Alese moves to Fratton Park on-loan to the end of this season.
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"He's a strong defender who is also comfortable on the ball and has experience of playing Championship football," Portsmouth manager John Mousinho told the club's website.
"We are short on centre-backs at the moment and he exactly fits the profile of what we have been looking for."
Sunderland chief Kristjaan Speakman also stated: "Aji has been incredibly patient and worked diligently to prepare himself for this next opportunity.
"The Championship is a league he's familiar with, and this move is the ideal step for him in the second half of the season."