  The evolution of a large-scale azimuthal velocity field in a rotating
cylindrical layer of fluid with meridional convective circulation was studied
experimentally. Two cases were considered: direct circulation provided by a rim
heater at the periphery and indirect circulation provided by a central heater.
The detailed 3D structure of the mean large-scale velocity field is
reconstructed using the PIV technique. Due to the action of the Coriolis force
the meridional flow results in differential rotation. Differential rotation is
characterized by the mean values of radial and vertical gradients of azimuthal
velocity. Strong negative mean radial gradient which grows with the Grasshoff
number is provided by direct circulation. In the case of indirect circulation a
pronounced negative gradient arises at moderate Grashoff number. The behavior
of the mean vertical gradient is quite different: a positive vertical gradient
grows logarithmically with Grasshoff number under direct circulation, whereas a
weak negative gradient characterizes the indirect circulation. This difference
follows from the structure of the flow -- the direct circulation provides a
large cyclonic area localized above the anticyclonic flow, while indirect
circulation leads to a strong separation of these two areas in the radial
direction (the central part is occupied by the cyclonic flow and the periphery
by the anticyclonic flow). Meridional circulation leads to substantial
variation of the integral angular momentum. Direct circulation results in the
growth of the integral angular momentum and indirect circulation causes it to
decrease. At the same heating power, the increase of angular momentum at direct
circulation is much stronger than its decrease at indirect circulation.
