Explanation:

The reversal of direction in the lower-atmosphere moisture-laden winds happens primarily due to the southward movement of Inter Tropical Convergence Zone (ITCZ) during the withdrawal phase. During the monsoon season, this ITCZ is located over the Indian landmass. By September, as the temperature in the northern hemisphere begins to go down, the ITCZ starts moving southwards, towards the Equator, and further into the southern hemisphere where the summer season begins to take shape.

The months of October, November and December are supposed to comprise the northeast monsoon season, though the normal date for the onset of this monsoon is only around October 20. The southern peninsular region receives rains in the first half of October as well, but that is attributable to the retreating summer monsoon. 

The northeast monsoon is particularly important for Tamil Nadu, which receives almost half its annual rainfall (438 mm of the annual 914.4 mm) during this season. The southwest monsoon contributes just 35 per cent to Tamil Nadu’s annual rainfall (the rest comes in other non-monsoon months).