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

Detail

1. Mechanism and timing of the southwest monsoon season

• Southwest monsoon season (June–September): the main rainy season of India. Moist winds blow from the south-west, from the Indian Ocean onto the land.
• How the winds are pulled in:
• In May–June the land of north-west India heats strongly. A deep thermal low forms there (a low-pressure area made by heat).
• By early June this low is strong enough to pull the southern trade winds (steady winds that blow towards the equator from the south-east in the southern hemisphere) across the equator.
• After they cross the equator, the Coriolis force (the turning effect of the Earth's spin, which bends winds to the right in the northern hemisphere) turns them. They now blow from the south-west.
• They pass over the warm Arabian Sea and Bay of Bengal and pick up a lot of moisture.

• Relief (the height and shape of the land) and the thermal low then decide where they go and how much rain falls.

• Two branches: the Indian Peninsula splits the incoming flow into the Arabian Sea branch and the Bay of Bengal branch.

• Share of rain:
• The season gives 868.6 mm of rain for India as a whole. That is 74.9% of the normal annual rainfall of 1,160.1 mm (IMD normal, 1971–2020) [2].
• Month-wise share of the season's rain (1971–2020): June 19.1%, July 32.3% (the wettest month), August 29.4%, September 19.3% [2].

• Gujarat region gets 96% of its yearly rain in this season (1971–2020) [2].

• Onset and spread (2025 example):

• The monsoon reached Kerala on 24 May 2025, eight days before the normal date of 1 June. This was the earliest onset since 2009 [3].
• It covered the whole country by 29 June 2025 (normal: 8 July) [3].
• Withdrawal began from west Rajasthan on 14 September 2025 (normal: 17 September) [3].
• Season total: 937.2 mm, which is 108% of the LPA (Long Period Average: the 50-year normal rainfall used as the baseline). This made 2025 the 5th wettest season since 2001 [3].

2. The Arabian Sea branch: three arms

• Arabian Sea branch of the monsoon: the part of the south-west monsoon that comes over the Arabian Sea and strikes the west coast of India. It is stronger than the Bay branch. On reaching India it splits into three arms.

Arm	Where it strikes	Path	Rain result
(i) Western Ghats arm	Western Ghats (Sahyadri), from the Tapi mouth to Kanyakumari	Forced 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	Up the Narmada and Tapi valleys into central India	Rain over central India; only 15 cm at Chotanagpur; joins the Bay branch in the Ganga plains
(iii) Kachchh–Rajasthan arm	Saurashtra peninsula and Kachchh (Gujarat)	Over west Rajasthan, alongside the Aravallis	Scanty rain; joins the Bay branch in Punjab and Haryana

(i) Western Ghats arm and the rain-shadow area

• How the windward rain forms:
• The Western Ghats run north–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).

• Result: 250–400 cm of rain on the Sahyadri and the west coast (Konkan, Goa, coastal Karnataka, Kerala).

• Rain shadow area: a dry zone on the leeward side of a mountain (the side away from the wind).

• The wind crosses the crest and goes down the eastern slopes.
• Sinking air is squeezed and gets warmer, so it can hold more water and gives little rain.
• Result: a dry belt on the eastern slopes and the Deccan plateau, for example Pune (Maharashtra) and Bengaluru (Karnataka).
• Exam comparison: Kozhikode (Kerala) and Mangalore (Karnataka) on the windward coast get far more rain than Pune and Bengaluru, which lie in the rain shadow.

(ii) Narmada–Tapi arm

• It strikes the coast north of Mumbai and moves east along the Narmada and Tapi valleys.
• The Narmada rises at Amarkantak (Madhya Pradesh) and flows west through a rift valley (a long valley formed where land sank between two faults) to the Gulf of Khambhat.

• The Tapi rises near Multai (Betul, Madhya Pradesh) and flows west to the Gulf of Khambhat near Surat.

• These east–west valleys act as corridors, so the wind reaches deep into central India.

• By the time it reaches the Chotanagpur plateau (Jharkhand), it gives only 15 cm, because most of its moisture has already fallen as rain.
• It then joins the Bay of Bengal branch over the Ganga plains.

(iii) Kachchh–Rajasthan arm

• It strikes Saurashtra and Kachchh (Gujarat) and passes over west Rajasthan.
• The Aravallis run south-west to north-east, parallel to the wind. Nothing forces the air to rise, so it gives very little rain. This is one reason why the Thar is a desert.
• It joins the Bay branch over Punjab and Haryana. Together the two branches give rain to the western Himalaya.

3. The Bay of Bengal branch

• Bay of Bengal branch of the monsoon: the part of the south-west monsoon that comes over the Bay of Bengal and reaches north-east and north India.
• Path, step by step:
• It first strikes the coast of Myanmar and south-east Bangladesh.
• The Arakan Hills (Arakan Yoma, along the Myanmar coast) turn a large part of it towards India.
• So it enters West Bengal and Bangladesh from the south and south-east, not from the south-west.
• The Himalaya to the north and the thermal low over north-west India then split it into two arms:
  • West arm: up the Ganga plain to Punjab. Rain decreases from east to west, because the air loses moisture as it moves inland.
  • North-east arm: up the Brahmaputra valley, giving heavy rain in Assam and the north-east.
• A sub-branch strikes the Garo and Khasi hills of Meghalaya.

Mawsynram and Cherrapunji

• Mawsynram: a village on the crest of the Khasi hills (East Khasi Hills district, Meghalaya, about 25°N). It gets the highest average yearly rainfall in the world (NCERT: about 11,000 mm).

• Why so much rain: the Khasi hills face the moist Bay wind. Funnel-shaped valleys squeeze the wind and force it upward, so the rain is very heavy.

• 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. It is wet but poor in vegetation in places.

Why the Tamil Nadu coast stays dry in this season

• Dry Tamil Nadu coast in the southwest monsoon: the Coromandel coast of Tamil Nadu gets little rain in June–September, for two reasons:
• It lies parallel to the Bay of Bengal branch, so the wind blows along the coast and is not forced to rise.

• It lies in the rain shadow of the Arabian Sea branch, behind the Western Ghats.

• Data: Tamil Nadu gets 48% of its yearly rain in the northeast monsoon (October–December) and only 36% in the southwest monsoon (1971–2020) [2].

4. Breaks in the monsoon

• Break in the monsoon: a dry spell of a week or more that comes after some days of rain, in the middle of the rainy season.

Region	Cause of the break
North India	Few rain-bearing storms form along the monsoon trough, or the trough shifts north to the Himalayan foothills
West coast	Winds blow parallel to the coast, so they are not forced to rise

• Monsoon trough: a long low-pressure belt that stretches from north-west India (Rajasthan) to the head of the Bay of Bengal during the monsoon. It is the Indian part of the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt where winds from the two hemispheres meet).
• When the trough moves north:
• The trough shifts to the Himalayan foothills.
• Heavy rain and floods hit the foothills and the Himalayan rivers.

• The plains of central and north India stay dry.

• 2025 example: the monsoon trough moved north of its normal position, causing a break-like spell from 2 to 12 August 2025. The monsoon became active again from 19 to 25 August 2025 [3].

• Active–break cycles: spells of heavy rain alternate with dry spells. They are linked to the MJO (Madden–Julian Oscillation: a belt of cloud and rain that moves eastward along the equator every 30–60 days). See the section on monsoon variability.

5. Monsoon depressions: the main rain-makers

• Monsoon depression: a low-pressure system that forms over the head of the Bay of Bengal. It moves west-north-west along the monsoon trough across central India and brings much of the season's rain.

• (NCERT: in July–August, such storms come every third to fifth day.)

• How it brings rain:

• Low pressure forms over the warm north Bay.
• Moist winds spiral in, rise and form thick clouds.

• The system drifts west-north-west over Odisha, Chhattisgarh and Madhya Pradesh, giving heavy rain along its track.

• On the west coast, offshore troughs and vortices (small low-pressure systems just off the coast) bring spells of very heavy rain.

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

6. 2025 case: Punjab floods

• In 2025 Punjab was hit by severe floods. Heavy monsoon rain was made worse by western disturbances (rain-bearing storms that come from the Mediterranean region with the westerly winds). Both natural and human causes added to the damage (NCERT Class 9 case study).
• IMD data:
• Northwest India got 747.9 mm (127% of LPA), its highest since 2001 and 6th highest since 1901 [3].
• Five western disturbances affected India in July 2025 and five more in August 2025 [3].
• East and northeast India got only 1,089.9 mm, the 2nd lowest since 1901 [3]. Uneven rain is a key feature of the monsoon.

Prelims Hooks

• The southwest monsoon gives about 75% of India's yearly rain: 868.6 mm out of 1,160.1 mm (1971–2020 normal). July is the wettest month [2].
• Rain shadow pairs: windward = Mangalore, Kozhikode; leeward (rain shadow) = Pune, Bengaluru.
• The Aravallis run parallel to the Arabian Sea arm, so Rajasthan gets scanty rain. This is a common "why is the Thar dry" question.
• The Arakan Hills (Myanmar) turn the Bay branch, so it enters West Bengal from the south and south-east.
• Mawsynram and Cherrapunji are in the Khasi hills, Meghalaya. They are fed by the Bay of Bengal branch, not the Arabian Sea branch.
• The Coromandel coast (Tamil Nadu) is dry in June–September: it lies parallel to the Bay branch and in the rain shadow of the Arabian Sea branch. It gets most of its rain from the northeast monsoon (48%) [2].
• Monsoon depressions form at the head of the Bay of Bengal and move west-north-west. Trap: they do not move north-east.
• Break in north India: the monsoon trough shifts to the Himalayan foothills, which floods the foothills and leaves the plains dry.
• Chotanagpur gets only 15 cm from the Narmada–Tapi arm.
• 2025: onset over Kerala on 24 May, the whole country covered by 29 June, season rain 108% of LPA [3].

Mains Points

• Relief controls where the rain falls:
• Mountains at a right angle to the wind (Western Ghats, Khasi hills) give very heavy windward rain.
• Ranges parallel to the wind (the Aravallis, the line of the Coromandel coast) give little rain.

• Leeward rain-shadow belts (interior Maharashtra and Karnataka) are drought-prone and need irrigation and watershed management (GS-I and GS-III).

• Breaks and the monsoon trough drive floods and droughts:

• When the trough shifts north, the Himalayan foothills flood while the plains are dry. The break of 2–12 August 2025 is a recent case [3].

• Long breaks damage kharif crops in rain-fed areas. This makes the case for medium-range forecasts and contingency crop plans.

• Uneven rain across regions, and its causes:

• 2025: northwest India at 127% of LPA against 1,089.9 mm (2nd lowest since 1901) for the north-east [3].

• The Punjab floods of 2025 show heavy monsoon rain combining with western disturbances, made worse by human causes such as building on floodplains. This is relevant to GS-III disaster management.

• Monsoon depressions are a resource and a hazard:

• They supply most of central India's rain.
• The 69 low-pressure-system days in 2025 against a normal of 55 [3] show that their number decides whether a season ends up wet or dry.