  In this paper, we explore optimal disturbances of blockings in the equivalent
barotropic atmosphere using the conditional nonlinear optimal perturbation
(CNOP) approach. Considering the initial blocking amplitude, the optimal
disturbance exhibits a solitary wave-like pattern. As the size increases
incrementally, the spatial pattern becomes more concentrated, and the nonlinear
evolution becomes more pronounced. During the evolution, it only focuses on
gradually intensifying the blocking amplitude without any other influence.
Additionally, based on the medium-range experiments, the time-delay optimal
disturbance appears to lead to larger errors, making it more challenging to
predict. Considering the preexisting synoptic-scale eddies, the optimal
disturbance displays a sharply concentrated pattern, even more concentrated by
increasing the size. However, it is worth noting that the nonlinear evolution
undergoes significant changes, compared to disturbances of the initial blocking
amplitude. Meanwhile, we find that the optimal disturbance not only strongly
impacts the amplitude of blockings but also their shape, making eddy straining
and wave breaking more chaotic and predominant, further influencing the
development of weather extremes. This suggests that blockings are more
sensitive to perturbations of preexisting synoptic-scale eddies than initial
blocking amplitudes. Furthermore, the perturbations of the synoptic-scale
eddies are more likely to lead to the development of weather extremes, making
them less predictable. In medium-range experiments, it is also found that
time-delay disturbances result in larger errors, particularly during the decay
period. Finally, we discuss how the variations of westerly wind influence
optimal disturbances in spatial patterns and nonlinear evolution as well as
their relation to predictability.
