  Radial transport of particles, elements and fluid driven by internal stresses
in three-dimensional (3D) astrophysical accretion disks is an important
phenomenon, potentially relevant for the outward dust transport in
protoplanetary disks, origin of the refractory particles in comets, isotopic
equilibration in the Earth-Moon system, etc. To gain better insight into these
processes, we explore the dependence of meridional circulation in 3D disks with
shear viscosity on their thermal stratification, and demonstrate strong effect
of the latter on the radial flow. Previous locally isothermal studies have
normally found a pattern of the radial outflow near the midplane, switching to
inflow higher up. Here we show, both analytically and numerically, that a flow,
which is inward at all altitudes, is possible in disks with entropy and
temperature steeply increasing with height. Such thermodynamic conditions may
be typical in the optically thin, viscously heated accretion disks. Disks in
which these conditions do not hold should feature radial outflow near the
midplane, as long as their internal stress is provided by the shear viscosity.
Our results can also be used for designing hydrodynamical disk simulations with
a prescribed pattern of the meridional circulation.
