Axisymmetric instability in the viscous regime when the quadrupolar field given by Eq. (23) and illustrated in Fig. 1, is initially imposed. First two panels: snapshots of the latitudinal component of the velocity field U_θ taken during the developing of the instability, at t = 14.1τ_Ap when the contraction does not act on the field lines (first panel) and at t = 127.4τ_Ap when it does (second panel). In these cuts are also represented the contours of the latitudinal shear ∂lnΩ/∂θ in black, with dashed lines corresponding to a negative shear and full lines to a positive one. In the second panel, three control points (in light blue, blue and black) are chosen at the location where the instability grows. Third panel: temporal evolution of the square of the latitudinal component of the velocity field at these selected points (with the same colour code), as a function of the Alfvén poloidal time. In each subplot, a coloured straight line is also presented as the result of a linear regression used to deduce a growth rate associated with the instability. The parameters are those of the runs Q4 and Q10 of Table 1 namely, Re_c = 5 (first panel) and Re_c = 1 (two last panels), with E = 10⁻⁴, P_r(N₀/Ω₀)² = 10⁴, P_m = 10², and L_u = 10⁴.