Long-term variations in the mean meridional motion of the sunspot groups

We find that the mean meridional motion of the spot groups varies considerably on a time scale of about 5-20 years. The maximum amplitude of the variation is about 10-15 m/s in both the Sun's northern and the southern hemispheres. Variation in the mean motion is considerably different during different solar cycles. At the maximum epoch (year 2000) of the current cycle~23, the mean motion is relatively strong in the past 100 years and northbound in both the northern and the southern hemispheres. The power spectral analyses suggest the existence of approximate 3.2- and 4.3-year periodicities in the mean motion of the spot groups in the southern hemisphere, whereas a 13-16 year periodicity is found to exist in the mean motion of the northern hemisphere. There is strong evidence for a latitude-time dependency in the periodicities of the mean motion. The north-south difference in the mean motion also varies by about 10 m/s. During the recent cycles, the north-south difference is negligibly small. Approximate 12- and 22-year periodicities are found to exist in the north-south difference.