Recent findings suggested the possibility that planetary waves play a role in the occurrence of midlatitude sporadic E layers. To account for this, we propose here a new mechanism for large-scale accumulation of metallic ions in the midlatitude E region ionosphere driven by planetary waves in the lower thermosphere. In this process, the plasma is forced to converge horizontally and accumulate inside areas of positive vorticity set up by cyclonic neutral wind shears within a planetary wave. In its simplest form, the proposed model is similar to the well-known vertical wind shear mechanism of Es formation, but with the geometry turned on its side. Because of the long times required for ambipolar diffusion, the new mechanism can lead to significant plasma accumulation, acting as complementary to the vertical wind shear process so that dense Es can form more efficiently and frequently. The present model provides a physical base for understanding the long-term periodicities in occurrence and also the seasonal dependence of strong sporadic E layers at midlatitude. ¿ 1999 American Geophysical Union