  We formulate the quasi-Lagrangian fluid transport dynamics of mass density
$\rho$ and the projection $q=\bom\cdot\nabla\rho$ of the vorticity $\bom$ onto
the density gradient, as determined by the 3D compressible Navier-Stokes
equations for an ideal gas, although the results apply for an arbitrary
equation of state. It turns out that the quasi-Lagrangian transport of $q$
cannot cross a level set of $\rho$. That is, in this formulation, level sets of
$\rho$ (isopychnals) are impermeable to the transport of the projection $q$.
