Why in News?

The IMD declared monsoon onset over Kerala on May 24, eight days earlier than the normal June 1 schedule. This onset signals the start of the June–September southwest monsoon, which delivers over 70% of India's annual rainfall, making it vital for agriculture and the economy.

The last time the monsoon arrived this early was in 2009, on May 23.

What’s in Today’s Article?

• Monsoon Onset Declaration: Criteria and Process
• Factors Behind Early Monsoon Onset in India

Monsoon Onset Declaration: Criteria and Process

• The IMD attempts to declare the schedule for southwest monsoon onset any time after May 10. To do so, certain essential criteria are considered.
• Key Criteria for Onset
  • Rainfall
    • At least 60% of 14 designated southern meteorological stations (including Thiruvananthapuram, Kochi, Mangalore, etc.) must report ≥2.5 mm rainfall for two consecutive days.
  • Wind Field
    • Westerly winds must dominate up to 600 hPa pressure level.
    • Wind speed at 925 hPa must range between 15–20 knots (27–37 km/h).
  • Outgoing Longwave Radiation (OLR)
    • The Earth absorbs and reflects solar energy, affecting global temperature and climate.
    • Aerosol particles in the atmosphere can absorb sunlight, contributing to atmospheric warming.
    • The absorbed energy is re-emitted as longwave infrared radiation (OLR).
    • OLR mainly originates from the warmed upper atmosphere, with some from the Earth's surface.
    • This radiation helps warm the lower atmosphere, which in turn warms the Earth’s surface.
    • OLR values must be below 200 W/m², indicating favorable atmospheric conditions for monsoon onset.
• IMD Declaration
• The monsoon onset is declared on the second day after all above conditions are met.
• This Year’s Onset
  • A simultaneous onset occurred over Kerala, Lakshadweep, Mahe (Puducherry), parts of the Arabian Sea, Bay of Bengal, and southern Karnataka and Mizoram.

Factors Behind Early Monsoon Onset in India

• The early onset of the southwest monsoon was driven by favourable atmospheric and oceanic conditions.
• A low-pressure area over the Arabian Sea and a trough over Vidarbha enhanced moisture inflow and atmospheric convection, accelerating the monsoon’s advance.
• Madden-Julian Oscillation (MJO):
  • MJO is a significant and complex ocean-atmosphere phenomenon that originates in the Indian Ocean and plays a crucial role in influencing the Indian monsoon.
  • It involves disturbances in clouds, winds, and pressure that move eastward at a speed of 4–8 metres per second.
  • These disturbances, known as MJO wind bands, can circle the globe within 30 to 60 days and lead to major weather changes along their path.
  • When the MJO is in a favourable phase, it can enhance rainfall over India during the monsoon season.
• Mascarene High
  • The IMD describes the Mascarene High as a high-pressure area found around the Mascarene Islands (in the south Indian Ocean) during the monsoon period.
  • The variation in the intensity of high pressure is responsible for heavy rains along India’s west coast.
• Convection and Rainfall
  • An increase in convective activity—vertical movement of heat and moisture in the atmosphere—can lead to rainfall.
  • For example, a convective system over Haryana recently moved southeastward and caused rain in the Delhi region.
• Somali Jet and Monsoon Strength
  • The Somali jet is a low-level, cross-equatorial wind system originating near Mauritius and north Madagascar.
  • By May, it crosses the east coast of Africa and reaches the Arabian Sea and the west coast of India.
  • A strong Somali jet enhances the monsoon winds, contributing to a robust monsoon.
• Heat-Low and Moisture Inflow
  • With the Sun’s shift to the northern hemisphere during summer, a low-pressure zone forms over the Arabian Sea.
  • A heat-low over Pakistan and nearby regions acts like a suction pump, drawing moist air into the monsoon trough and intensifying monsoon rainfall.
• Monsoon Trough
  • The monsoon trough is an elongated low-pressure area stretching from the heat low over northwest India to the north Bay of Bengal.
  • Its north-south oscillation brings rainfall across the core monsoon zone during June to September.
  • Additionally, the pressure gradient and the monsoon onset vortex—a cyclonic system in the Arabian Sea—contribute to the timely and strong onset of the monsoon.