The Inter Tropical Convergence Zone (ITCZ) is a very prominent summer feature in both the Northern as well as the Southern Hemisphere. Practically, it runs around the earth in the tropics. The Northern Hemisphere ITCZ during the summer months is the Monsoon trough.
Broadly, the Monsoon regime lies between latitude 5°N of the northernmost surface position of the ITCZ in July and the Latitude 5°S of the southernmost position of the ITCZ during the month of January. The ITCZ is more prominent in the Afro-Asia Australian region.
The onset of Monsoon is predominantly manifested by the cross-equatorial flow of winds. These winds crossover from south to north of Equator and change direction from southeast to southwest which is why the name Southwest Monsoon. The stronger the flow of the winds, more aggressive is the wind surge during the onset phase.
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Monsoon over the Indian sub-continent begins over the Andaman Sea followed by Sri Lanka and the Central Bay of Bengal before making onset over the mainland which normally happens around June 1.
This year, the onset date over the Indian Islands i.e. the North Andaman Sea ad Port Blair is expected on May 20. On May 24, it is expected to reach Sri Lanka and the East Central Bay of Bengal. The Indian mainland is likely to see the Monsoon arrival on May 28 with an error margin of +/-2 days.
Meanwhile, IMD has forecast the onset date to be on May 29 with an error margin of +/-4 days.
Criteria for declaring the onset of Monsoon
The onset of Monsoon over the Indian mainland is never attempted before May 10. Onset of Monsoon is always subjected to fulfilment of specific criteria. Following are the same:
- Rainfall: If after May 10, 60% of the pre-decided 14 stations such a Minicoy, Amini, Thiruvananthapuram Kannur Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thaiassery, Kannur, Kudulu and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the second day provided the following criteria are also in concurrence.
- Wind field: Depth of westerlies should be maintained upto 600 hPa, in the box equator to latitude 10°N and longitude 55°E-80°E. The zonal wind speed over the area bounded by latitude 5°N to 10°N, longitude 70-80°E should be of the order of 15-20 KTS at 925 hPa.
- Outgoing long radiation (OLR): It should be below 200 wm-2 in the box confined by latitude 5°N to 10°N and longitude 70°E to 75°E.
The onset of Monsoon on most occasions takes place under the influence of either a strong Monsoon surge or the formation of low pressure areas on either side of the coast i.e. the Bay of Bengal or the Arabian Sea.
This time, Monsoon onset will take place in view of a system in the Arabian Sea which will push the Monsoon current towards the mainland.
Monsoon covers its entire journey in about 40-45 days and the entire country comes under Monsoon’s influence by July 15. Skymet will keep you updated with regards to the Monsoon arrival and advancement as and when time passes by.
