Northeast Monsoon mainly affects five subdivisions of Peninsular India namely, Tamil Nadu, Kerala, Coastal Andhra Pradesh, Rayalaseema and South Interior Karnataka. Among these subdivisions, Tamil Nadu and Kerala are the rainiest with respective average rainfall of 438 mm and 481 mm during the Northeast Monsoon season. Coastal areas receive about 60% of total rainfall.
Northeast Monsoon and Southwest Monsoon have several fundamental differences in terms of origin, structure and characteristics. Firstly, Southwest Monsoon rains are observed throughout the country. Northeast Monsoon has limited extent.
Secondly, Southwest Monsoon winds are part of the equatorial westerlies, which are very humid and unstable. On the other hand, Northeast Monsoon winds are relatively dry and stable air mass.
Thirdly, Southwest Monsoon winds originate in the sea, while the winter Monsoon winds are land based winds.
The Summer Monsoon generally receives +/- 10% rains than the LPA of 887 mm. But, Northeast Monsoon witnesses large variations. The long period average NEMR (Northeast Monsoon Rainfall) is 312 mm, with a standard deviation of 84 mm.
Northeast Monsoon winds are related to low level circulations in the seas, easterly waves and sometimes cyclones and depressions. During Southwest Monsoon season, the scale of weather systems are large. Monsoon troughs, off shore troughs, low pressures areas and depressions.
During El Nino years, Southwest Monsoon brings poor rains in India but Northeast Monsoon brings normal or above normal rains. Of course, there are exceptions. In fact, the relation between the two monsoons become significant during El Nino. The year 2014 was a strong El Nino year when the country received 12% excess rains during Northeast Monsoon, but 22% less rains than normal during the Summer Monsoon. In 2009, another El Nino year, Northeast Monsoon recorded 12% more rains than normal, while Southwest Monsoon remained 22% rain deficit.
Image credit - Hindu