  Manifestations of the solar magnetic activity through periodicities of about
11 and 2 years are now clearly seen in all solar activity indices.In this
paper, we add information about the mechanism driving the 2 year period by
studying the time and latitudinal properties of acoustic modes that are
sensitive probes of the subsurface layers. We use almost 17 years of high
quality resolved data provided by the Global Oscillation Network Group (GONG)
to investigate the solar cycle changes in p-mode frequencies for spherical
degrees l from 0 to 120 and 1.6 mHz < nu < 3.5 mHz. For both periodic
components of solar activity, we locate the origin of the frequency shift in
the subsurface layers and put in evidence for a sudden enhancement in amplitude
just in the last few hundred kilometers. We also show that, in both cases, the
size of the shift increases towards equatorial latitudes and from minimum to
maximum of solar activity, but, in agreement with previous findings,the
quasi-biennial periodicity (QBP) causes a weaker shift in mode frequencies and
a slower enhancement than the one caused by the 11 year cycle. We compare our
observational findings with the features predicted by different models that try
to explain the origin of this QBP and conclude that the observed
propertiescould result from the beating between a dipole and quadrupole
magnetic configuration of the dynamo.
