Long-term variations in meridional flows

Abstract

Using sunspot group data from Greenwich and from the Solar Optical Observation Network during 1879–2008, we find that the mean meridional motion of the observed spot groups varies considerably on a time scale of about 10–20 years. During the current cycle 23, which is anomalous in the sense that the cycle pair 22 and 23 violated the Gnevyshev and Ohl rule, the mean meridional speed of the spot groups is substantially higher than it was during the last 10–11 cycles. We also find a roughly 18-year periodicity in the mean meridional motion of spot groups in the northern hemisphere, and a roughly 25-year periodicity in the north–south asymmetry of the mean meridional motion.