ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation B-Citation I-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: by magenta, green, and orange lines, respectively, with solid (dashed) lines indicating results for the northern (southern) hemisphere. The SSN is shown for each hemisphere. 6. Discussion Taken at face value, the results as illustrated by Figures 10 and 11 are unexpected in that the shallow return flow present in the northern hemisphere appears to be inconsistent with most prior determinations of the meridional circulation that employ multiyear observations and attempt to remove the effects of center-to-limb related artifacts. Specifically, most analyses involving modeling time–distance travel-time perturbations show a poleward flow in both hemispheres that persists from the surface down to approximately 0.9 R ⊙ (i.e., about 70 Mm below the surface). This includes the analyses of Zhao et al. ( 2013 ), Jackiewicz et al. ( 2015 ), Rajaguru Antia ( 2015 ), and Chen Zhao ( 2017 ). A notable exception is the recent analysis of Gizon et al. ( 2020 ), which is discussed below. In general, uncertainties in the inference of the flow increase with the depth of those inferences. This makes the discrepancy with other helioseismic analyses at a relatively shallow depth all the more puzzling. If we assume that the results for at least the northern hemisphere are spurious, then the challenge is to understand the cause of what appears to be an uncorrected artifact. Of course, real hemispheric differences in the meridional circulation may very well exist, but it is worth considering the possible nature of pseudoflows, which are themselves asymmetric. The comparison of the time variation of the frequency shifts with the sunspot numbers for both hemispheres (Figure 12 ) indicates that the observed hemispheric differences in the 88 month interval persist regardless of the level of solar activity or its preference for one hemisphere over the other. While this does not rule out the presence of real solar-cycle variations in flows (e.g., Chou Dai 2001 ; Zhao Kosovichev 2004 ; González Hernández et al. 2008 ; Hathaway Rightmire 2010 ) or potential pseudoflow artifacts related to magnetic regions (e.g., Liang Chou 2015 ), there is no direct correlation between hemispheric differences in solar activity and the differences in the frequency shifts. Errors in the control measurements due to uncertainties of the leakage discussed in Appendix B appear to be too small to account for the hemispheric differences observed in Figure 9 . In particular, we note that likely errors in the leakage factor amounting to 20% of the selected value of 0.84 (see Appendix B ) cause uncertainties in the scaled frequency shifts of the deepest modes of around 1 m s −1 . These are smaller than the standard errors of the observations shown in Figure 9 and substantially smaller than the 4 m s −1 discrepancy observed at the highest phase speeds. It is plausible that the results may be compromised by incorrect assumptions regarding the azimuthal invariance of the center-to-limb correction. At