  The poleward migration of the active regions' magnetic flux on the solar
surface plays an important role in the development of the large-scale field
development, especially the polar field reversal, which is a key process in the
Babcock-Leighton-type solar dynamos. The poleward flux transport is nonuniform,
centered around poleward surges as suggested by previous observations. The
strong, long-lasting surges are related to activity complexes, and often result
in violent polar field reversal. However, the nonuniformity of poleward flux
transport has not been evaluated quantitatively. We propose a statistical
method to analyze the poleward flux transport during solar cycles 21-24 by
considering the frequency distributions of the magnetic field at latitudes of
poleward surges occurrence during solar cycles. The nonuniformity is quantified
as the kurtosis statistics representing the tailedness of the distributions. We
test the method on results of surface flux transport simulations, and apply to
WSO, NSO, MWO, and HMI data. We confirm that the poleward surges are of
significance during solar cycles 21-24 in general. The kurtosis within a solar
cycle is affected by different latitudes of the magnetic field and different
data sources. The southern hemisphere of cycle 24 exhibits the largest
kurtosis, agreeing the super surge concept from previous work. The significant
nonuniformity of poleward flux transport originates from the nonrandomness of
active regions, which favors the activity complexes origin of poleward surges.
