  Thermal evolution of the axial anomaly is investigated in the system of the
linear sigma model for $2+1$ flavors. We explore the functional form of the
effective potential and the coefficient of the `t Hooft determinant term. It is
found that the latter develops a non-trivial structure as a function of the
chiral condensate and grows everywhere with respect to the temperature. This
shows that mesonic fluctuations strengthen the axial anomaly at finite
temperature and it does not get vanished at the critical point. The phenomenon
has been found to have significance in the thermal properties of the mesonic
spectra, especially concerning the $\eta-\eta'$ system.
