The country witnesses two Monsoon seasons -Southwest Monsoon and Northeast Monsoon. The earlier is a four-month long season spanning from June to September, while the latter one is three months season beginning from October to December.
While Southwest Monsoon covers entire country, Northeast Monsoon affects only five sub-divisions of Tamil Nadu, Kerala, South Interior Karnataka, Rayalaseema and Coastal Andhra Pradesh.
However, both the seasons do not co-exist i.e. until and unless Southwest Monsoon retreats from the entire country, the onset of Northeast Monsoon is not declared. According to Skymet Weather, as soon as withdrawal line of Southwest Monsoon reaches up to 15°N, it is considered as complete.
Other criteria for the onset of Northeast Monsoon is the change of surface winds to easterly-northeasterly beginning with Tamil Nadu, along with substantial increase in rainfall intensity and spread beginning with Tamil Nadu and Andhra Pradesh.
Unlike Southwest Monsoon, there is no official onset date for Northeast Monsoon as well as the Monsoon sets in one go over the region. Normally, the season sets in around October 20, with an error margin of +/-5 days. Irrespective of its onset date, Northeast Monsoon comes to an end on December 31. The season normally records 332 mm of rains, as compared to 887 mm of rain during the Southwest Monsoon.
Apart from this, Northeast Monsoon, also known as post-Monsoon season does not see much variations in terms of rainfall categories. While rainfall to the tune of 96% to 104% is considered normal in Southwest Monsoon, the normal rainfall for Northeast Monsoon spans from 89% to 111%.
Rainfall variation is much more in Northeast Monsoon as compared to Southwest Monsoon. 2010 saw surplus rains to the tune of 54% during the post-Monsoon season, while 2016 saw deficit rains of 65%.
However, Southwest Monsoon does not see much variation which keeps on settling between 5% to 10%. 2009 saw highest deficit Monsoon to the tune of 21.8%, which was declared as severe drought year. Since 2000, there have been 14 years have reported less than normal rainfall, five years of above normal rainfall and excess rainfall on two occasion in 2007 and 2013 by the margin of 5.7%.
Both the Monsoon seasons perform differently during the presence of El Niño. El Niño has the power of corrupting the entire Southwest Monsoon, while Northeast Monsoon is linked with good Monsoon rainfall during the season.
This year, Northeast Monsoon is likely to be normal and that too on the positive side.
Any information taken from here should be credited to skymetweather.com