Due to their limited resolution, numerical ocean models need to be interpreted as representing filtered or averaged equations. How to interpret models in terms of formally averaged equations, however, is not always clear, particularly in the case of hybrid or generalized vertical coordinate models. We derive the averaged hydrostatic Boussinesq equations in generalized vertical coordinates for an arbitrary thickness weighted-average. We then consider various special cases and discuss the extent to which the averaged equations are consistent with existing model formulations. As previously discussed, the momentum equations in existing depth-coordinate models are best interpreted as representing Eulerian averages (i.e., averages taken at fixed depth), while the tracer equations can be interpreted as either Eulerian or thickness-weighted isopycnal averages. Instead we find that no averaging is fully consistent with existing formulations of the parameterizations in semi-Lagrangian discretizations of generalized vertical coordinate ocean models. Perhaps the most natural interpretation of generalized vertical coordinate models is to assume that the average follows the model’s coordinate surfaces. However, the existing model formulations are generally not consistent with coordinate-following averages, which would require “coordinate-aware” parameterizations that can account for the changing nature of the eddy terms as the coordinate changes. Alternatively, the model variables can be interpreted as representing either Eulerian or (thickness-weighted) isopycnal averages, independent of the model coordinate that is being used for the numerical discretization. Existing parameterizations in generalized vertical coordinate models, however, are usually not fully consistent with either of these interpretations. We discuss what changes are needed to achieve consistency.

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/GFDLâ\euro™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.

A document by Nora Loose. Click on the document to view its contents.

The mechanical interaction between ice floes in the polar sea-ice packs plays an important role in the state and predictibility of the ice cover. Using a Lagrangian-based numerical model we investigate the mechanics of sea ice floe-floe interactions. Our simulations show that elastic and reversible deformation offers significant resistance to compression before ice floes yield with brittle failure. When pressure ridges start to form, compressional strength dramatically decreases, implying thicker sea ice is not necessarily stronger compared to thinner ice. These effects are not accounted for in current sea-ice models that describe ice strength by thickness alone. As our results show, the observed transition in mechanical state during ridging initiation may lead to biases in simulated ridge building rates and sea-ice thickness. We propose a parameterization that describes failure mechanics from fracture toughness and Coulomb sliding, improving the representation of ridge building dynamics in particle-based and continuum sea-ice models.

We examine the role of transient and standing eddies in an idealized model with a Southern Ocean analog and two basins in the Southern hemisphere. We use the new MOM6 code base in an entirely adiabatic configuration, which allows for full equilibrium integration in only 100 years of simulation time. Fine resolution eddy-resolving simulations at 1/16- and 1/8-degree resolutions are performed in cases with and without continental barriers and/or bottom roughness. A time- and zonal-Reynolds averaging approach is used to decompose eddy fluxes into transient eddies and standing meanders. For a flat bottom channel the wind driven overturning circulation is balanced by transient eddies, consistent with previous studies. However, when continental barriers or bottom roughness are present, we find that standing meanders are dominant in the Southern Ocean.