Transient and localized severe total ozone reduction events, ozone miniholes, have been sometimes observed in both hemispheres. Dynamical contributions to the minihole formation are estimated for 22 events and the mean features are explored in each hemisphere. For the estimation, global distributions of ozone mixing ratio are reconstructed dynamically by using three kinds of satellite ozone data and isentropic distributions of potential vorticity. As a result, most of the total ozone reductions are explained by dynamical processes in both hemispheres. On miniholes in the Northern Hemisphere, about a half of the dynamical reduction of column ozone in the upper troposphere and the stratosphere is due to isentropic transport of ozone-poor air which is poleward around the tropopause and equatorward in the middle stratosphere. The remaining half is explained by loss of air masses in isentropic layers, which is caused by vertical uplift of isentropes in the lower and middle stratosphere. The same type of miniholes is also found in the Southern Hemisphere. The other type in the Southern Hemisphere is caused by displacement or stretching of the Antarctic ozone hole toward midlatitudes, where the total ozone is reduced by the equatorward transport of ozone depleted polar air throughout the stratosphere, while the local uplift of isentropes due to the associated advection of cold temperature also contributes to reduce total ozone.