The stopper left Middlesbrough to secure first-team football under the management of Derek McInnes and finds himself sharing a dressing room with fellow countrymen Stephen Pearson, Mark Wilson and Richard Foster.
He said: "There are a few familiar faces around the place which is nice.
"I'm looking forward to hopefully helping the club perform well for the remainder of the season.
"I've had a loan spell at Bristol City in the past and I enjoyed working under Derek McInnes so, when the opportunity came along to come back and get playing, I decided to take it.
"I left Tony Mowbray on good terms at Middlesbrough and my focus is now on playing regularly for the rest of the season and taking things from there.
"My Middlesbrough contract expires at the end of the season and then I'll look at my options."