Bay of Bengal branch of the monsoon

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

Meaning

The Bay of Bengal branch of the monsoon is the part of the south-west monsoon that blows over the Bay of Bengal. It first hits Myanmar and south-east Bangladesh. The Arakan Hills then turn it towards India, so it enters West Bengal and Bangladesh from the south and south-east. After that it splits into two arms. One goes west up the Ganga plain to Punjab. The other goes up the Brahmaputra valley. A sub-branch hits the Garo and Khasi hills of Meghalaya.

This branch brings the monsoon rain of north-east and north India. It also feeds Mawsynram and Cherrapunji, two of the wettest places on Earth. UPSC often asks about its path, the hills that turn it, and why rain falls less and less as it moves west.

Explanation

How the branch forms

• The pull from the land:
• In May–June the land of north-west India gets very hot. A deep thermal low (an area of low pressure made by heat) forms there.

• By early June this low pulls the southern trade winds (steady winds that blow towards the equator from the south-east in the southern hemisphere) across the equator.

• The turn to south-west:

• The Coriolis force (the turning effect of the Earth's spin, which bends winds to the right in the northern hemisphere) turns these winds after they cross the equator.

• They now blow from the south-west.

• The split: the Indian Peninsula divides the incoming wind into two branches, the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch is the stronger one.

• Moisture: the Bay branch blows over the warm Bay of Bengal and picks up a lot of water vapour.

The path, step by step

• Step 1, first landfall: it hits the coast of Myanmar and south-east Bangladesh.
• Step 2, the turn: the Arakan Hills (Arakan Yoma, along the Myanmar coast) turn a large part of the wind towards India.

• So it enters West Bengal and Bangladesh from the south and south-east, not from the south-west.

• Step 3, the split: the Himalaya to the north and the thermal low over north-west India divide it into two arms:

• West arm: it moves up the Ganga plain as far as Punjab.

• North-east arm: it moves up the Brahmaputra valley and gives heavy rain in Assam and the north-east.

• Step 4, the Meghalaya sub-branch: a sub-branch hits the Garo and Khasi hills of Meghalaya.

• Step 5, meeting the other branch:
• The Narmada–Tapi arm of the Arabian Sea branch joins the Bay branch over the Ganga plains.
• The Kachchh–Rajasthan arm joins it over Punjab and Haryana.
• Together, the two branches bring rain to the western Himalaya.

Why rainfall changes along the path

• Rain decreases from east to west in the Ganga plain:
• The wind drops its moisture as rain while it moves inland.

• So the air holds less and less water as it goes west, and places further west get less rain.

• Very heavy rain on the Khasi hills:

• The hills face the moist Bay wind.
• Funnel-shaped valleys squeeze the wind and force it upward.

• Rising air cools, its water vapour condenses, and very heavy rain falls on the side facing the wind (the windward side).

• Little rain on the Tamil Nadu coast:

• The Coromandel coast runs parallel to the Bay branch. The wind blows along the coast, so nothing forces it to rise.
• The coast also lies in the rain shadow (the dry side of a mountain, away from the wind) of the Arabian Sea branch, behind the Western Ghats.

Rain-making systems linked to the Bay

• Monsoon depression: a low-pressure system that forms over the head of the Bay of Bengal.
• Moist winds spiral in, rise and form thick clouds.
• It moves west-north-west along the monsoon trough (a long low-pressure belt from Rajasthan to the head of the Bay of Bengal, which is the Indian part of the ITCZ, the belt where winds from the two hemispheres meet).
• It brings heavy rain over Odisha, Chhattisgarh and Madhya Pradesh.

• NCERT: in July–August these systems come every third to fifth day.

• 2025: 7 monsoon depressions formed, and one became a deep depression. India had 69 low-pressure-system days, against a normal of 55 [3].

Where it occurs

• Myanmar coast: where the branch first strikes. The Arakan Hills (Arakan Yoma) here turn it towards India.
• South-east Bangladesh and West Bengal: the branch enters from the south and south-east.
• Ganga plain, from West Bengal west to Punjab: path of the west arm. Rain decreases from east to west. It meets the Arabian Sea branch over the Ganga plains and over Punjab and Haryana.
• Brahmaputra valley (Assam and the north-east): path of the north-east arm, with heavy rain.
• Garo and Khasi hills (Meghalaya): hit by the sub-branch.
• Mawsynram: a village on the crest of the Khasi hills (East Khasi Hills district, about 25°N). It gets the highest average yearly rainfall in the world (NCERT: about 11,000 mm).

• Cherrapunji (Sohra): a town close to Mawsynram on the same southern Khasi slopes. It is famous for historic rainfall records. Heavy rain has washed away its soil, so its surface is bare and rocky in places.

• Western Himalaya: gets rain from the Bay branch and the Arabian Sea branch together.

• Coromandel coast (Tamil Nadu): the dry exception. Tamil Nadu gets only 36% of its yearly rain in the south-west monsoon, but 48% in the northeast monsoon of October–December (1971–2020) [2].
• 2025 contrast: east and northeast India got only 1,089.9 mm, the 2nd lowest since 1901 [3]. Northwest India got 747.9 mm (127% of LPA, the Long Period Average, which is the 50-year normal rainfall) [3].

Don't confuse with

• Arabian Sea branch: it hits the west coast (Western Ghats, Gujarat) and splits into three arms. It is the stronger branch. The Bay branch hits Myanmar and Bangladesh first and splits into two arms.
• Northeast monsoon (October–December): it gives Tamil Nadu most of its rain (48%) [2]. The Bay branch of the south-west monsoon (June–September) leaves the Tamil Nadu coast mostly dry.
• Western Ghats rain vs Khasi hills rain: both come from wind hitting a mountain and being pushed up. The Western Ghats are fed by the Arabian Sea branch. The Khasi hills (Mawsynram, Cherrapunji) are fed by the Bay of Bengal branch, with funnel-shaped valleys that make the rain stronger.
• Monsoon depression vs the Bay branch itself: a depression is a low-pressure system that forms at the head of the Bay and moves west-north-west. The branch is the wind flow, which enters India from the south and south-east.

Prelims Hooks

• The Arakan Hills (Myanmar) turn the Bay branch, so it enters West Bengal from the south and south-east, not the south-west.
• Two arms after the split: west up the Ganga plain to Punjab, and north-east up the Brahmaputra valley. The Himalaya and the thermal low over north-west India cause the split.
• Rain in the Ganga plain decreases from east to west, because the air loses moisture as it moves inland.
• Mawsynram and Cherrapunji are in the Khasi hills, Meghalaya. They are fed by the Bay of Bengal branch, not the Arabian Sea branch. Mawsynram gets the world's highest average yearly rainfall (NCERT: about 11,000 mm).
• Trap: the Coromandel coast is dry in June–September. It lies parallel to the Bay branch and in the rain shadow of the Arabian Sea branch [2].
• Monsoon depressions form at the head of the Bay of Bengal and move west-north-west, not north-east.

Mains Points

• Relief decides where the Bay branch drops its rain (GS-I):
• The Arakan Hills turn the wind towards India.
• The Himalaya guide it west and north-east.
• The Khasi hills and their funnel-shaped valleys force it up, so the rain is very heavy. Cherrapunji's bare, rocky surface shows that heavy rain can also wash away soil.

• The Coromandel coast runs parallel to the wind and stays dry.

• East-to-west fall in rain shapes farming and water needs:

• The eastern Ganga plain and the north-east are wet. The western plain gets less rain and depends more on the meeting of the two branches over Punjab and Haryana.

• This helps explain regional differences in crops and irrigation needs across the Ganga plain.

• Uneven rain and hazards (GS-III disaster management):

• In 2025, east and northeast India got only 1,089.9 mm (2nd lowest since 1901), while northwest India got 127% of LPA [3].
• Monsoon depressions from the head of the Bay bring rain but also floods along their track. The 69 low-pressure-system days in 2025, against a normal of 55 [3], show how their number decides whether a season is wet or dry.

Related concepts

• Southwest monsoon season
• Arabian Sea branch of the monsoon
• Rain shadow area
• Mawsynram
• Cherrapunji
• Dry Tamil Nadu coast in the southwest monsoon
• Break in the monsoon
• Monsoon depression

Read more

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