Didn’t the Spirit descend upon Jesus when Jesus was being baptized in the river?
Yes. In sensus plenior, the book of Jonah tells the Holy Spirit’s role in the life of Christ. He was joined with him at the baptism, separated from him upon the cross, and grieved, and reunited in resurrection.
He took counsel from the Son, which is why he is called “Another Comforter”. The Son is the first comforter.
The first verse of Jonah, in sensus plenior, has Christ sending the Holy Ghost, which agrees that he is sent from both the Father and the Son.
Discerning sensus plenior is child’s play. God speaks in riddles and double entendre.
Jonah 1:1 ¶ Now the word of the LORD came unto Jonah the son of Amittai,
Who is the Word of the Lord? Christ
Jonah means ‘dove’ which represents the Holy Ghost
son also means ‘builder’ in word play
Amittai means ‘truth’
It is a riddle in double entendre: Christ came to dove, the builder of truth.
The rules that constrain sensus plenior from being free-for-all allegory are here:
sensusplenior.net/?page_id=379
So in the context of the OP: The Holy Ghost was to reluctant to leave the Son again. Imagine 3 days of separation as experienced by an eternal Deity. God himself was split, and it seemed to be an eternity.
With Christ returned to eternity, the Holy Ghost could fill us without separating from Him again.