On the Periodicities in Solar Rotation and Activity Determined from Sunspot-Group Data and Possible Connection with Rossby-Like Waves

nalyses of solar rotation and activity, and their variations, can improve the understanding of the formation and interaction of the solar-activity bands and therefore give important observational constraints on the modelling of the solar dynamo and improve the forecast capability in solar activity and space weather. Sunspot-position data obtained from the Greenwich and Debrecen Photoheliographic data for the years 1874 to 2016 were used to calculate the rotational and meridional velocities of the solar plasma. The velocities were calculated from daily shifts of sunspot groups. By analysing the rotation-rate residuals, covariance of meridional velocities and rotation-rate residuals, as well as daily total sunspot number, taken from WDC-SILSO, for periodicity in the range of periods larger than two days, significant periods of their changes were found. It was demonstrated that the frequencies of the quasi-annual variations of solar activity, with periods from 230 to 400 days, match those of classical Rossby modes with m≥17 . There is no match between the frequencies of Rossby modes and the peaks in the rotation-rate residual and horizontal component of the Reynolds-stress-tensor periodograms. These variations are most probably connected with the existence of active longitudes. It was demonstrated that the algorithms for detecting periodic signals should be used with caution when analysing complex periodic data.