What is this fuselage-side aerial used for?
One of the features on this American Airlines Airbus A319 is the Gogo Inflight Internet.


  The A319 will be the first aircraft type in American’s fleet to use Gogo’s ATG-4 Wi-Fi solution. ATG-4 uses more antennas and better data compression technology to provide more robust and reliable service (August 15, 2013).


There's another one on the other side. It's part of the air-to-ground (ATG) internet setup that utilizes Gogo's cellular towers in the US and Canada, as opposed to satellite internet.

(Left; Right) Click to view.



Related external links:


Inside a Gogo ATG4 installation
How Does In-Flight Wi-Fi Really Work?


Thanks to @mins for their help on the answer.