Abstract: A two dimensional numerical simulation ofnatural convection in a differentially heated cavity is performed. Theevolution of the horizontal hot intrusion is observed and its physicalproperties are described. The numerical results show that, after the hotintrusion reaches the cold wall, some accumulates in the cold top corner,since the cold boundary layer flow cannotentrain all of the hot intrusion. Due to the hot fluid accumulation and the buoyancyeffect, a reverse flow (opposing the cold boundary layer flow) first arisesin the cold top corner, and then forms a clockwise local circulation. Thiscirculation may move back to the hot wall, and ultimately a cavity-scaleoscillation (i.e. internal gravity wave) is resulted between the hot and coldwalls underneath the ceiling of the cavity. Such oscillation significantlydecays with the passage of time, and almost disappears after twoperiods.