This is an interesting idea, one that I have spent a little time pondering. The problem you will face is equilibrium. The surface water falling as it cools would work only if the water below is warmer than the surface water. You would either need to find a way to warm the bottom water column or find a way to cool the upper water column by quite a bit to make it more dense than the layers beneath. This would be easier than heating the upper layer, but at some point the whole mass will reach equilibrium and be the same temperature throughout, shutting down your circulation system.
The black pipe would need to be outside the tank in order to heat up enough to warm the water more than it's surrounding mass of water, alternately what if you painted the bottom outside surface of the IBC black? This might heat up the water directly inside the tank on that edge to cause some circulation.
I don't see a way to achieve circulation with 0 inputs, but I could see this system working with passive inputs like geothermal or solar heat harvesting. But then again I am only an armchair physicist.
I would be interested in twinning this system with a trombe wall or environmental earth battery to provide the passive heating.