Inter Tropical Convergence Zone

Geography glossary

Also called: ITCZ, Equatorial low, Doldrums, Equatorial trough · Topic: Climate of India and the Monsoon · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

The ITCZ (Inter Tropical Convergence Zone) is a belt of low pressure near the equator. Here the trade winds of the northern and southern hemispheres meet, the air rises, the surface winds are calm (the doldrums), and heavy convectional rain falls.

  • The ITCZ is not fixed. It moves north and south with the sun.
  • In July it sits over north India as the monsoon trough. This shift pulls in the south-west monsoon, so the ITCZ is at the centre of how India's monsoon works.

Explanation

How the ITCZ forms (cause → process → result)

  • Cause: the trade winds meet.
  • Trade winds are steady surface winds. They blow from the subtropical high-pressure belts (about 30°N and 30°S) towards the equator.

  • The trade winds from both hemispheres meet (converge) near the equator.

  • Process: the air rises.

  • When the two flows meet, the air can only go up.

  • The rising warm air cools. Its water vapour condenses (turns from gas into water drops).

  • Result: calm winds and heavy rain.

  • The air moves mostly upward, so winds at the surface are weak and calm. Sailors called this calm belt the doldrums.
  • Convectional rain falls. This is rain from warm air that rises because the ground heated it.

The ITCZ follows the sun

  • Summer (July): the ITCZ moves north to about 20°–25°N, over the Gangetic plain.
  • Over India it is called the monsoon trough.

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

  • Winter: the ITCZ moves south of India.

  • High pressure builds over north India and Central Asia.

  • The winds over India reverse and blow from land to sea.

  • Link with the upper air: the ITCZ shift over India is linked to the westerly jet (a fast upper-level wind blowing from west to east) moving away from the north Indian plain to north of the Tibetan Plateau. Only after that does the tropical easterly jet set in. The monsoon then begins suddenly with heavy rain, called the burst of the monsoon.

How the ITCZ shift creates the south-west monsoon

  • Pull from the north:
  • The 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. A strong heat low goes with a good monsoon over India [2].

  • Crossing the equator:

  • The south-east trade winds of the southern hemisphere cross the equator between 40° and 60°E, off the East African coast.

  • The Coriolis force (the turning effect of Earth's spin) bends winds to the right in the northern hemisphere. So the south-east winds become south-west winds.

  • Result:

  • These winds reach India from the Arabian Sea and the Bay of Bengal as the south-west monsoon.

  • They bring moisture picked up over thousands of kilometres of warm ocean.

  • Winter reversal:

  • The ITCZ moves south, and winds blow outward from land to sea as the north-east monsoon.
  • This monsoon is dry over most of India. It is wet over the Tamil Nadu coast, because the winds pick up moisture while crossing the Bay of Bengal.

The monsoon trough: the Indian part of the ITCZ

  • Monsoon trough: a long, narrow belt of low pressure over north India. It is the Indian part of the ITCZ in summer.
  • Low-pressure systems (depressions from the Bay of Bengal) move along it and bring rain to the plains.
  • Its north–south swing controls active and break spells:
Eastern end of the trough Effect on rain Where it rains
Normal or moves south Active monsoon Most of India gets good rain [2]
Moves north to the Himalayan foothills Break in the monsoon The plains go dry. Heavy rain falls in the Himalayan foothills [2]
Weak trough Rain becomes weak or stops Long dry spells in central India
  • The eastern side of the trough swings north and south through the season [2]. Because of this, the monsoon comes in "pulses" and not as steady rain.

Where it occurs

  • World: a belt near the equator that circles the globe, where the trade winds of the two hemispheres meet. It moves north in the northern summer and south in the northern winter.
  • India in summer (July): about 20°–25°N, over the Gangetic plain. Here it is the monsoon trough.
  • Extent: from the Thar Desert (Rajasthan) in the north-west to Patna (Bihar) and the Chotanagpur plateau (Jharkhand) in the east.

  • Centre: about 25°N in July.

  • Heat low: over north and north-west India.

  • Cross-equatorial flow: the south-east trades cross the equator between 40° and 60°E, off East Africa, over the western Indian Ocean.
  • During break spells: heavy rain in the Himalayan foothills, with flood risk in Bihar, Assam and north Bengal [2].
  • India in winter: the ITCZ lies south of India. The returning north-east monsoon brings rain to the Tamil Nadu coast.

Don't confuse with

  • Monsoon trough: this is not a separate belt. It is the Indian part of the ITCZ in summer, stretching from the Thar Desert to Patna and Chotanagpur.
  • Subtropical high-pressure belt: this lies at about 30°N and 30°S. It is the high-pressure source that trade winds blow out of. The ITCZ is the low-pressure belt where the trade winds end and meet.
  • Mascarene High: this is a high-pressure area in the southern Indian Ocean near Mauritius. It pushes air north across the equator. The ITCZ and heat low are lows that pull the air in.
  • Tropical easterly jet: this is an upper-air wind at about 150 hPa [2], near 15°N. The ITCZ and monsoon trough are surface low-pressure features.

Prelims Hooks

  • ITCZ = low-pressure belt where trade winds converge → rising air → calm doldrums + convectional rain.
  • In July the ITCZ lies at about 20°–25°N over the Gangetic plain, where it is called the monsoon trough.
  • Monsoon trough extent: Thar Desert → Patna and Chotanagpur plateau. Centre about 25°N in July.
  • Southern hemisphere trades cross the equator between 40° and 60°E and turn south-west due to the Coriolis force.
  • Trough moves north to the Himalayan foothills → break in the plains + heavy rain in the foothills. Trough moves south → active monsoon [2].
  • Trap: the ITCZ is a low-pressure belt. The Mascarene High (southern Indian Ocean, near Mauritius) is the high-pressure "push" from the south. It is not over the Arabian Sea or the Bay of Bengal.

Mains Points

  • The ITCZ shift is the "pull" behind the south-west monsoon.
  • Strong summer heating moves the ITCZ north and deepens the heat low over north India.
  • This draws cross-equatorial winds, turned by the Coriolis force, onto India as moist south-west winds. A strong heat low goes with a good monsoon [2].

  • The full picture also needs upper-air features. The westerly jet moves north of Tibet, then the tropical easterly jet sets in, which explains the sudden burst.

  • The swing of the monsoon trough causes active and break spells (GS-I and GS-III disaster management).

  • A northward shift dries the plains and causes floods in the Himalayan foothills (Bihar, Assam, north Bengal) [2].

  • This affects kharif sowing, reservoir planning and flood preparedness.

  • Seasonal reversal of the ITCZ shapes India's rainfall calendar.

  • Summer: a northward ITCZ gives the south-west monsoon across most of India.
  • Winter: a southward ITCZ gives the dry north-east monsoon, which is wet only on the Tamil Nadu coast.
  • So tracking the ITCZ is central to forecasting for food security, water management and power demand.

Related concepts

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Sources

  1. 1Class 11, Ch 4 "Climate"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 11 "World Climate and Climate Change" (primary)
  2. 2IMD, Monsoon FAQmausamaudit.imd.gov.in · tier 1