  We studied the temporal evolution of quasi-biennial oscillations (QBOs) using
acoustic mode oscillation frequencies from the Global Oscillation Network
Group. The data used here span over more than 25 yr, covering solar cycles 23
and 24 and the ascending phase of cycle 25. The analysis reveals that the
QBO-like signals are present in both the cycles, but with different periods.
The dominant QBO period in cycle 23 is found to be about 2 yr while it is about
3 yr in cycle 24. Furthermore, the quasi-biennial oscillatory signals are
present only during the ascending and high-activity phases of cycle 23 and
quickly weaken around 2005 during the declining phase. In comparison, the QBO
signals are present throughout the cycle 24, starting from 2009 to 2017. We
also explored the depth dependence in QBO signals and obtained a close
agreement at all depths, except in the near-surface shear layer. A detailed
analysis of the near-surface shear layer suggests that the source region of
QBOs is probably within a few thousand kilometers just below the surface.
