Let's look at this convection from the side. This image is a certain vertical cross section of the fluid. Despite a temperature gap at both the upper and lower boundaries, the distribution of temperature forms a vertical stripe pattern. In other words, there is very little change in temperature vertically. For better or worse the position of the vortices is changing, so we can see in one instant many different vertical cutoffs of a vortex. Looking at a cutoff that grazes a rim of the vortex, we can see that the vortex is rotating in the reverse direction top and bottom like a cloth being squeezed out.