From idealized modeling experiments with a depth level coordinate ocean circulation model (GFDL MOM3), the structure of interdecadal variability from thermally driven circulations has been investigated. The model ocean is driven by a zonally uniform surface heat flux at the absence of surface wind stresses. As reported in earlier studies, variability is strongest in a flat bottom case and weakest in a bowl shaped bottom case, in which continental slopes are placed along the side boundaries except the southern boundary. In the flat bottom case, temperature and velocity anomalies circulate cyclonically over the polar ocean mainly due to interactions between meridional temperature gradient anomalies and subsequent zonal velocity anomalies on a horizontal plane. As a consequence of the variability, a phase lag between the meridional temperature gradient and overturning circulation, which is believed to be the main cause of such variability in earlier studies, is established. When a continental slope of exponential shape is added along the northern or eastern wall, the variability is reduced significantly, because the bottom topography guides boundary currents that remove anomalies effectively. A continental slope along the western wall, however, cannot weaken the variability because the slope cannot modify the boundary currents along the eastern or the northern walls.