  The empirical finding with data from the Solar Dynamics Observatory (SDO) of
positive (negative) horizontal Reynolds stress at the northern (southern)
hemisphere for solar giant cells (Hathaway et al. 2013) is discussed for its
consequences for the theory of the solar/stellar differential rotation. Solving
the nonlinear Reynolds equation for the angular velocity under neglect of the
meridional circulation we show that the horizontal Reynolds stress of the
northern hemisphere is always negative at the surface but it is positive in the
bulk of the solar convection zone by the action of the Lambda effect. The
Lambda effect, which describes the angular momentum transport of rigidly
rotating anisotropic turbulence and which avoids a rigid-body rotation of the
convection zones, is in horizontal direction of cubic power in the rotation
rate $\Omega$ and it is always equatorwards directed. Theories without Lambda
effect which may also provide the observed solar rotation law only by the
action of a meridional circulation lead to a horizontal Reynolds stress with
the opposite sign as observed.
