  Stellar variability due to magnetic activity and flows at different spatial
scales strongly impacts radial velocities. This variability is seen as
oscillations, granulation, supergranulation, and meridional flows. The effect
of this latter process is poorly known but could affect exoplanet
detectability. We aim to quantify its amplitude when integrated over the disc
and its temporal variability, first for the Sun, seen with different
inclinations, and then for other solar-type stars. We used long time series of
solar latitudinal meridional circulation to reconstruct its integrated
contribution. We then used scaling laws from HD simulations relating the
amplitude of the meridional flow variability with stellar mass and rotation
rate to estimate the typical amplitude expected for other solar-type stars. We
find typical rms of the order of 0.5-0.7 m/s (edge-on) and 1.2-1.7 m/s
(pole-on) for the Sun, with a minimal jitter for an inclination of 45-55 deg.
This is significant compared to other stellar activity contributions and is
much larger than the radial-velocity signal of the Earth. The variability is
strongly related to the activity cycle. Extension to other solar-type stars
shows that the variability due to meridional flows is dominated by the
amplitude of the cycle of those stars. The meridional flow contribution
sometimes represents a high fraction of the convective blueshift inhibition
signal, especially for quiet, low-mass stars. Our study shows that these
meridional flows could be critical for exoplanet detection. Low inclinations
are more impacted than edge-on configurations, but these latter still exhibit
significant variability. Meridional flows also degrade the correlation between
radial velocities due to convective blueshift inhibition and chromospheric
activity indicators. This will make the correction from this signal challenging
for stars with no multi-cellular patterns.
