Like Southwest Monsoon the Northeast Monsoon also has a criterion for its onset. Tamil Nadu, Coastal Andhra Pradesh, Rayalaseema, South Interior Karnataka, and Kerala are the five sub-divisions which observe the Northeast Monsoon season.
The withdrawal of Southwest Monsoon paves way for the onset of Northeast Monsoon.
Commencement of Northeast Monsoon rains
i) Withdrawal of Southwest Monsoon up to Latitude 15°N.
ii) The onset of persistent surface easterlies over the Tamil Nadu coast.
iii) Depth of easterlies up to 850 hPa over the Tamil Nadu coast.
iv) Fairly widespread rainfall over coastal Tamil Nadu, south coastal Andhra Pradesh and adjoining areas.
Criteria for Declaring Onset of Northeast Monsoon
For declaring the onset of Northeast Monsoon following criteria may be
considered:
(1) Withdrawal of southwest Monsoon up to 15º N.
(2) Onset of persistent surface easterlies over the Tamil Nadu coast.
(3) Depth of easterlies up to 850 hPa/ 5000 feet over the Tamil Nadu coast.
(4) Fairly widespread rainfall over the coastal Tamil Nadu and adjoining areas.
(5) Onset is not to be declared before October 10 even, if the conditions described above exist
Generally, the normal date for the change in the pattern of winds from southwesterly to easterlies or northeasterly is around October 14 and it is observed for four to five days. While the average date for the onset of Northeast Monsoon is October 20 (+/- five days)
Also, the onset of Northeast Monsoon takes place mostly under the influence of weaker systems like low-level Cyclonic Circulations in the Southwest Bay of Bengal or in the wake of easterlies from east to west.
It might just not be a thumping onset all the time, it could be a milder one as well.
There are some systems that may impede or inhibit the progress of the Northeast Monsoon and this is the Cyclone season as well.
Like in 2018, two storms were formed and they both were very strong. Cyclone Titli was formed in the Bay of Bengal and was active from October 8 to October 12. Almost during the same time that is from October 6 to October 15, Cyclonic Storm Luban was formed in the Arabian Sea. The experts here have to say that the storms in the Arabian Sea cause more damage because it prolongs the stay of southwesterlies.
Last year in 2018, because of the strong system in the Arabian Sea the southwesterly winds regime continued. Hence, resulting in the late onset of Northeast Monsoon.
During that time, Tamil Nadu had excess rainfall which even extended the Southwest Monsoon activity and led to a lot of rainfall in October over Tamil Nadu. Since the winds could not change under the influence of these two systems, thus Northeast Monsoon got delayed and it was then somewhere around November 1 or 2.
By the time Cyclone vacated, rains reduced, and the Southwest Monsoon then paved way for the Northeast Monsoon.
The winds were too under the influence of the storm and they took time to get converted into the easterlies/northeasterly and thus delayed the onset of Northeast Monsoon.
This year, we don’t expect any Cyclone to brew in any of the seas. And already indication is there, and the onset of Northeast Monsoon can be seen anytime soon, most probably by the end of this week.
In wake of this, widespread rains are expected over Tamil Nadu, mainly over the coastal parts of Tamil Nadu for the next three to four days.
Image Credits – The Indian Express
Any information taken from here should be credited to Skymet Weather