Trevor Rosenthal has been tremendous in 2015

Seventy-six games into the 2015 season, St. Louis Cardinals closer Trevor Rosenthal possesses the second lowest earned run average (ERA) among qualified relief pitchers at 0.51, behind only Wade Davis (0.27) of the Kansas City Royals. In an attempt to convey as minimal bias as possible on a Cardinals-centric blog, I will state that there may be one closer I would choose to be on my imaginary team over Rosenthal at this time and it's the flame-throwing Aroldis Chapman. Per usual, Chapman is striking out hitters at an incredibly high rate (40.7%), but, merited or not, it is increasingly difficult for me to look past his 13.1% walk rate, an issue he has dealt with his entire career. No matter who you would choose (feel free to discuss this in the comments below), let's now take a look at just how good Rosenthal has been thus far in 2015, including a few possible reasons as to why he has improved from last season.

2015 Statistics (through June 30, 2015)

His 2015 strikeout rate is very similar to 2014's (28.3%), and though this is down from his career average of 30.5%, his decrease in walks, particularly from 2014 (13.6%) but also from his career rate of 9.6%, helps make up for the difference. While this table includes very shiny All-Star-Game-worthy statistics, there are two components worth keeping a close eye on as 2015 progresses because they will be key determinants in Rosenthal's future performance.

Up to this point, hitters have only been able to muster a .184 batting average against Rosenthal, which is down from his career batting average against of .208. While this does not seem like too big of a difference, it is enough to worry about the net result of one more flare falling in or one ground ball finding a hole in a high leverage situation. The other statistic to focus on is his left-on-base rate (LOB%) which is unsustainably high at 98.2%. Rosenthal's career LOB rate is 80.4%, and the league leader last season was Huston Street at a considerably lower 93.3%. Yet, even when we eventually do see a decrease in LOB rate, Rosenthal has created such a wide margin up this point that he will still be viewed as supremely effective.

Pitch Velocity

Seeing that his fourseamer velocity has thus far returned to where it was in 2012 is definitely a welcome observation, especially considering the team is just now entering the summer (read: hot weather) months of the season. On an even more positive note, May 2015 (99.00 MPH) was the third fastest month for his fourseamer in his career, behind October 2012 (99.30 MPH) and October 2014 (99.12 MPH).

Not to be ignored, his changeup velocity appears to have "followed" the increase in his fourseamer velocity—helping make sure that the gap between the two remains consistent so that it is not detrimental to either pitch's effectiveness. It must be noted that while Rosenthal is getting fewer swings and misses on his changeup this season, hitters are pounding it into the ground to the tune of a small-sample-sized (26 at bats) ISO of .000.

An uptick in velocity is helpful, but it can be counterproductive should the pitcher lose his ability to command the pitch. Fortunately, as you will see below, this has not yet been an issue for Rosenthal.

Pitch Location

In 2014, Rosenthal threw pitches in the bottom two rows (the area outlined in yellow) 39.77% of the time. Well, so far in 2015, Rosenthal has located pitches in this same zone 55.84% of the time, a ~40% increase from last season. Subsequently, this noticeable shift downward in pitch location has led to an abundance of ground balls this season. At present, his ground ball rate is 46.0%, as compared to 38.0% in 2014 and 43.5% for his career. Plus, when Rosenthal is not inducing ground balls on pitches low in the zone, he is painting outside corners with a 97 MPH fourseamer:

That being said, Rosenthal still knows when he can climb the ladder with his fastball. Just ask tied-up Mike Olt about this 99.2 MPH heater from opening night:

Finally, another benefit from Rosenthal's increased command is the fact that he is "gaining" many more strikes than he is "losing." Sure, Yadier Molina is a big part of this, but it also helps umpires out to have pitches end up close to Yadi's target:

So, among active closers, who would you choose to be on your imaginary team?

As usual, credit to PitcherList.com for the GIFs and BrooksBaseball.net/BaseballSavant.com for the graphs/charts.