  The south-north travel-time differences are measured by applying
time-distance helioseismology to the MDI and HMI medium-degree Dopplergrams
covering May 1996-April 2017. Our data analysis corrects for several sources of
systematic effects: P-angle error, surface magnetic field effects, and
center-to-limb variations. An interpretation of the travel-time measurements is
obtained using a forward-modeling approach in the ray approximation. The
travel-time differences are similar in the southern hemisphere for cycles 23
and 24. However, they differ in the northern hemisphere between cycles 23 and
24. Except for cycle 24's northern hemisphere, the measurements favor a
single-cell meridional circulation model where the poleward flows persist down
to $\sim$0.8 $R_\odot$, accompanied by local inflows toward the activity belts
in the near-surface layers. Cycle 24's northern hemisphere is anomalous:
travel-time differences are significantly smaller when travel distances are
greater than 20$^\circ$. This asymmetry between northern and southern
hemispheres during cycle 24 was not present in previous measurements (e.g.,
Rajaguru & Antia 2015), which assumed a different P-angle error correction
where south-north travel-time differences are shifted to zero at the equator
for all travel distances. In our measurements, the travel-time differences at
the equator are zero for travel distances less than $\sim$30$^\circ$, but they
do not vanish for larger travel distances. This equatorial offset for large
travel distances need not be interpreted as a deep cross-equator flow; it could
be due to the presence of asymmetrical local flows at the surface near the end
points of the acoustic ray paths.
