  Helioseismic observations have provided valuable datasets with which to
pursue the detailed investigation of solar interior dynamics. Among various
methods to analyse these data, normal-mode coupling has proven to be a powerful
tool, used to study Rossby waves, differential rotation, meridional
circulation, and non-axisymmetric multi-scale subsurface flows. Here, we invert
mode-coupling measurements from Helioseismic Magnetic Imager (HMI) and
Michelson Doppler Imager (MDI) to obtain mass-conserving toroidal convective
flow as a function of depth, spatial wavenumber, and temporal frequency. To
ensure that the estimates of velocity magnitudes are proper, we also evaluate
correlated realization noise, caused by the limited visibility of the Sun. We
benchmark the near-surface inversions against results from Local Correlation
Tracking (LCT). Convective power likely assumes greater latitudinal isotropy
with decrease in spatial scale of the flow. We note an absence of a peak in
toroidal-flow power at supergranular scales, in line with observations that
show that supergranulation is dominantly poloidal in nature.
