Langmuir monolayers of an amphiphilic azobenzene derivative exhibit easily perturbable mesophases with long-range orientational order that couples strongly to linearly polarized light, resulting in symmetric photoaligned textures. Controlled rotation of the polarization plane induces continuous collective precession of the molecular field, which reveals the shear-thinning nature of the surface rotational viscosity. The dynamics is controlled by a balance between the strength of the optical coupling and viscous dissipation so that an increase in the rotational velocity results in a dampening of the synchronous anharmonic oscillations of the orientational field, as revealed by a space-time analysis of the Brewster angle microscopy images.