Theoretical threshold value for the differential rotation amplitude, above which t_w<t_cont throughout the region below the hydrogen-burning shell, as a function of log  g. The curve obtained with the default set of plume parameters, ν_p = ω_c and b = b_R95 (see Sect. 3.2), and a smooth rotation profile is represented by the blue thick solid line. The effect of variations in ν_p and b is also displayed, as well as the results obtained with a sharp rotation profile. For each curve, the value of only one single parameter is changed while keeping the others at their default values. The vertical red dashed line symbolizes the transition between the subgiant phase to the beginning of the RGB.