<p>The results of direct numerical simulation (DNS) of the effect of a horizontal surface temperature gradient on the dynamics of a thermal convective flow are presented. A series of simulations was performed at moderate Rayleigh numbers (10⁵–10⁸). The computational domain was rectangular and its two horizontal walls had a specified mean temperature difference, leading to the formation of a free-convective flow. The temperature on the lower wall was nonuniformly distributed along one of the horizontal axes. Convective cells formed due to the vertical density gradient were subjected to a weak horizontal temperature gradient, which led to their deformation and transfer towards the central axis of the computational domain. As a result, intense coherent horizontal oscillations of the position of the central updraft at a low frequency were observed. This frequency depended weakly on the Rayleigh number, while the oscillations amplitude was increasing with the increase in Ra. This low-frequency oscillation effect is not evident in two-dimensional modeling and arises from the interaction of adjacent three-dimensional convective cells entering the central updraft. The authors propose an asymptotic estimate for the period of these fluctuations. It is shown that the fluctuations do not disappear with increasing Rayleigh number, suggesting their influence on heat and mass transfer in natural flows.</p>