Solar-Cycle Variation of Subsurface Meridional Flow Derived with Ring-Diagram Analysis

We study the solar-cycle variation of the meridional flow in the near-surface layers of the solar convection zone from the surface to a depth of 16 Mm. We have analyzed Global Oscillation Network Group (GONG) Dopplergrams with a ring-diagram analysis covering about 13 years (July 2001 -aEuro parts per thousand October 2014), from the maximum of Cycle 23 through the rising phase of Cycle 24, and Helioseismic and Magnetic Imager (HMI) Dopplergrams covering more than four years (May 2010 -aEuro parts per thousand January 2015). GONG and HMI lead to similar meridional flows during common epochs and latitudes. The meridional flow averaged over a Carrington rotation is poleward up to about in both hemispheres at all depths after correcting for systematic effects. The flow amplitude peaks at about latitude with an amplitude of about 16 to depending on depth. The meridional flow varies with the solar cycle; the flow amplitudes are larger during cycle minimum than during maximum at low- and mid-latitudes. The flows are mainly faster or more-poleward-than-average on the equatorward side of the mean latitude of activity and slower or less-poleward-than-average on its poleward side. The residual meridional flow converges near the mean latitude of activity. A comparison with the corresponding zonal flow derived from GONG and HMI data shows that the bands of more-poleward-than-average meridional flow coincide with the bands of faster-than-average zonal flow and that the bands of less-poleward-than-average meridional flow coincide with the bands of slower-than-average zonal flow. This implies that the residual flows are cyclonic. The bands of fast meridional flow appear at mid-latitudes about three years before magnetic activity of Cycle 24 is present in synoptic maps.