Why do the north-east trade winds change their direction while blowing through the Ganga valley?

(i) A feeble high pressure area develops over the north-western part of
India in the cold weather season. Light winds begin to blow outwards.
(ii) These dry north-westerlies winds come in contact with the Indian trades
(north-easterlies) over the Ganga valley’
(iii) The direction of north-easterlies changes as a result of this contact as
well as under the influence of topography’
(iv) Their direction is north-westerlies down the Ganga valley and northerlies
over the Ganga-Brahmaputra Delta’
(v) Over the Bay of Bengal, the trade winds retain their original north-easterlies direction, as they are free from the influence of any topography
over the sea.