ATLANTA — Southwest Airlines said it is buying AirTran Airlines for $1.4 billion. “Both companies have dedicated people with kindred Warrior Spirits, who care about each other, and who care about serving Customers. We will continue to enhance our award-winning Customer experiences and high-quality operations,” Gary Kelly, Southwest Airlines’ chairman, president, and chief executive officer, said in a statement. “We believe this deal can benefit all Stakeholders through an expansion of low fares for customers,