  Activity-related variations in the solar oscillation properties have been
known for 30 years. However, the relative importance of the different
contributions to the observed variations is not yet fully understood. Our goal
is to estimate the relative contribution from sunspots to the observed
activity-related variations in the frequencies of the acoustic modes. We use a
variational principle to relate the phase differences induced by sunspots on
the acoustic waves to the corresponding changes in the frequencies of the
global acoustic oscillations. From the sunspot properties (area and latitude as
a function of time), we are able to estimate the spot-induced frequency shifts.
These are then combined with a smooth frequency shift component, associated
with long-term solar-cycle variations, and the results compared with the
frequency shifts derived from the Global Oscillation Network Group (GONG) data.
The result of this comparison is consistent with a sunspot contribution to the
observed frequency shifts of roughly 30 per cent, with the remaining 70 per
cent resulting mostly from a global, non-stochastic variation, possibly related
to the changes in the overall magnetic field. Moreover, analysis of the
residuals obtained after the subtraction of the model frequency shifts from the
observations indicates the presence of a 1.5-yr periodicity in the data in
phase with the quasi-biennial variations reported in the literature.
