A plume-based high-order closure scheme for upper ocean turbulence*

Turbulent mixing in the ocean surface boundary layer (OSBL) plays a key role in the evolution of the Earth system on various scales. The current state-of-the-art mixing schemes show systematic biases in representing this mixing in general circulation models (GCMs). In this study, we present a new physically motivated mixing scheme that effectively captures the OSBL turbulent mixing produced by a wide range of realistic oceanic forcing, including surface waves, wind, and buoyancy. This new scheme is a hybrid mass-flux and high-order closure that maintains full energetic constraints by evolving a subset of second- and third-order turbulent moments utilizing a plume-based closure assumption. It inherently captures the non-diffusive mixing produced by both buoyancy-driven convective and wave-driven Langmuir turbulence. We have evaluated the scheme against a suite of large eddy simulations and other existing schemes and find that the new scheme compares well to the large eddy simulation results, has little sensitivity to vertical resolution and time step, making it feasible for GCMs. We extended the scheme to capture even more complex realistic oceanic forcing, such as time-varying diurnal forcing, sea ice melt effects, and testing in global simulation by optimizing the numerical implementation of the scheme. Finally, the new scheme is also well suited for implementation on GPU systems, allowing ocean models to be more amenable to emerging high-performance computing architectures.