  Isoscalar (IS) monopole and dipole excitations in $^{16}$O were investigated
by the method of shifted basis antisymmetrized molecular dynamics combined with
the generator coordinate method. Significant strengths of the IS monopole and
dipole transitions were obtained in the low-energy region below the giant
resonances. In addition to the compressive mode, which mainly contributes to
the high-energy strengths for the IS dipole giant resonance, we obtained a
variety of low-energy dipole modes such as the vortical dipole mode in the
$1^-_1$ state of the vibrating tetrahedral $4\alpha$ and the $^{12}$C+$\alpha$
cluster structure in the $1^-_2$ state. The $1^-_1$ state contributes to the
significant low-energy strength of the IS dipole transition as 5\% of the
energy-weighted sum rule, which describes well the experimental data observed
by the $\alpha$ inelastic scattering.
