  We present results of two simulations of the convection zone, obtained by
solving the full hydrodynamic equations in a section of a spherical shell. The
first simulation has cylindrical rotation contours (parallel to the rotation
axis) and a strong meridional circulation, which traverses the entire depth.
The second simulation has isorotation contours about mid-way between cylinders
and cones, and a weak meridional circulation, concentrated in the uppermost
part of the shell.
  We show that the solar differential rotation is directly related to a
latitudinal entropy gradient, which pervades into the deep layers of the
convection zone. We also offer an explanation of the angular velocity shear
found at low latitudes near the top. A non-zero correlation between radial and
zonal velocity fluctuations produces a significant Reynolds stress in that
region. This constitutes a net transport of angular momentum inwards, which
causes a slight modification of the overall structure of the differential
rotation near the top. In essence, the {\it thermodynamics controls the
dynamics through the Taylor-Proudman momentum balance}. The Reynolds stresses
only become significant in the surface layers, where they generate a weak
meridional circulation and an angular velocity `bump'.
