Onset of Monsoon

Onset of Monsoon

News: The southwest monsoon is likely to set in over Kerala on May 27, well ahead of its normal date of June 1. If the forecast turns out to be accurate, this will be the earliest onset of the monsoon over Kerala since at least 2009.

When is the onset of south-west monsoon announced by IMD?
• The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record at least 2.5 mm of rain for two consecutive days at any time after May 10.
• The depth of westerlies should be upto 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. The zonal wind speed over the area bound by 5-10ºN latitude and 70-80ºE longitude should be of the order of 15-20 knots (28-37 kph) at 925 hPa.
• Outgoing Longwave Radiation (OLR) value (a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere) should be below 200 watt per sq m (wm2) in the box confined by 5-10ºN latitude and 70- 75ºE latitude.

Factors influencing the onset on Southwest Monsoon:
• The low-pressure area or depression in the Bay of Bengal during the last days of May or the beginning of June.
• There are such systems in the Arabian Sea as well around the same time which results in onset over the mainland.
• ‘Cyclonic Vortex’ is another factor which appears in the Southeast Arabian Sea, off Kerala and Lakshadweep region. They also shift along the west coast to push the monsoon current.
• The formation of ‘trough’ off the west coast due to temperature differential between land and sea. This situation could be for a mild start and weak progress.
• Lastly the cross-equatorial flow, wherein the trade winds from the Southern Hemisphere crossover to the Northern Hemisphere can bring a strong monsoon surge towards the Indian mainland.

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