Arsene Wenger's side have kept two successive clean sheets away from home this week, in 2-0 wins over Bayern Munich and Swansea City.
Mertesacker believes that the sudden improvement is partly linked to time spent studying defensive deficiencies on video.
He told the club's official website when asked about extra time spent watching films: "We do [this] before every game. But the defenders did it on our own and I think there has been quite an improvement.
"We saw ourselves as a unit on the television and I think it was a good thing to do because sometimes you have a different view on the pitch and you feel different. It was everybody's decision [to do it]. We all had the feeling there was something to do so everybody was involved and everyone felt we needed to improve and speak more.
"The most important thing we learned was to cover each other more. There were a lot of situations where there was no pressure on the ball and the back four was not prepared for any balls between us. We are always watching videos. That is not the point. Sometimes it is good to see situations and I think the very important scenes against Tottenham made it more clear for everyone."