Abstract Langmuir circulation (LC) is believed to be one of the leading causes of turbulent mixing in the upper ocean. Large eddy simulation (LES) models that solve the Craik–Leibovich equations are used to study LC in the upper ocean, yielding new insights that could not be obtained from field observations or turbulent closure models alone. The present study expands our previous LES modeling investigations of LC to real ocean conditions with large-scale environmental motion due to strong horizontal density gradient, which is introduced to the LES model through scale separation analysis. The model is applied to field observations in the Gulf of Mexico when a measurement site was impacted by fresh water inflow. Model results suggest that LC can enhance turbulence in the water column and deepen the mixed layer (ML) with or without the large scale motions, being consistent with previous studies. The strong salinity gradient is shown to be able to reduce the mean flow in the ML, align Langmuir cells with the pressure gradient direction and inhibit turbulence in the ocean surface boundary layer.