Monsoon trough

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

Meaning

The monsoon trough is a long, narrow belt of low pressure over north India in summer. It is the Indian part of the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt where the trade winds of the two hemispheres meet). In July it stretches from the Thar Desert to Patna and the Chotanagpur plateau.

It matters because it pulls moist monsoon winds into India. Storms move along it and bring rain to the plains. When it shifts north or south, rain starts and stops in "pulses", which gives the monsoon its active and break spells.

Explanation

How it forms: the ITCZ follows the sun

• The ITCZ is not fixed. It moves with the sun.
• Near the equator, the trade winds meet. Trade winds are steady surface winds that blow from the subtropical high-pressure belts (about 30°N and 30°S) towards the equator.

• Where they meet, the air can only go up. So the air rises, cools and gives heavy convectional rain (rain from warm air that rises because it was heated).

• Summer shift (cause → process → result):

• Cause: the summer sun heats north India strongly.
• Process: the ITCZ moves north to about 20°–25°N, over the Gangetic plain.

• Result: over India, this belt is called the monsoon trough. In July its centre lies at about 25°N.

• It also helps build a thermal low (heat low: low pressure made by strong heating of the ground) over north and north-west India.

• Winter: the ITCZ moves south of India. The trough disappears. High pressure builds over north India, and the winds reverse into the north-east monsoon.

How it works: the "pull" of the monsoon

• The trough and heat low act like a pump.
• Low pressure over north India pulls in air from the high pressure of the southern Indian Ocean.
• A strong heat low works like a suction device. It pulls moist air in along the monsoon trough [2].

• A strong heat low goes with a good monsoon over India [2].

• The trough is a path for storms.

• Low-pressure systems and depressions from the Bay of Bengal move along the trough.

• They bring rain to the north Indian plains.

• Push and pull together:

• Push from the south: the Mascarene High in the southern Indian Ocean.
• Pull from the north: the heat low and the monsoon trough.

Active and break spells: the swing of the trough

• The eastern end of the trough swings north and south during the season [2]. Because of this swing, the monsoon comes in "pulses" and not as steady rain.

Position of the trough's eastern end	Effect on rain	Where it rains
Normal or moves south	Active (strong) monsoon	Most of India gets good rain [2]
Moves north to the Himalayan foothills	Break in the monsoon	Plains go dry. Heavy rain falls in the Himalayan foothills, so there is flood risk in Bihar, Assam and north Bengal [2]
Weak trough	Rain becomes weak or stops	Long dry spells in central India

Conditions it needs

• Strong summer heating of north and north-west India. This makes a deep heat low.
• Early spring heating of the land. If there is a lot of snow over Eurasia (Europe and Asia together) and the Himalaya in winter and spring, the spring heating is delayed. This gives a weak thermal low and weak monsoon westerlies over India [4].
• A steady supply of moist air: the south-east trade winds cross the equator between 40° and 60°E. The Coriolis force (the turning effect of Earth's spin; it bends winds to the right in the northern hemisphere) turns them into south-west winds, which flow towards the trough.

Where it occurs

• India (July position): a belt across the north Indian plain at about 20°–25°N, with its centre near 25°N.
• Western end: the Thar Desert (Rajasthan), the heat-low region of north-west India.
• Eastern end: Patna (Bihar) and the Chotanagpur plateau (Jharkhand).

• Region it lies over: the Gangetic plain.

• During a break: the eastern end shifts north to the Himalayan foothills. Floods hit Bihar, Assam and north Bengal [2], and the hill areas of Uttarakhand are also affected.

• Moisture sources that feed it: the Arabian Sea and the Bay of Bengal. Bay of Bengal depressions travel along it.
• World context: it is the summer Indian part of the global ITCZ, which lies near the equator for most of the year. The doldrums (the calm, windless belt that sailors named) are found in this same equatorial ITCZ.

Don't confuse with

• ITCZ: the ITCZ is the whole belt around the globe, and it moves with the sun. The monsoon trough is only its summer part over north India, at about 20°–25°N.
• Tibetan High: a high-pressure area in the upper air over Tibet, centred near 30°N, 90°E at 300 hPa [2]. The monsoon trough is a low-pressure belt at the surface. Remember the pattern: low below, high above.
• Mascarene High: a surface high in the southern Indian Ocean near Mauritius. It pushes air north. The monsoon trough pulls air in from over north India.
• Tropical easterly jet: a fast upper-air wind at about 150 hPa and about 15°N, linked to the burst of the monsoon [2]. The monsoon trough is a surface low-pressure belt at about 25°N that controls active and break spells.

Prelims Hooks

• Monsoon trough = the ITCZ over north India in summer. In July it lies at about 20°–25°N, with its centre near 25°N.
• Extent: from the Thar Desert in the west to Patna and the Chotanagpur plateau in the east.
• Trough's eastern end moves north to the Himalayan foothills → break in the plains + heavy rain and floods in the foothills [2].
• Trough is normal or moves south → active monsoon over most of India [2].
• A strong heat low works like a suction device along the trough → good monsoon [2].
• Trap: the monsoon trough is a surface low. The Tibetan High is an upper-air high. The Mascarene High is a surface high in the southern Indian Ocean, not over north India.

Mains Points

• Active and break spells come from the swing of the trough.
• Northward shift → dry spells in the plains and floods in the Himalayan foothills (Bihar, Assam, Uttarakhand).

• This affects kharif sowing, reservoir planning and flood preparedness (GS-III disaster management).

• The trough is the "pull" part of a three-dimensional monsoon system.

• Surface heat low and trough (pull) + Mascarene High (push) + upper-air Tibetan High and tropical easterly jet (outflow).

• A land–sea heating idea alone cannot explain why the rain comes in pulses. The swing of the trough explains it.

• Forecasting depends on the strength of the heat low.

• The April mean surface temperature over the central Indian heat-low region and winter Eurasian snow cover are both significantly linked to the date of monsoon onset over Kerala [3].
• Good forecasts help food security, power demand planning and water management.

Related concepts

• Inter Tropical Convergence Zone
• Tibetan Plateau heating and the monsoon
• Mascarene High

Read more

• Pressure, winds and the upper air: the ITCZ, jet streams, Tibet and the Mascarene High