Helioseismic measurements of the subsurface meridional flow

We measure the mean frequencies of acoustic (p-mode) waves propagating toward and away from the poles of the Sun from observations made with the Solar Oscillations Investigation-Michelson Doppler Imager on board the Solar and Heliospheric Observatory and the ground-based Global Oscillations Network Group. We demonstrate that there is a significant frequency shift between poleward- and equatorward-traveling waves measured over solar latitudes 20°-60°, which is consistent with the Doppler effect of a poleward meridional flow on the order of 10 m s^-1. From the variation of the frequency shifts of p-modes with degree ℓ between 72 and 882 as a function of the lower turning point depth, we infer the speed of the meridional flow, averaged over these latitudes, over a range in depth extending over the top half of the solar convection zone. We find no evidence for a significant equatorward return flow within this depth range.