Monday, October 17, 2016

With the 2016 rainy seasons hesitating to change between the SWM and NEM, a brief Summary of the NEM.

-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 withdrawing 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 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 sysytems 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 around the 24th. taking into considerations the withdrawal of the SWM. The SWM lingers on, and even today, on the 17th, 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 long overdue 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.


Unknown said...

Wow.superb explained sir

Nilay Wankawala said...

Very informative on nem.

Unknown said...

Rajesh thats a power packed punch of an explanation !! A real delight when you make it lucid like this for us to understand. The Indian SWM and NEM is too complex with so many variables indeed. Lets hope the NEM is as good as the SWM this year.


Narayanan Ky said...

Thanks a lot for NEM coverage and explanation in a simple way

Prabhakaran Subramaniyam said...

This year thundershower is also absent in Erode district of west tamilnadu.when can we expect thundershower in my region? We are already buying water for our coconut trees and mulberry cultivation also affected seriously. Will NEM be normal for us?!

sset said...

fear repetition of 2012,2013 NEM haunts horrible NEM failure wherein even palmyara trees could not survive.... Many areas of SE India (intersection region of TN/KAR/AP) are drier than even Jaiselmar west RAJ.