  Solar torsional oscillations are migrating bands of slower- and
faster-than-average rotation, which are strongly related to the Sun's magnetic
cycle. We perform a long-term study (16 yr) of hemispherical asymmetry in solar
torsional oscillation velocity using helioseismic data for the first time. We
study the north-south asymmetry in the velocity using the zonal flow velocities
obtained by ring diagram analysis of the Global Oscillation Network Group
(GONG) Doppler images. We find significant hemispherical asymmetry in the
torsional oscillation velocity and explore its variation with respect to depth,
time, and latitude. We also calculate the hemispherical asymmetry in the
surface velocity measurements from the Mount Wilson Observatory and the zonal
flow velocities obtained from the Helioseismic and Magnetic Imager ring diagram
pipeline. These asymmetries are found to be consistent with the asymmetry
obtained from GONG observations. We show that the asymmetry in near-surface
torsional oscillation velocity is correlated with the asymmetry in magnetic
flux and sunspot number at the solar surface, with the velocity asymmetry
preceding the flux and sunspot number asymmetries. We speculate that the
asymmetry in torsional oscillation velocity may help in predicting the
hemispherical asymmetry in sunspot cycles.
