Using a depth level coordinate ocean circulation model (GFDL MOM3), the structure of interdecadal variabilities 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. The effect of bottom topography is also investigated. As reported in earlier studies, variability occurs over the polar ocean and is strongest in a flat bottom case and weakest in a bowl shaped bottom case, which has exponential continental slopes 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 zonal velocity anomalies on a horizontal plane. As a consequence of the variabilities, a phase lag between the meridional temperature gradient and overturning circulation, which is believed to be the main cause of such variabilities in many 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 western boundary is the downstream.