Ocean surface gravity waves, especially the resulting Langmuir turbulence, affect the ocean surface vertical mixing, yet are missing in most of the global climate models (GCM). The lack of explicit representation of such effects in GCMs may contribute to persistent biases in the simulated ocean mixed layer depth (MLD), air-sea fluxes and temperature distribution and tracer concentrations in the upper ocean.
To assess the effects of Langmuir turbulence on the simulation of global climate, parameterizations are developed based on large eddy simulations (LES) of the ocean surface boundary layer (OSBL), which are able to simulate some of the key features of Langmuir turbulence. In particular, two effects of Langmuir turbulence are distinguished: enhanced vertical mixing within the OSBL, and enhanced entrainment at the base of the OSBL. The former is parameterized by a wave-related enhancement factor on the turbulent velocity scale in the K-Profile Parameterization (KPP) based on the scaling law of the root-mean-square vertical velocity from previous studies. The latter is parameterized by a wave-related unresolved shear term in KPP based on the scaling law of the entrainment buoyancy flux from a new set of LES experiments. The latter approach is supported by further analyses of the structure and anisotropy of Langmuir turbulence, which show no significant impact of Langmuir turbulence on the predominant processes that drive entrainment at the base of the OSBL except the extra energy through enhanced downward TKE transport.
The modified KPP is then implemented and tested in the NCAR earth system model, CESM. A state-of-the-art ocean surface wave model, WAVEWATCH III, is incorporated into CESM to provide the necessary wave information. It is found that accounting for both effects of Langmuir turbulence in CESM significantly reduces the shallow MLD biases in the Southern Ocean, and improves the simulated intermediate water ventilation and ocean subsurface temperature.
To avoid large computational and code development expenses of coupling a prognostic wave model with a climate model, two statistical approaches to parameterize the effects of Langmuir turbulence are explored and tested in CESM. Both appear to reproduce the effects of Langmuir turbulence as estimated using WAVEWATCH III with significantly less computational cost.
"Langmuir Turbulence and Its Effects on Global Climate"
Earth, Environmental and Planetary Sciences Theses and Dissertations.
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