Relationships between eddy representation and vertical structure

The representation of energetic transfers associated with ocean mesoscale eddies is a leading challenge in the development of modern climate models. Here we investigate the relationships between eddy representation and vertical structure. We employ the GFDL-MOM6 in an idealized, one-basin, stacked-shallow water configuration and consider four resolutions of otherwise-identical simulations: 1/4, 1/8, 1/16, and 1/32 degree. We assess the degree of eddy representation by: (1) the ratio of the deformation scale to the model grid spacing; and (2) using linear QG instability analysis to compute the fastest growing wavenumber and comparing it to the model resolution. We then analyze the available potential energy (APE) and kinetic energy (KE) distributions for each simulation. KE is found to broadly increase with increasing resolution. The KE is decomposed into barotropic (BT) and baroclinic (BC) components, which are further split into temporally-defined “eddy” and “mean” parts. The dominant trend in eddy representation vs. vertical structure is an increasing fraction of KE going into the BT mode, particularly the BT-eddy component, as eddy representation increases. We attribute this to the inaccurate representation of BC energy transfers in the low-resolution models which leads to buildup of BC energy and lack of barotropization. The end goal of this work is contributing to the development of scale-aware, energetically-consistent mesoscale eddy parameterizations by constraining the vertical structure of eddy energy.