Numerical simulations of an asymptotically reduced version of the Craik–Leibovich (CL) equations are described. By filtering surface waves, the CL equations facilitate simulations of surface-wave driven or surface-wave modified phenomena in the upper ocean—most notably, Langmuir circulation (LC)—with time scales long relative to the period of the dominant waves. Although numerical simulations of the fully three-dimensional CL equations are now routine, simulations in spatially extended domains, hundreds of times the characteristic width of individual LC vortices, require immense computing resources. The reduced CL (rCL) equations render numerical simulations in very long domains more feasible by exploiting the strongly anisotropic structure of LC that emerges in the strong CL “vortex force” limit. Here, simulations of the rCL equations are performed to explore the dynamics admitted by the reduced system. Spatially quasi-coherent, three-dimensional flow structures that drift downwind are found to be a prominent feature of the reduced dynamics in the parameter regime explored. The spatial form and temporal variability of these structures are associated with a resonance between pairs of LC vortices with cross-wind (transverse) wavenumbers in a 2:1 ratio. Depending upon the strength of the wind and surface-wave forcing, this spatial resonance leads to either periodic or aperiodic temporal oscillations in the number of LC vortices crossing a fixed downwind station, a process that is accompanied by the formation of Y-shaped features in Lagrangian surface patterns. The stems of these “Y-junctions” are preferentially oriented downwind, as is commonly reported in field observations of LC.
