Langmuir turbulence (LT) plays an important role in enhancing vertical mixing in the ocean surface boundary layer (OSBL). Such enhanced mixing is strongly affected by the diurnally varying heat flux, especially in the early morning when there is a transition from cooling to heating. In this period, turbulence is weakened, yet the surface heat flux is changing rapidly, such that the deviation of transient turbulence from its equilibrium state is large. This may lead to biases in the parametrization of turbulent mixing due to LT in large-scale ocean circulation models, in which an equilibrium of the turbulence state with the surface forcing is often assumed. In this study, we investigate the transient response of LT to an abrupt onset of surface heating using idealized large eddy simulations, and compare it with the transient response of wind-driven shear turbulence (ST). Near the surface, the destabilizing Stokes shear force competes with the stabilizing surface heating, resulting in a gradual decay of the turbulence intensity, in contrast to ST whose intensity decreases rapidly at first and then partially recovers due to the formation of a stronger jet in the surface warm layer. Below the surface, the decay of coherent downwelling plumes of LT occurs faster than shear turbulence, resulting in a quicker response of LT than ST at depth. The vertical velocity variance of LT at depth decays at a rate initially following $t^{-1}$ and later transitioning to $t^{-2}$. We also examine the impact of details of the Stokes forcing on the transient response of LT. These results may help improve vertical mixing parametrizations in the OSBL.

Close

PrevNext

View figure in article

• Oceanography
• Turbulent boundary layers
• Turbulent mixing
• Wave-turbulence interactions