Since the drag caused by bottom friction reduces the total vorticity in the water column, it also reduces the divergence in the lower layers.
(the drag; caused; by bottom friction) (the drag caused by bottom friction; reduces; the total vorticity in the water column) (it; reduces; the divergence in the lower layers)
the drag the drag caused by bottom friction it
caused reduces reduces
by bottom friction
the total vorticity in the water column
the divergence in the lower layers