In this study we have investigated the behaviour of natural convection flow in open cavities, with both homogeneous and thermally stratified ambient, using direct numerical simulation. The cavity is insulated at the top and bottom boundaries, heated from the left-hand side boundary and open at the right-hand side. A wide range of Rayleigh numbers were considered (5 × 106 to 1 × 1010) with Pr = 0.7 for all cases. It was found that the homogeneous flow is steady for all Rayleigh numbers considered, whereas the stratified flow with a high enough Rayleigh number exhibits low- and high-frequency signals of the same type as are observed for closed cavity flow. The thermal boundary layer is examined in detail and it is shown that both low- and high-frequency signals are located predominantly in the upper region of the heated plate and are associated with a reverse-S-flow formed by the boundary layer exit jet interacting with the stratified interior. The low-frequency signal is associated with standing waves in the boundary layer, whereas the high-frequency signal is associated with travelling waves. The high-frequency signal occurs initially as a harmonic of the base low-frequency signal. A corner jet with the same inlet characteristics as the natural convection boundary layer exit jet is also examined and shown to exhibit a similar bifurcation, but with the low frequency always dominant. It is suggested that the generation mechanism for the bifurcation of the natural convection flow is the same as that for the corner jet.