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Features of South West Monsoon Withdrawal and the advance of the North East Monsoon:
This note is not a forecast nor a quantum estimate. It is just how to begin a Monsoon Watch for the NEM.
In the very initail para, we begin with the basic fact of the Monsoon origins. SWM has its origin around the Mascarene Highs off Madagascar, and the NEM has its origin round a large anticyclone over Siberia.
However, coming to the advances of these systems, the mode of setting of the 2 monsoons differ. The onset of SWM is well defined, and follows progressively Northwards.This can be monitored quite accurately on weather charts.
On the other hand the onset of the NEM is not clearly defined. In fact, in some years, there has been no clear indication between the withdrawal of the SWM in peninsula India and the onset of the NEM.
I would, on the above point, normally hesitate to actually present or believe a "normal date" for the onset of the NEM in the Southern States.
Another point of comparison between the 2 monsoons is that, the NEM, during its life cycle, is normally with interrupted rainfall. The rainfall occurs in spells, accompanied by storng surge of cold air from the Siberain "origin".
Many a times, these surges are associated with the formation of lows and systems in the Equatorial South China Seas. These systems move westwards. Occasionally, these systems intensify.
SWM generally has an un-interrupted rainfall in its four months tenure in some regions or the other.
Now, the commencement of the NEM , as stated above, is directly corelated to the withdrawal of the SWM.
Hence, to start monitoring and "watching" the NEM, we must follow the withdarawl.
Factors are to be considered while studying or monitoring the SWM withdrawal and NEM commencement.
1. An anticyclone starts forming at sea level and 850 hpa level initially in Sindh and spreading to NW India. Originally, the seasonal low forms in May as a result of thermal highs, that is high temperatures. Now, we should observe and record the rate of (night) temperature drop, to see the diminishing of the low. A prelude to monsoon withdrawal.
2. The Monsoon axis starts moving Southwards, and for the NEM, rotates the wind directions. Both points, 1. and 2. are related and occur when the axis movement starts towards the South. For this, we have to monitor the wind change direction and dissolving of the "supporting" low in Sindh (Pakistan).
3. The ITCZ, simultaneously starts shifting Southwards. See Position of ITCZ in SWM(Diagram shows extreme break Monsoon Position) and in NEM (Winter).
4. An anticyclone moves towards central India at the 200 hpa level.
5. Extreme High pressure initial formation starts in Lower Siberia regions. Now, like the Mascrene Highs are the "power House" of the SWM. the Siberian/Tibetan high is the "power house" of the NEM. Force winds rush from these highs towards the Bay and south China Seas in 2 seperate branches (Hadley Cell). Cooling of the region results in a good High formation.
6. Upper air stability visible observations to be seen. Cloud formations and clear (moisturless) skies are a strong indicator of the lack of monsoon atmosphere. For this actual observations and on-the-spot reports help.
7. The SW cross equatorial winds weaken. Normally in a full SWM season, the depth of the SW winds coming into India. The reversal of wind fields occur at 6 kms.
8. Once these commence, we can start chasing the NEM...
NEM: After actual setting, the general prevailing weather pattern should be stable like:
1.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.Another factor are the jet streams.
In the SWM, the Tropical Easterly Jet stream dominates the Peninsula, while the Sub-tropical Westerly Jet Stream is the feature of the NEM.
2.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.
3.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. Most of the NEM rains depend on depressions and cyclones from the Bay towards the East Coast of India. This is more so in the coastal Districts.The highest seasonal rainfall (October-December)of around 75 cms occurs along the SE coast of Tamil Nadu and adjoining South AP.
4.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