) and (iii): why the rain becomes less inland

Narmada–Tapi arm: the wind loses moisture as it goes:
The Narmada and the Tapi flow from east to west. Their valleys work like corridors, so the wind can travel deep into central India.
The Narmada flows west through a rift valley to the Gulf of Khambhat. A rift valley is a long valley formed where land sank between two faults.
Most of the moisture falls as rain on the way. By the time the wind reaches the Chotanagpur plateau, it gives only 15 cm.
Kachchh–Rajasthan arm: nothing pushes the air up:
The Aravallis run from south-west to north-east, parallel to the wind.
The air is not forced to rise, so it does not cool enough to give much rain.
This is one reason why the Thar is a desert.
This arm joins the Bay branch over Punjab and Haryana. Together, the two branches bring rain to the western Himalaya.

Where it occurs

Western Ghats (Sahyadri): from the Tapi mouth to Kanyakumari. This arm brings heavy rain to the Konkan, Goa, coastal Karnataka and Kerala.
Windward towns: Kozhikode (Kerala) and Mangalore (Karnataka) get very heavy rain.
Rain-shadow towns: Pune (Maharashtra) and Bengaluru (Karnataka), on the Deccan plateau to the east of the Ghats.
Narmada valley: the Narmada rises at Amarkantak (Madhya Pradesh) and flows west to the Gulf of Khambhat.
Tapi valley: the Tapi rises near Multai (Betul, Madhya Pradesh) and reaches the Gulf of Khambhat near Surat.
Chotanagpur plateau (Jharkhand): this is the far eastern end of the Narmada–Tapi arm, where it gives only 15 cm.
Gujarat: Saurashtra and Kachchh are struck by the third arm. Gujarat region gets 96% of its yearly rain in the southwest monsoon season (1971–2020) [2].
West Rajasthan and the Thar: this area gets scanty rain because the Aravallis lie parallel to the wind.
Meeting zones with the Bay branch: the Ganga plains (Narmada–Tapi arm) and Punjab and Haryana (Kachchh–Rajasthan arm).
Coromandel coast (Tamil Nadu): it lies in the rain shadow of the Arabian Sea branch, so it is dry in June–September. Tamil Nadu gets only 36% of its yearly rain in the southwest monsoon and 48% in the northeast monsoon (1971–2020) [2].

Don't confuse with

Bay of Bengal branch: it strikes Myanmar and south-east Bangladesh first. The Arakan Hills turn it, so it enters West Bengal from the south and south-east. The Arabian Sea branch strikes the west coast of India directly and is the stronger branch.
Mawsynram and Cherrapunji rainfall: this rain comes from the Bay of Bengal branch hitting the Khasi hills in Meghalaya. It does not come from the Arabian Sea branch, even though both cases are examples of relief-forced rain.
Northeast monsoon: this blows in October–December and gives Tamil Nadu most of its rain. The Arabian Sea branch belongs to the southwest monsoon (June–September) and leaves Tamil Nadu dry.
Break on the west coast versus a break in north India: a west-coast break happens when the winds blow parallel to the coast, so the air is not forced up the Ghats. In north India, a break happens when the monsoon trough shifts north to the Himalayan foothills.

Prelims Hooks

Three arms in order from south to north: Western Ghats arm → Narmada–Tapi arm (coast north of Mumbai) → Kachchh–Rajasthan arm (Saurashtra and Kachchh).
The Western Ghats force the wind up to 900–1,200 m and get 250–400 cm of rain on the windward side.
Windward and rain-shadow pairs: Mangalore and Kozhikode (wet) versus Pune and Bengaluru (rain shadow).
"Why is the Thar dry?" The Aravallis lie parallel to the Arabian Sea arm, so the air is not forced to rise.
The Narmada–Tapi arm gives only 15 cm at the Chotanagpur plateau and then joins the Bay branch over the Ganga plains.
Trap: the Kachchh–Rajasthan arm meets the Bay branch over Punjab and Haryana, not over the Ganga delta. Together the two branches bring rain to the western Himalaya.

Mains Points

Relief controls where the rain falls:
The Western Ghats lie at a right angle to the wind, which gives very heavy rain on the west coast.
The Aravallis lie parallel to the wind, which leaves the Thar dry.
The east-west Narmada and Tapi valleys carry the rain into central India, but the rain becomes weaker as the wind moves inland.
Rain-shadow belts are drought-prone:
Interior Maharashtra and Karnataka lie on the leeward side of the Ghats.
Farming there needs irrigation and watershed management (GS-I and GS-III).
The Coromandel coast also lies in this rain shadow, so it depends on the northeast monsoon (48% of Tamil Nadu's yearly rain) [2].
Western India depends heavily on one season:
Gujarat region gets 96% of its yearly rain from the southwest monsoon [2].
Long breaks, which happen when winds blow parallel to the coast, can damage kharif crops in rain-fed areas. This is why medium-range forecasts and contingency crop plans matter.

Related concepts

The southwest monsoon season: the two branches, rain shadow and breaks

Sources

1Class 11, Ch 4 "Climate" (primary)
2IMD Press Release, "Updated Rainfall Normal based on data of 1971-2020" (14 April 2022)internal.imd.gov.in · tier 1