As May gives way to June it is that time of the year, Southwest Monsoon Onset, a weather phenomenon that not only holds fascination for weather watchers across the globe but is critical to the growth prospects of India thanks to the large parts of the country that is still dependent on the monsoon for the agricultural prospects. After two years of not so good monsoon it is essential this year’s Southwest Monsoon picks up pace fast with many areas of the country still reeling under severe drought and water scarcity. The Monsoon Onset over Kerala (MOK) signifies the arrival of Monsoon over Indian Mainland while Andaman Islands are privy to the monsoon onset about a fortnight earlier.
As things stand the Monsoon Onset is currently stalled over Bay of Bengal. Thanks to the depression that became Cyclone Roanu the Bay Branch of Southwest Monsoon picked up around the middle of May and covered up the Andaman Islands by 20th May almost on track with the monsoon calendar derived by IMD through statistical models. Subsequently as one can observe in the map it has stalled for the next 10 days. The Sri Lankan Met Department has confirmed Southwest Monsoon has set over the Island nation while IMD is awaiting possible conditions to be met for concurring with the same.
With the current status listed and the official Southwest Monsoon season starts from today it is time to see what is the current status of the Monsoon Onset over Kerala. IMD has a certain set of parameters that is used to declare monsoon onset, these parameters primarily signify the Rainfall status, strength of the monsoon current & the depth of the monsoon.
While OLR (Outgoing Longwave Radiation) Maps are primarily used to understand the energy emitted from the Earth it is a tool that is used to understand cloudiness and probability of rainfall. As one can observe in the given below map the region which IMD monitors should see OLR Values of less than 200 W/m2 but as things stand large areas of the region still does not show consistent clouding to indicate the strength of monsoon.
The next set of maps we are going to see are the wind charts which indicate the depth & strength of the monsoon. As most of us know Southwest Monsoon is primarily driven by the Low Level Jet or the Somali Jet as some prefer to call. This Low Level Jet is strongest at around 1 km height above MSL (Mean Sea Level). IMD uses 925 hPa level (760 mts above MSL) to understand the strength of the monsoon surge around the tip of Southern Peninsular India consistently. The below chart clearly indicates except for the region around Sri Lanka the winds are not strong enough to indicate the monsoon surge.
The other wind factor which IMD considers is the depth of the Westerlies which basically indicates the consistency in the monsoon mechanism. IMD expects the winds to be Westerlies from ground to a height of 4.2 kms above MSL (600 hPa) from equator till 10N latitude right across Arabian Sea up to parts of Southern Peninsular India. The winds around equator are still scrambled at 600 hPa and there are a fair bit of Northerlies as well along with a good amount of Westerlies in the region
The last parameter is the rainfall criteria under which 60% of a select list of 14 stations should receive 2.5 mm rains or above for two days consecutively. As things stand many places of Kerala has been receiving rains for the last few days so this criteria is unlikely to be an issue. Considering other criteria have still not been met IMD has been considering these rains as pre monsoon rains.
Is there something to worry with monsoon still not marking its attendance on the Indian Sub Continent? No going by past data there is no consistent correlation between delayed monsoon and the actual rainfall performance of Southwest Monsoon for the year. The models are also indicating some progress over the next few days in terms of couple of disturbances evolving either side of the Peninsular India providing impetus to the monsoon current to reach the Indian Sub Continent