Time Dependence of Solar Meridional Circulation

A recent re-analysis of the digital archive record of Magnetogram and Dopplergram observations of the synoptic program at the 150-foot solar tower telescope at the Mt. Wilson Observatory (MWO) has shown that the processes known as meridional circulation and torsional oscillations are closely related (Ulrich, Tran, and Boyden 2023). The meridional circulation velocity is along lines of constant heliographic longitudes while the torsional oscillations are changes in the rotation rate. Previously an extension of the meridional circulation analysis of Ulrich (2010) revealed that the polar rotation slowdown periods of the torsional oscillations also coincide with the polar meridional circulation flow being toward the equator instead of the usual toward the pole pattern. We interpret these two polar velocity patterns as indicating that the material flow during the solar minimum periods has an upwelling that pushes matter toward the equator. It is generally believed that the polar region magnetic fields interact with the meridional flow in the following sequence: a) at or before sunspot minimum the meridional flow brings additional magnetic flux of the same polar sign to the pole increasing the polar field strength; b) after solar minimum the new cycle field reverses and opposes the polar field, reversing it. The polar upwelling is not part of this sequence and our observations could indicate the need for modification of the models. This paper provides details about the time dependence of the solar surface Meridional Circulation.