  The sunspots and other solar activity indicators tend to cluster on the
surface of the Sun.These clusters very often occur at certain longitudes that
persist in time.It is of general interest to find new and simple ways to
characterize the observed distributions of different indicators and their
behaviour in time. In the present work we use Greenwich sunspot data to
evaluate statistical but not totally coherent stability of sunspot distribution
along latitudes as well as longitudes. The aim was to obtain information on the
longitudinal distribution of the underlying spot-generating mechanism rather
than on the distribution and migration of sunspots or sunspot groups on the
solar surface. Therefore only sunspot groups were included in the analysis, and
only the time of their first appearance was used. We use simple nonparametric
approach to reveal sunspot migration patterns and their persistency. Our
analysis shows that regions where spots are generated tend to rotate
differentially as the spots and spot groups themselves do. The activity areas,
however, tend to break down relatively fast, during 7-15 solar rotations. This
study provides a challenge for solar dynamo models, as our results are
consistent with the presence of a non-axisymmetric spot-generating mechanism
experiencing differential rotation (known as phase mixing in dynamo theory).
The new nonparametric method introduced here, completely independent of the
choice of the longitudinal distribution of sunspots, was found to be a very
powerful tool for spatio-temporal analysis of surface features.
