Early Onset of Southwest Monsoon

The India Meteorological Department (IMD) declared the onset of the southwest monsoon over Kerala eight days earlier than the climatological norm of June 1. This early arrival is significant as the southwest monsoon delivers over 70% of India’s annual rainfall, directly impacting agriculture, water resources, and the economy. The last comparable early onset was recorded in 2009. Understanding the dynamics of monsoon onset is critical for planning and mitigating climate-related risks.

Monsoon Onset Declaration: Criteria and Process

• Rainfall Threshold: At least 60% of 14 key southern meteorological stations must report ?2.5 mm rainfall on two consecutive days.
• Wind Field Requirements: Westerly winds must prevail up to 600 hPa pressure level, with surface wind speeds at 925 hPa between 15–20 knots (27–37 km/h).
• Outgoing Longwave Radiation (OLR): OLR values below 200 W/m² indicate atmospheric conditions favorable for convection and monsoon onset.
• IMD Procedure: The onset is officially declared on the second day after these criteria are simultaneously met.

Atmospheric and Oceanic Drivers Behind the Early Onset:

• Low-Pressure Systems: A low-pressure area over the Arabian Sea combined with a trough over Vidarbha intensified moisture inflow and convection.
• Madden-Julian Oscillation (MJO): This eastward-moving atmospheric-oceanic wave enhances monsoon rainfall during its favorable phase by influencing clouds and winds.
• Mascarene High: A high-pressure zone in the south Indian Ocean modulates wind patterns and moisture transport critical to monsoon rains.

Monsoon Dynamics and Strengthening Factors

• Convection and Rainfall: Vertical movement of heat and moisture increases cloud formation and rainfall, as recently observed with convective systems affecting northern India.
• Somali Jet Stream: A strong cross-equatorial wind from near Madagascar to the Arabian Sea boosts monsoon circulation and rainfall potential.
• Heat-Low and Moisture Inflow: Summer heating forms low-pressure zones that draw moist air from the oceans, intensifying rainfall across the monsoon trough.
• Monsoon Trough and Onset Vortex: These low-pressure systems facilitate rainfall over the core monsoon zone, ensuring the timely and robust establishment of monsoon conditions.

Impact of Early Onset of Monsoon in India

Agricultural Impacts

• Positive Impact:
  • Timely sowing: Early rains facilitate early sowing of kharif crops like paddy, maize, pulses, and cotton.
  • Extended growing period: Can improve crop yields due to longer vegetative phases.
  • Better groundwater recharge: Early rains may allow more time for aquifer replenishment before the post-monsoon dry season.
• Negative Impact:
  • False onset risk: If early rains are not sustained (i.e., a temporary wet spell), farmers may suffer due to seed damage and poor germination.
  • Crop calendar misalignment: Traditional sowing schedules may be disrupted, particularly for marginal farmers lacking access to timely advisories.
  • Pest and disease risk: Prolonged wet conditions may increase the incidence of pests and crop diseases.

Water Resources Management

• Positive Impact:
  • Early reservoir filling: Helps in early replenishment of major reservoirs and tanks, particularly in drought-prone states.
  • Improved irrigation prospects: Reduces pressure on groundwater and canal irrigation systems.
• Negative Impact:
  • Flood risk escalation: If the early onset is followed by heavy rainfall events, it may lead to localized flooding, especially in low-lying or urban areas.
  • Reservoir mismanagement: If water is released based on normal calendar assumptions, it may not align with actual hydrological patterns.

Economic Implications

• Agriculture-driven GDP fluctuations: Improved agricultural output due to timely rains can positively influence rural incomes and demand.
• Commodity price impacts: Early harvests may affect supply chains and market prices, especially for perishables.
• Insurance sector challenges: Crop insurance schemes may face new claims if false onset leads to agricultural loss.

Health and Disease Patterns

• Vector-borne diseases: Longer wet conditions due to early monsoon increase the breeding period for mosquitoes, elevating the risk of malaria, dengue, chikungunya, etc.
• Water-borne diseases: Inundation and poor drainage infrastructure can cause outbreaks of diarrhea, cholera, and leptospirosis.

Disaster Management and Preparedness

• Early flood risk: Sudden, heavy rainfall in unprepared regions may cause flash floods and landslides, especially in the Western Ghats and northeastern states.
• State readiness concerns: Disaster preparedness efforts, typically scheduled around June, may lag behind, straining relief mechanisms.

Impact on Energy Sector

• Hydropower benefits: Longer wet season may improve hydropower generation capacity.
• Cooling effect: Early rains reduce high pre-monsoon temperatures, cutting power demand from air-conditioning and irrigation pumps.

Urban Planning and Infrastructure Stress

• Drainage bottlenecks: Cities with poor pre-monsoon drainage preparations (like Mumbai, Delhi) may face urban flooding.
• Construction delays: Infrastructure and road-building projects may face work disruptions due to early rains.

Ecological and Environmental Implications

• Forest regeneration: Early rains support forest growth and biodiversity in monsoon-dependent ecosystems like the Western Ghats and central Indian forests.
• Wetland recharge: Crucial for bird nesting and aquatic biodiversity in areas like the Chilika Lagoon and Bharatpur.