India’s Monsoon Arrives Early: Unpacking the Reasons Behind the Sudden Shift

India’s Monsoon Takes an Early Plunge

The India Meteorological Department (IMD) has declared the arrival of the southwest monsoon over Kerala, eight days ahead of its normal schedule. This early onset, last seen in 2009, marks the beginning of the crucial four-month rainy season that brings over 70% of India’s annual rainfall. The IMD typically announces the monsoon onset date any time after May 10, based on specific criteria.

Declaring the Monsoon Onset

The IMD considers several factors to declare the monsoon onset, including:

• Rainfall: At least 60% of 14 southern meteorological stations must record 2.5 mm or more rainfall for two consecutive days.
• Wind Field: Westerly winds must prevail between latitudes 30 and 60 degrees in both hemispheres, with a depth of up to 600 hectoPascals and speeds ranging from 15-20 knots (27-37 km/hr) at 925 hPa.
• Outgoing Longwave Radiation (OLR): Satellite-derived OLR values should be below 200 Watts per square meter.

If these criteria are met, the IMD declares the monsoon onset over Kerala on the second day of observations.

What Fueled This Early Arrival?

This year’s early monsoon is attributed to a confluence of favorable factors:

• Madden-Julian Oscillation (MJO): This complex ocean-atmospheric phenomenon, originating in the Indian Ocean, enhances rainfall in India during its favorable phase.
• Mascarene High: This high-pressure area over the Mascarene Islands influences rainfall along India’s west coast. Variations in its intensity can cause heavy or light rains.
• Convection: Increased convective activity, the vertical transport of heat and moisture, leads to rainfall, as seen in the recent rains in Delhi caused by a convective system over Haryana.
• Somali Jet: This low-level cross-equatorial wind band, originating near Mauritius, strengthens monsoon winds upon reaching the Arabian Sea and west coast of India.
• Heat-Low: The development of a low-pressure area in the Arabian Sea, following the Sun’s northward movement, acts as a suction device for moist air, influencing monsoon rainfall.
• Monsoon Trough: This elongated low-pressure area extending from the heat-low to the north Bay of Bengal causes rainfall across the core monsoon zone.

The IMD also highlights pressure gradients and the monsoon onset vortex, a cyclonic formation in the Arabian Sea, as contributing factors.

Monsoon’s Expanse on Onset Day

On its onset day, the southwest monsoon covered a wide area, including the southwest and east-central Bay of Bengal, Maldives and Comorin regions, south and central Arabian Sea, Kerala, Lakshadweep, and Mahe. Notably, it also reached northeast India (Mizoram), southern and coastal Karnataka, and Tamil Nadu (except its northern regions). This early advancement into Karnataka is particularly significant, as the onset is usually around June 5th.