The rotational properties of sunspots during the time interval 1977-1986 (solar cycle 21) are studied; only sunspot groups older than 4 days have been the object of this research. We have looked systematically for any kind of 'anomaly' or fine structure in the differential rotation latitudinal profile and any significant change occurring during the course of the solar cycle. Some latitudinal bands are found where the angular rotation rate, rather than decreases according to its overall tendency, increases or is constant with the latitude. The differential rotation profile is, therefore, finely structured. The whole fine-structure pattern is affected by a slow equatorward shift. It is suggested that these fine structure features are due to the Coriolis forces acting on the meridional motions associated with giant toroidal convective cells. Some of the properties of such cells are inferred. Moreover, while the spot zone shifts equatorward, it is found to expand poleward; this expansion occurs by the 'addition' of new belts of activity on the poleward side of the pre-existing active zone. The active zone is therefore found to consist, at the maximum activity epoch, of three different 'belts' of spot production, each of them being centered around a local maximum-activity latitude; each of these 'centroids' of activity is hypothesized to lie where a couple of meridional streams - associated with giant cells - converge. The activity belts are independent of each other as far as their activation, maximum, and end time, as well as their lifetime and level of activity are concerned. The angular rotation rate is correlated, in each belt, with the local level of activity. © 1990 Kluwer Academic Publishers.
