  Measurements of the large cellular flows on the Sun were made by local
correlation tracking of supergranules seen in full-disk Doppler images obtained
by the HMI instrument on the NASA SDO satellite. The hourly measurements were
averaged over 34 days to produce daily maps of the latitudinal and longitudinal
velocities. While flows at all latitudes are largely in the form of vortices
with left-handed helicity in the north and right-handed helicity in the south,
there are key distinctions between the low latitude and high latitudes cells.
The low latitude cells have roughly circular shapes, lifetimes of about one
month, rotate nearly rigidly, do not drift in latitude, and do not exhibit any
correlation between longitudinal and latitudinal flow. The high latitude cells
have long extensions that spiral inward toward the poles. They have lifetimes
of several months, rotate differentially with latitude, drift poleward at
speeds approaching 2 m s$^{-1}$, and have a strong correlation between prograde
and equatorward flows. Spherical harmonic spectral analyses confirm that the
flows are dominated by the curl component with RMS velocities of about 12 m
s$^{-1}$ at wavenumber $\ell$ = 10. Fourier transforms in time indicate two
notable components - an $m = \pm\ell$ feature representing the low latitude
component and an $m = \pm1$ feature representing the high latitude component.
The dispersion relation for the low latitude component is well represented by
that derived for Rossby waves. The high latitude component has a constant
temporal frequency for all $\ell$ indicating features advected by differential
rotation at rates representative of the base of the convection zone high
latitudes. The poleward motions of these features further suggest that the high
latitude meridional flow at the base of the convection zone is poleward - not
equatorward.
