Influence of the upper mixed layer depth on Langmuir turbulence characteristics

The upper mixed layer depth ( h ) has a significant seasonal variation in the real ocean and the low-order statistics of Langmuir turbulence are dramatically influenced by the upper mixed layer depth. To explore the influence of the upper mixed layer depth on Langmuir turbulence under the condition of the wind and wave equilibrium, the changes of Langmuir turbulence characteristics with the idealized variation of the upper mixed layer depth from very shallow ( h =5 m) to deep enough ( h =40 m) are studied using a non-hydrostatic large eddy simulation model. The simulation results show that there is a direct entrainment depth induced by Langmuir turbulence ( h LT ) within the thermocline. The normalized depth-averaged vertical velocity variance is smaller and larger than the downwind velocity variance for the ratio of the upper mixed layer to a direct entrainment depth induced by Langmuir turbulence h/h LT <1 and h/h LT >1, respectively, indicating that turbulence characteristics have the essential change (i.e., depth-averaged vertical velocity variance (DAVV)DADV for Langmuir turbulence) between h/h LT <1 and h/h LT >1. The rate of change of the normalized depth-averaged low-order statistics for h/h LT <1 is much larger than that for h/h LT >1. The reason is that the downward pressure perturbation induced by Langmuir cells is strongly inhibited by the upward reactive force of the strong stratified thermocline for h/h LT <1 and the effect of upward reactive force on the downward pressure perturbation becomes weak for h/h LT >1. Hence, the upper mixed layer depth has significant influences on Langmuir turbulence characteristics.