The effect of ocean downwind swells on the harvesting of offshore wind energy is studied using large-eddy simulation of fully developed wind turbine array boundary layers, which is dynamically coupled with high-order spectral simulation of sea-surface wave field with and without the presence of a downwind swell. For the two moderate wind speeds of 7 m/s and 10 m/s considered in this study, the swell is found to induce a temporal oscillation in the extracted wind power at the swell frequency, with a magnitude of 6.7% and 4.0% of the mean wind power output, respectively. Furthermore, the averaged wind power extraction is found to be increased by as much as 18.8% and 13.6%, respectively. Statistical analysis of the wind field indicates that the wind speed in the lower portion of the boundary layer oscillates periodically with fast wind above the swell trough and slow wind above the swell crest, resulting in the observed wind power oscillation. The wind above the swell accelerates due to the strong wave forcing, causes a net upward flux of kinetic energy into the wind turbine layer, and thus acts to increase the extracted wind power of the turbines. For a high wind speed of 17 m/s, the wave-induced motion becomes relatively weak and the swell effect on the wind turbine performance diminishes. (C) 2014 Elsevier Ltd. All rights reserved.