Using NIW Observations to Assess Mixed Layer Parameterizations: A Case Study in the Tropical Atlantic

Abstract Tropical sea surface temperature (SST) biases can cause atmospheric biases on global scales, hence SST needs to be represented well in climate models. A major source of uncertainties is the representation of turbulent mixing in the oceanic boundary layer, or mixed layer (ML). In the present study we focus on near‐inertial wave (NIW) induced mixing. The performance of two mixing schemes, Turbulent Kinetic Energy and K‐profile parameterization (KPP), is assessed at two sites (11.5°N, 23°W and 15°N, 38°W) in the tropical Atlantic. At 11.5°N, turbulence observations (eddy diffusivities, shear and stratification) are available for comparison. We find that the schemes differ in their representation of NIWs, but both under‐represent the observed enhanced diffusivities below the observed ML. However, we find that the models do mix below the ML at 15°N when a storm passes nearby. The near‐inertial oscillations remain below the ML for the following 10 days. Near‐inertial kinetic energy (NIKE) biases in the models are not directly correlated with the wind speed, the MLD biases, or the stratification at the ML base. Instead, NIKE biases are sensitive to the vertical mixing scheme parameterization. NIKE biases are lowest when the KPP scheme is used. Plain Language Summary The surface temperature of the ocean is highly dependent on the depth of the mixed layer (ML), the uppermost layer in the water column, where density, temperature and salinity are approximately constant. In climate models, the vertical mixing processes cannot be resolved, and instead they are computed with the use of vertical mixing schemes. We assess how well two of such schemes can represent the mixing induced by a specific type of ocean waves, near‐inertial waves (NIWs). We compare recent observations of turbulent mixing induced by NIWs in the tropical Atlantic with numerical simulations that resolve storms. Our results show that the models are able to reproduce the observed NIWs, but underestimate their mixing and amplitude. Our analysis also shows that NIWs are a driver of mixing below the uppermost ocean layer in the models. The strength of the near‐inertial currents is sensitive to the vertical mixing parameterization. Key Points Observations of inertial oscillations are used to evaluate the performance of two vertical mixing schemes in two high‐resolution models Both the K‐profile parameterization and the Turbulent Kinetic Energy closure underestimate the NIW‐induced mixing Near‐inertial kinetic energy biases are sensitive to the vertical mixing parameterization