"Periodic oscillations" refers to regular back-and-forth movements or fluctuations that occur in a repeated pattern over time. It describes something that continuously goes through cycles or swings, typically at regular intervals. Full definition
High frequency combustion instabilities are periodic oscillations of combustion that occur due to the coupled interaction between unsteady heat release from combustion, acoustic fluctuations within the combustor, and perturbations of the incoming propellant. (americaspace.com)
Weaker equatorial winds would respond by stopping and reversing their flow thus triggering the ENSO and other periodic oscillations. (drtimball.com)
The phenomenon was dubbed the Quasi Periodic Oscillation (QPO). (sciencedaily.com)