  Transient quasi-periodic oscillations (QPOs) are extremely interesting
observational phenomena. However, the precise physical mechanisms leading to
their generation are still hotly debated. We performed a systematic search for
transient QPO signals using Weighted Wavelet Z-transforms on the gamma-ray
light curves of 134 bright blazars with peak flux exceeding
$1\times10^{-6}$~ph~cm$^{-2}$~s$^{-1}$ as monitored by Fermi-LAT. Artificial
light curves were generated from the power spectral density and probability
distribution functions of the original light curves to assess the significance
level of transient QPO. We discuss several physical mechanisms that produce
transient QPOs, with the helical jet model providing the best explanation. This
study identified four new transient QPO events. Interestingly, repetitive
transient QPOs are observed in PKS 0537-441, and nested transient QPOs are
detected in PKS 1424-41. Additionally, we find that transient QPOs tend to
occur in the flare state of the blazar. Finally, we estimate the incidence of
transient QPO events to be only about 3\%.
