|On 30th June 2017, IMD declared monsoon set over Kerala and various parts of Arabian Sea. Monsoon entered the peninsular India two days early to it’s schedule time.
Onset of monsoon over Kerala is a very important date for Indians. This date marks the beginning of rainy season for the country. Kerala extends from Latitude 8 deg 15 minutes to 12 deg 50 minutes north. The meaning of giving extension of Kerala in latitude only because monsoon progress from south to north along west coast of India. Here are some facts that you should know about onset of monsoon over Kerala.
Conditions of onset date (Figure is shown below for more understanding):
1) If 60% of the enlisted 14 stations report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala may be declared on second day provided the following criteria are also satisfied in concurrence. 14 stations are Minicoy Amini Thiruvanathpuram Puram Punalur Kollam Aallapuzha Kottayam Kochin Trissur Kozhikode Talassery Cannur Kasargode Mangalore.
2) Depth of westerlies should be maintained up to 4.2 km (600 hpa) in the box of 00 degree to 10 degree north and longitude 55 deg 80 deg E. The zonal wind speed bounded by 05 deg to 10 deg N and longitude 70 deg to 80 deg E should be of the order of 15 to 20 knots at altitude of 0.8 km (925 hpa) level.
3) INSAT derived OLR value should be below 200 W/sq m in the box 05 deg N to 10 deg N and 70 deg E to 75 deg E.
|OLR – It is Ongoing Longwave Radiation. Earth gets heated and emit electromagnetic infrared radiation. It’s value is high when sky is clear and value reduces when cloudy weather or dusty atmosphere. Hence during monsoon due to cloudy condition it suppose to reduce to certain value which is 200 Watts/sq m.
Westerlies – It is a wind direction flowing from west to east.
In these criteria the emphasis has been given to sharp increase in rainfall over Kerala. However setting up of large scale of monsoon flow and extent of westerlies upto altitude of 4.2 km (600 hpa) are also confirmed before the declare of monsoon date over Kerala.
The favorable conditions of progress of southwest monsoon over Indian latitude is:
1. Mascarene high to supply moisture laden wind to cross equator. Hence it results in strong cross equatorial flow over Indian ocean to create Southwest monsoon winds over Indian subcontinent.
2. System in Bay of Bengal helps to giving pulling effects to South Westerly wind along west coast.
3. Sea Surface Temperature of Indian ocean rise helps in forming cyclonic circulation over South Bay of Bengal.
4. Land heating over North West India and Central India helps to create heat low over land which drives air from sea to land.
5. Off shore trough over west coast.