  Recent analysis revealed that supergranules (convection cells seen at the
Sun's surface) are advected by the zonal flows at depths equal to the widths of
the cells themselves. Here we probe the structure of the meridional circulation
by cross-correlating maps of the Doppler velocity signal using a series of
successively longer time lags between maps. We find that the poleward
meridional flow decreases in amplitude with time lag and reverses direction to
become an equatorward return flow at time lags > 24 hours. These
cross-correlation results are dominated by larger and deeper cells at longer
time lags. (The smaller cells have shorter lifetimes and do not contribute to
the correlated signal at longer time lags.) We determine the characteristic
cell size associated with each time lag by comparing the equatorial zonal flows
measured at different time lags with the zonal flows associated with different
cell sizes from a Fourier analysis. This association gives a characteristic
cell size of ~50 Mm at a 24 hour time lag. This indicates that the poleward
meridional flow returns equatorward at depths > 50 Mm -- just below the base of
the surface shear layer. A substantial and highly significant equatorward flow
(4.6 +/- 0.4 m/s) is found at a time lag of 28 hours corresponding to a depth
of ~70 Mm. This represents one of the first positive detections of the Sun's
meridional return flow and illustrates the power of using supergranules to
probe the Sun's internal dynamics.
