Langmuir Turbulence in the HYCOM Ocean Model
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- Geophysical Institute 
The impact of parameterising the turbulent mixing induced by surface wave-driven Langmuir turbulence is investigated for a basin-scale configuration of the hybrid-coordinate ocean circulation model HYCOM. Two-year, non-data assimilative model simulations are performed within the North Atlantic model domain of the operational ocean forecasting system TOPAZ4, with surface wave parameters acquired from a hindcast produced using the WaveWatch III spectral wave model. The model runs consist of one control simulation in which explicit surface wave effects are neglected, and four additional simulations, each of which implements a different modification of the K profile parameterisation (KPP) upper-ocean mixing scheme to account for the Langmuir turbulence effects. The model response to the mixing scheme modifications is analysed in terms of the mixed layer depth (MLD), the sea surface temperature (SST) and vertical temperature profiles. The largest improvements in model performance attributed to the inclusion of the Langmuir turbulence parameterisations are observed in the summer season, when the standard model configuration is shown to underestimate the mixing in the upper ocean boundary layer (OBL). In the winter, the introduction of the parameterisations tends to create exaggerated levels of near-surface mixing, leading to increased errors and biases in the model temperature fields when compared to observational datasets. It is concluded that the present set of Langmuir turbulence parameterisations implemented in the KPP code of HYCOM is inadequate for the purpose of improving the operational forecasting skill of HYCOM in a year-round, realistic North Atlantic setting—continued development and testing of alternative parameterisations is, therefore, required. It is proposed that developers seeking improved parameterisations of the process focus on validating dimensionless scaling laws of surface wave-forced boundary layers, incorporating parameters to account for varying water mass stratification, and developing methods to allow for stabilising effects in conditions of opposing surface waves and currents.