Effects of stratification on an ocean surface Ekman layer

Large-eddy simulations are used to investigate the effects of stratification on structural and turbulent dynamics of an upper-ocean Ekman layer that is driven by a constant wind stress (friction velocity $u^*$) at low latitude with Coriolis parameter $f$. The surface layer evolves in the presence of interior stratification whose buoyancy frequency varies among cases, taking three values: $N/f = 19, 60$ and $192$. At quasi-steady state, a stratified turbulent Ekman layer forms with a surface current veering to the right of the wind direction. The thickness of the Ekman layer decreases with increasing $N$ and is found to scale with $u^*$, $f$, and $N$, similar to the neutral atmospheric boundary layer of Zilitinkevich \& Esau (2002) that is capped by a stratified layer with buoyancy frequency, $N$. As $N$ increases, the speed of the Ekman current increases but the Ekman transport is invariant. The surface veering angle also increases with larger $N$. The shear rate and buoyancy frequency are elevated at the base of the Ekman layer. The peak of down-wind Reynolds stress occurs near the surface and scales with $u^{*2}$ in all cases while the peak of cross-wind Reynolds stress occurs in the middle of the Ekman layer and decreases with increasing $N$.