Sunday, September 19, 2010

With the season about to change between the SWM and NEM, an attempt at a brief summary of meteorological facts 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 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.

For the SWM, its origin source, 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 Nortward 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 intansity 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.

some points taken from P.K.Das book.

Dehra Dun breaks a 44-year monsoon record by registering a whopping 3018.3 mm of rainfall this season

The monsoon rainfall broke a 44-year-old record in Dehra Dun by registering a whopping 3018.3 mm of rainfall this season. Met Director Anand Sharma told PTI that the previous highest record was in 1966 when the Doon valley recorded 2930 mm of rainfall. Before 1966, records regarding monsoon rains are not available, he said. With heavy rains lashing Dehra Dun, several rivers in the valley were on spate which include Rispana, Bindal and Asan. Nine of the total 13 districts of Uttarakhand have received excess rainfall this monsoon season. During the past 24 hours, Dehra Dun has received 149.3 mm of rainfall.
A dry MJO phase from 3-Oct-2010 tp 23-Oct-2010 might delay the onset of North-East monsoon over Tamilnadu ... http://ow.ly/i/3WvJ
What is Climate Risk Management? ... http://ow.ly/2Gpbb
2nd time in 2 months, IRI - predicts that North East monsoon may fail,... below normal rain forecast for OCT, NOV & DEC. http://ow.ly/i/3Wvz
GFS predicts, Heavy widespread showers for S. Tamilnadu, S. Kerala, Bengal and E. India from 24-Sep till 28-Sep-2010
All over India, showers will taper off from 22 or 23-Sep.
In 24hrs, Heavy showers forecast for Uttarakand, New Delhi and N. Madhyapradesh... http://ow.ly/i/3Wu5
Today, LOW pressure and circulation is located over Chatisgarh and E-S-E Madhyapradesh... http://ow.ly/i/3Wu0
Chennai - Having a warm day.. now 11:58am, temp now is at 33.1°C and rising, mild wind is from S-W
Ah ha... 80% clear looking India with.. activity N. Madhyapradesh and along N-E Orissa, coastal Bengal... http://ow.ly/i/3WlR