In other words, the jumpers will stack up in the lower air mass. The safe formula here would be to know for sure that the earlier group was well below opening altitude before the next group leaves the aircraft. That would mean that the jumpers would be spaced 2640 feet apart at opening altitude for each ten second exit delay. This would also be a cross wind relative to the ground. Again, a relative measuring point only.
Strange things happen here in an extreme wind shear model.
I will, of course, start with the strange working back to more everyday conditions. As with many things in physics, starting with an extreme and simple model can make it much easier to conceptualize. The fluid dynamics of these wind-shear models are quite complex in their total picture. A computer animation would reveal much more understandable models along with some accurate numerical data. This would indeed be a fun future project.