Strong trading revenue helped JP Morgan and Citigroup turn in solid third-quarter earnings. Such episodes have been short-lived in recent years, but there is reason to believe that the good times may linger.

At JP Morgan, fixed-income trading revenue surged by 48% from a year earlier to $4.3 billion. At Citi it rose 35% to $3.5 billion. Though overall net profit declined from a year earlier at both banks, they both solidly beat analyst expectations.