  We have carried out inversions of travel times as measured by Gizon et al.
(2020) to infer the internal profile of the solar meridional circulation (MC).
A linear inverse problem has been solved by the regularized least-squares
method with a constraint that the angular momentum (AM) transport by MC should
be equatorward (HK21-type constraint). Our motivation for using this constraint
is based on the result by Hotta and Kusano (2021) where the solar equator-fast
rotation was reproduced successfully without any manipulation. The inversion
result indicates that the MC profile is a double-cell structure if the
so-called HK21 regime, in which AM transported by MC sustains the equator-fast
rotation, correctly describes the physics inside the solar convective zone. The
sum of the squared residuals computed with the inferred double-cell MC profile
is comparable to that computed with the single-cell MC profile obtained when we
exclude the HK21-type constraint, showing that both profiles can explain the
data more or less at the same level. However, we also find that adding the
HK21-type constraint degrades the resolution of the averaging kernels. Although
it is difficult for us to determine the large-scale morphology of the solar MC
at the moment, our attempt highlights the relevance of investigating the solar
MC profile from both theoretical and observational perspectives.
