This paper deals with the numerical analysis of transient thermosolutal natural convection in a cavity containing a binary mixture. The configuration under study refers to the experimental situation where the composition gradient inducing the solutal buoyancy force is created by melting of pure ice in a salty solution. The time evolution of the flow structure and the heat transfer characteristics is studied for opposing body forces over a range of Rayleigh numbers. The numerical results allow to provide a better insight in the mechanisms that drive the formation of the flow structure by thermohaline convection.