I think this is an interesting topic.The term "Ace" is used a lot, but never defined.I think that is because there are at least two ways of looking at it: staff ace and ace in general.Santana is definitely our staff ace.He wouldn't be the staff ace for many other teams, but he is pitching like a staff ace this year.
I looked into the definition of ace a few years back when I was arguing for Jack Morris being inducted into the Hall of Fame on sites where everyone was against him.He was definitely considered an ace for the three teams that he helped win the World Series.Part of that is subjective: he started every opening day for 14 straight years; managers set him up to pitch as often as possible, and specifically games 1 of the ALCS and WS in 1984, 1991 and 1992.
However, Morris also exemplified two other less subjective aspects of an ace.First, it seems to me that your ace ought to lead your team in getting outs (IP), and your team should win more when your ace is pitching.
Second, during the course of a season, your ace should step up when needed.This means he leads the staff when the team has a winning streak, he stops losing streaks, and he wins important games against division rivals and down the stretch when the pennant or the playoffs are on the line.Then, finally, he wins big games in the playoffs.
So far, Ervin is ticking these boxes for this season for this team.I hope he continues to through what would be an improbable season.