Vertical velocity asymmetries in high resolution simulations of the surface Ekman layer

Most formulations that have been suggested for the nonlinear Ekman layer predict a growing asymmetry between up- and downwelling regions as the Rossby number (Ro) approaches unity. Here, a similar asymmetry is considered using Large Eddy simulations of wind-driven flow over a deep mixed layer. A sinusoidal wind stress is applied at the surface of a 10 km wide periodic domain. Resolution is uniform, with dx=dz=5 m and solutions show the asymmetry to be much stronger than predicted by any of the previous formulations. Moreover, it is characterized by an Ro that takes into account both the underlying currents and the Ekman flow itself. As this Rossby number approaches unity, upwelling becomes increasingly diffuse/weak and downwelling becomes increasingly narrow/strong (with maximum downwelling velocities growing exponentially in Ro). A spin-up from rest identifies two regimes, one characterized by an emmission of waves and another by the appearance of a focussed downwelling turbulent jet. A mechanistic interpretation is discussed. Finally, we note that while 10 km scale variance in the wind field is typically weak, sea ice and ocean current effects can introduce significant variance to the stress at these scales. Possible implications for vertical mixing and transport are discussed.