  The dynamical origins of the Sun's tachocline and near-surface shear layer
(NSSL) are still not well understood. We have attempted to self-consistently
reproduce a NSSL in numerical simulations of a solar-like convection zone by
increasing the density contrast across rotating, 3D spherical shells. We
explore the hypothesis that high density contrast leads to near-surface shear
by creating a rotationally unconstrained layer of fast flows near the outer
surface. Although our high-contrast models do have near-surface shear, it is
confined primarily to low latitudes (between $\pm15^\circ$). Two distinct types
of flow structures maintain the shear dynamically: rotationally
$\textit{constrained}$ Busse columns aligned with the rotation axis and fast,
rotationally $\textit{unconstrained}$ downflow plumes that deplete angular
momentum from the outer fluid layers. The plumes form at all latitudes, and in
fact are more efficient at transporting angular momentum inward at high
latitudes. The presence of Busse columns at low latitudes thus appears
essential to creating near-surface shear in our models. We conclude that a
solar-like NSSL is unobtainable from a rotationally unconstrained outer fluid
layer alone. In numerical models, the shear is eliminated through the advection
of angular momentum by the meridional circulation. Therefore, a detailed
understanding how the solar meridional circulation is dynamically achieved will
be necessary to elucidate the origin of the Sun's NSSL.
