  At sufficiently high concentrations, motile bacteria suspended in fluids
exhibit a range of ordered and disordered collective motions. Here we explore
the combined effects of confinement, periodicity and curvature induced by the
active motion of E. coli bacteria in a thin spherical shell (cortex) of an
oil-water-oil (O/B/O) double emulsion drop. Confocal microscopy of the
bacterial flow fields shows that at high density and activity, they exhibit
azimuthal zonal flows which oscillate between counterclockwise and clockwise
circulating states. We characterize these oscillatory patterns via their
Fourier spectra and the distributions of their circulation persistence times.
To explain our observations, we used numerical simulations of active particles
and characterize the two-dimensional phase space of bacterial packing fraction
and activity associated with persistent collective motions. All together, our
study shows how geometric effects lead to new types of collective dynamics.
