  Studying the onset of the deviation scale proposed by C. M. Mabena [C. M.
Mabena et al., Phys. Rev. A 99, 013828 (2019)] may facilitate us to make better
decisions on when we can employ the phase perturbation approximation to
simulate the effect of atmospheric turbulence on the orbital angular momentum
(OAM) carrying beam, which offers an valuable guideline to the optical
propagation procedures under specific scintillation conditions. Thanks to the
multiple phase screen (MPS) approach, we explain the physical meaning of the
deviation scale in this paper, it is demonstrated that when the turbulent
channel at weak scintillation scale, slight intensity modulation can cause the
vortex split into multiple individual vortices as well as generate the
vortex-antivortex pairs, after a spatial accumulation over a distance, the
deviation starts to emerge when the vortex splitting ratio passes this
threshold, which means that the occurrence of such circumstance happens to be
unpredictable by the scintillation index and needs to be measured with the help
of the vortex splitting ratio.
