From idealized numerical 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 oceans are driven by steady heat fluxes given at the surface 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 among temperature gradient anomalies, the subsequent velocity anomalies, and the continents that block the velocity anomalies over the northeastern corner. As a consequence of the variability, a phase lag between the meridional temperature gradient and overturning circulation is established. When a continental slope 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.