As the south-west monsoon draws to a close, the direction of the surface winds change and the stage is set for the north-east monsoon. Much of the country is heavily dependent on the south-west monsoon for most of its annual rainfall. However, many parts of southern India also receive considerable rain from the north-east monsoon.
Tamil Nadu, in particular, typically gets nearly half its annual rain during this monsoon. In fact, as much as 60 per cent of the rain that the State’s coastal regions receive in a year is from this monsoon.
-The SWM onset is well defined, and follows a well coursed out Northward progress. And can be discerned with reasonable accuracy on charts.
For the NEM, it is not so.
In fact, on many occasions, there is no clear indication between the widhrawing SWM and setting NEM. Often one tends to merge into another.
So, setting a date for the commencement of NEM is difficult, and sometimes not possible.
-The main has its origin, its "Power House", is at the Mascrene Highs off Madagascar. The NEM has its origin round a large anti-cyclone over Siberia.
-The Upper winds during the NEM across the Bay are in a clockwise direction.That is an anti-cyclone is formed. At a level of 500 mb, the centre of the anticyclone shifts to central Burma region.
-The Upper Air temperatures show a Northward gradient in the NEM. Near 10N it is normally warmest, and dropping by 8c at 30N.
-Jet streams at 200 hpa are common to both the monsoons.
In the SWM, the Tropical Easterly Jet stream dominates the Peninsula, while the Sub-tropical Westerly Jet Stream is the feature of the NEM.
- The Sub Tropical Ridge (STR) in the stream determines the track of cyclones during the NEM months.The STR osscillates East-West, and shifts along the North Bay.
-The normal Sea Level Pressure during the NEM is a large system of low pressure over the Central Bay. It can extend into the Indian Peninsula as a trough.Towards the end of November,there is a shift southwards of this extended low.
-The general variations and fluctuations in intensity of this low pressure, governs the rainfall. When the trough is well defined, and the low is well marked, rainfall over the southern peninsula is good.
-During the NEM, an occasional burst of cold air from the Siberian High develops a low pressure systems over the Equatorial regions of the South China Seas. These systems move slowly westwards thru Sarawak/Eastern Malaysia coast, and remnants of these cross over into the Bay and form depressions.
Most of the NEM rains depend on depressions and cyclones from the Bay towards the East Coast of India.
The advent of the NEM is getting delayed this year.
Vagaries predicts its advance by the 24th. taking into considerations the withdrawal of the SWM. The SWM lingers on, and even today, on the 14th, is still prevailing over parts of Karnataka and T.N.
The overall delayed actions, beyond forecast estimates, is mainly attributed to the stubborn SWM, and to some extent the weak MJO not "supporting" the fast formation of NEM currents in the Bay. Cooler SSTs are prevailing west of Java and Sumatra.
But, the upper winds are showing signs of "resigning" and changing. The Mascrene highs have lost their grip, and are seasonal and weaker, and the Tibetan/Siberian High is taking shape ,though late.
Look out for Saturday 17th October Posting: El Nino Position and Quantum of NEM.