Recent global seismological observations have revealed lateral variations in the thickness of the D″ layer. We have performed laboratory experiments to explore how undulations of various sizes in the D″ layer affect convection patterns. We find that topography on the lower boundary can induce plumes and that there is a critical height above which topography controls convection patterns. Observed undulations in the D″ layer exceed this critical height, suggesting that they may control mantle convection patterns. ¿ 1999 American Geophysical Union