Speaker

Brandon Reichl, NOAA Geophysical Fluid Dynamics Laboratory

Improved representation of upper ocean vertical mixing in equatorial regions in ocean climate models

Abstract

Deficiencies in upper ocean vertical mixing parameterizations contribute to equatorial 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 degree nominal ocean horizontal grid spacing OM4 (uncoupled ocean) configuration of the MOM6 ocean model, with various modifications that target improvements to biases in the original model. We identify a variety of enhancements to the existing mixing schemes that are evaluated using observational constraints from TAO moorings and Argo floats. In particular, we find that we can improve the diurnal variability of mixing in OM4 via modifications to its mixing scheme, and that we 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 and its diurnal deep-cycle variability, leading to more realistic time-mean tropical thermocline structure, mixed layer depths, SSTs, and a better Pacific Equatorial Undercurrent.