Improving Equatorial Upper Ocean Vertical Mixing in the NOAA/GFDL OM4 Model.

Deficiencies in upper ocean vertical mixing parameterizations contribute to tropical upper ocean biases in global coupled general circulation models, affecting their simulated ocean heat uptake and ENSO variability. To better understand these deficiencies, we develop a suite of ocean model experiments including both idealized single column models and realistic global simulations. The vertical mixing parameterizations are first evaluated using large eddy simulations as a baseline to assess uncertainties and evaluate their implied turbulent mixing. Global models are then developed following NOAA/GFDL's 0.25° nominal horizontal grid spacing OM4 (uncoupled) configuration of the MOM6 ocean model, with various modifications that target biases in the original model. We test several enhancements to the existing mixing schemes and evaluate them against observational constraints from Tropical Atmosphere Ocean moorings and Argo floats. In particular, we find that we can improve the diurnal variability of mixing in OM4 via modifications to its surface boundary layer mixing scheme, and can improve the net mixing in the upper thermocline by reducing the background vertical viscosity, allowing for more realistic, less diffuse currents. The improved OM4 model better represents the mixing, leading to improved diurnal deep‐cycle variability, a more realistic time‐mean tropical thermocline structure, and a better Pacific Equatorial Undercurrent. Plain Language Summary: Computational models of the oceanic and atmospheric circulation are critical tools for understanding and projecting changes in the Earth's climate. These models have errors that can arise from many sources, including model formulation or the choices in applying the model. One of the more well known sources of error is the representation of turbulent mixing. In this work we consider specially designed small‐scale models that simulate turbulent mixing, and use their results to improve the representation of turbulence and its induced mixing in large‐scale models. In particular, we investigate how the intensity of mixing varies throughout the day, considering the progression from deep mixing during cooler nighttime surface conditions to shallower mixing in the presence of strong solar heating during the day. We find some modifications to the mixing scheme in the ocean climate model that can improve the model solutions when compared to the real ocean. Key Points: Large eddy simulation results are used to evaluate the diurnal cycle of equatorial turbulent mixing in the OM4 ocean modelReducing vertical viscosity in an ocean model increases shear near the Equatorial Undercurrent (EUC) and can result in increased vertical mixingVertical grid spacing of a few meters helps to resolve shear mixing events within and below the EUC in ocean models [ABSTRACT FROM AUTHOR]