Southwest Monsoon normally arrives over Kerala on 01st June. Arrival dates of monsoon were revised in the year 2020 for various stopovers over Indian Sub-Continent. Data from 1960 - 2019 was utilised for this exercise. The Andaman Sea arrival has been postponed by 02 days (22nd May) as against the earlier date of 20th May. Island nation Sri Lanka gets it on 26th May. Onset for Kerala remained unchanged as of 01st June. The arrival over most central parts of the country was delayed by 3-7 days and preponed over Northwest India by about a week.
Skymet has predicted the onset of monsoon over Kerala on 07th June with a margin error of +/- 3 days. National Weather Agency has pegged it at 04th June with a margin error of +/- 4 days. One week as such is taken as the standard deviation for the arrival of the southwest monsoon.
Since the year 2000, the monsoon arrived earliest on 18th May 2004. The most delayed arrival was on 08th June in 2003 & 2019 during this period. Season 2004 happened to be a drought year and years 2003 & 2019 were observed as normal and above normal respectively. It suffices to allay fears that early or late arrival has any overall bearing on the status of the seasonal rainfall.
It is fair enough to examine why the monsoon is touching down late this year. Monsoon circulation is a vast global scale feature. Oceanic features and synoptic scale weather systems form a part of a conglomerate of monsoon dynamics. An attempt is being made to analyse the current scenario and its implications on the arrival of Monsoon.
Cross Equatorial Flow: Cross equatorial flow serves as a bridge in the inter-hemispheric transportation of atmospheric mass, moisture and energy. It significantly contributes to changes in tropical precipitation, air temperature and weather systems. There are trade winds on either side of the equator. Depending upon the season these winds cross over from one hemisphere to the other one. During the southwest monsoon of India, these winds cross the equator and turn right under the influence of the Coriolis Force ( due rotation of the earth). This should start happening anytime around 15-20 May. The presence of tropical storm Fabien in the south-central Indian Ocean is restricting the outflow. This category -3 equivalent hurricane has been sailing very close to the equatorial water, south of the Arabian Sea for the last 5 days. The storm has now been downgraded to category-1 and will further weaken to a tropical storm shortly. However, the cyclone will move slowly and remain in the proximity of the equator, in the Southern Hemisphere, abeam Lakshadweep Islands for the next 4 days. Cross-equatorial flow is getting impeded on account of this storm. Even after diffusing over the sea, the resumption of seasonal flow has a lag lasting for over one week.
Anticyclone Arabian Sea: Southwest Monsoon is synonymous with westerly winds blowing across the Arabian Sea and striking the West Coast. Transportation of moisture, essential for the monsoon rains, especially around the onset time is carried out by a stream of low level westerly winds. The presence of an anticyclone over the central parts of the Arabian Sea in the lower levels of the atmosphere is an aberration and hinders the free flow of westerly winds. This feature is not showing any early signs of vacating that area. The onset of the monsoon will have to wait till this anticyclone gets diluted and the wind flow gets aligned.
Tropical Cyclones Over West Pacific Ocean: India will need to be wary of an active West Pacific basin this year. Typhoons generated in the South China Sea and the Philippines Sea tend to subsume monsoon systems over the Bay of Bengal. Higher frequency of tropical cyclones over the West Pacific can cause weakening of the Indian Monsoon. Cyclogenesis over the Northwest and West-Central Pacific Ocean is stronger and more frequent during weak monsoon years. Numerical models indicate the likely formation of a tropical storm during the last week of May over the South China Sea and the Philippines Sea. There is a bit of uncertainty about the timings but chances of storm formation are bullish. Madden Jullian Oscillation (MJO) is quite active and currently sailing over the West Pacific. It may provide trigger towards the formation of tropical cyclone. Any powerful weather system over the West Pacific in the next week or 10 days does not augur well for the approach of the monsoon stream.
In view of the above, the southwest monsoon is likely to be delayed by about a week with a margin window of 3 days. The initial progress as such may also be sluggish. Sowing operations over South Peninsula to start with, have to take a judicious call.
