Turnbull handed Southport role
Turnbull combined his role as manager at the Bluebirds with his duties at Glanford Park and will do the same with this job.
Meanwhile, they have also signed towering former Iron loan striker Mat Bailey on loan from Crewe.
Bailey is yet to make a senior appearance for Dario Gradi's Championship side since signing last summer but has had spells with Northwich and Hereford.
He was on loan with United for a month at the beginning of last season following an injury to Steve Torpey on the eve of the campaign.