Monsoon
- The term “monsoon” denotes the seasonal reversal of wind direction and the climate system associated with it.
- The monsoons are a system of seasonal winds that travel from the sea to the land in the summer and from the land to the sea in the winter.
- The formation of Monsoon winds are caused by the seasonal shift in the Intertropical Convergence Zone (ITCZ).
Seasons of Monsoon
Monsoon Rainfall in India occupy 2 seassons, namely.
The southwest monsoon season:
- Southwest monsoon occurs between June and September.
The retreating monsoon season
- The months of October and November are known for retreating monsoons.
ITCZ
- The Intertropical Convergence Zone (ITCZ) is a low-pressure belt that determines precipitation in the tropics by its northward and southward movements along the equator.
- This convergence zone lies parallel to the equator but moves north or south with the seasonal movement of the sun.
Factors Influencing Monsoon:
The shift of the position of Inter Tropical Convergence Zone (ITCZ):
- The ITCZ i.e., the equatorial low-pressure zone which is normally positioned about 5°N of the equator, moves over the Ganga plain during summer.
- This results in the formation of what is known as the monsoon-trough during the monsoon season.
The differential heating and cooling of land and water:
- Due to the higher specific heat capacity of water, the sea is often much cooler than the landmasses during summer.
- This creates a low pressure on the landmass of India while the nearby seas experience comparatively high pressure.
High-pressure area in South Indian Ocean:
- High pressure area can be formed at the east of Madagascar, approximately at 20°S over the Indian Ocean.
- The intensity and position of this high-pressure area has a major effect on the Indian Monsoon.
Pressure in Tibetan plateau:
- If the Tibetan plateau gets intensely heated during summer, it will result in strong vertical air currents.
- This results in the formation of low pressure over the plateau at about 9 km above sea level.
- Higher temperature in the Tibetan plateau results in stronger monsoon.
Westerly jet stream:
- Westerly jet stream moves to the north of the Himalayas and tropical easterly jet stream moves over the Indian peninsula during summer.
- The Subtropical jet stream shifting to the north of Himalayas is often the indicator of the onset of Southwest Monsoon.
- While its return back to the south of Himalayas is often the indicator of withdrawal/retreating of monsoon.
Southern Oscillation (SO):
- Also known as El Nino, the presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.
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