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Context

It was on Saturday, May 24, that the India Meteorological Department (IMD) announced that the monsoon season has begun over Kerala. This is eight days earlier than the typical date schedule, which is June 1.

Criteria for monsoon onset

The IMD uses specific parameters to determine monsoon onset. This includes:

• Rainfall measurement requires at least 60% of 14 major meteorological stations to report 2.5 mm or more of rainfall over two consecutive days.
• Wind Field Analysis: Westerly winds must be maintained at a depth of up to 600 hPa, with speeds ranging from 15-20 knots.
• Satellite-derived OLR values must be less than 200 Watts per square metre to show adequate atmospheric moisture.

Factors for the early onset of monsoons

• The Madden-Julian Oscillation (MJO) is a tropical atmosphere-ocean-linked phenomenon that enhances cloud and wind patterns, promoting early rainfall.
• Mascarene High: A high-pressure system in the Mascarene Islands (in the south Indian Ocean) increased monsoon flow towards India.
• Convection: Increased convective activity, or the vertical movement of heat and moisture in the atmosphere, results in rainfall.
• Somali jet: A low-level, cross-equatorial wind band that formed near Mauritius and north Madagascar, strengthening monsoon currents in the Arabian Sea.
• Heat-low: An intense heat low (pressure departure below normal) over the continent around the Arabian Sea forms a shallow low-pressure zone that serves as a suction mechanism, pushing moist air down the monsoon trough and increasing rainfall in India.
• Monsoon trough: An elongated low-pressure area that runs from the hot low to the North Bay of Bengal. Its southerly movement causes vigorous monsoon conditions throughout India.

Regions Affected by Monsoon Onset

The southwest monsoon made an impact on various regions on its onset day. It covered:

• The southwest and east-central Bay of Bengal

• Southern and central Arabian Sea

• Kerala, Lakshadweep, and Mahe
• Parts of Karnataka and Tamil Nadu

The monsoon’s early arrival in Karnataka was particularly notable, occurring over ten days ahead of the usual schedule.

Source: Indian Express