The India Meteorological Department (IMD) has announced the arrival of the southwest monsoon. 

Key Highlights Onset of Monsoon

• The climatic conditions are favorable for the monsoon to reach the Kerala coast in the coming days.
  • The Southwest Monsoon is the primary rainy season for most of India, bringing much-needed water for agriculture.
  • The Northeast Monsoon provides some rainfall to the southern peninsula, particularly Tamil Nadu.
• Typical Monsoon Pattern
  • Andaman and Nicobar Islands: This region usually starts receiving monsoon rain between May 15 and May 20.
  • Kerala Coast: This region typically starts receiving rain in the last week of May.

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Meaning of Onset of Monsoon

• Start of Monsoon Season: The onset of the monsoon (June-September)marks the arrival of the southwest monsoon over the Indian subcontinent.
  • This occurs when the monsoon winds reach Kerala, the southernmost state of India.
• Starting months: This event usually happens around late May or early June.
• Annual Rainfall: The southwest monsoon provides over 70% of India’s annual rainfall.
  • It is crucial for agriculture, water resources, and overall economic well-being.

Criteria for Declaring Monsoon Onset

The IMD broadly checks for consistent rainfall, its intensity, and wind speed over a specific area.

• Rainfall Requirements
• • Consistency: At least 60% of 14 designated weather stations in Kerala and Lakshadweep must record a minimum of 2.5 mm of rain for two consecutive days after May 10.
  • The 14 designated stations are:

• Wind Speed and Direction: Westerly winds should reach up to 600 hectopascals (hPa) in the specified area from the equator to 10ºN latitude, and 55ºE to 80ºE longitude.
• • Zonal wind speed in the area from 5-10ºN latitude and 70-80ºE longitude should be around 15-20 knots (28-37 kph) at 925 hPa.
• Heat Requirements- Outgoing Longwave Radiation (OLR): The OLR value should be below 200 watts per square meter  (wm²) in the specified region to confirm the onset.
  • OLR measures the energy emitted to space by the Earth’s surface, oceans
    • This measurement should be within the region bounded by 5-10ºN latitude and 70-75ºE longitude, as per IMD guidelines.

Factors Affecting South West Monsoon Formation

• Differential Heating and Cooling
  • Land-Water Contrast: Variation in temperature between land and water creates low pressure over the Indian landmass and high pressure over the surrounding seas.
• Position of Inter Tropical Convergence Zone (ITCZ)
  • Summer Shift: During summer, the Inter Tropical Convergence Zone (ITCZ) moves over the Ganga plain, approximately 5°N of the equator. 
    • This zone is also called the monsoon-trough. 
      • It influences monsoon formation.
• High-Pressure Area over Indian Ocean
  • East of Madagascar: A high-pressure area exists east of Madagascar, around 20°S in the Indian Ocean. 

• • • The intensity and position of this area impact the Indian Monsoon.
• Tibetan Plateau Heating: The Tibetan plateau heats intensely during summer, generating strong vertical air currents and forming low pressure about 9 km above sea level.
• Influence of Jet Streams
  • Westerly Jet Stream: Movement of the westerly jet stream to the north of the Himalayas influences monsoon formation.
  • Tropical Easterly Jet Stream: Presence of the tropical easterly jet stream over the Indian peninsula during summer affects monsoon patterns.
• Southern Oscillation (SO)
  • Pressure Reversal: Normally, high pressure occurs in the tropical eastern south Pacific Ocean and low pressure in the eastern Indian Ocean. 
    • However, in some years, this pattern reverses which is known as the Southern Oscillation (SO).

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