the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Mar 2025
DINO: A Diabatic Model of Pole-to-Pole Ocean Dynamics to Assess Subgrid Parameterizations across Horizontal Scales
Abstract. Climate models are limited in resolution by computational constraints. The ocean component is currently resolved at spatial scales between approximately 10 to 100 km, which is too coarse to adequately capture the mesoscale. Eddies at these scales play a major role in the global energy cycle, and therefore it is crucial that they are accurately parameterized. In this context, we propose DINO (DIabatic Neverworld Ocean), an ocean-only model configuration of intermediate complexity designed as a test protocol for eddy parameterizations across a range of horizontal scales. It allows for affordable simulations, even at very high resolution, while crucial aspects of the global ocean like the Meridional Overturning Circulation (MOC), Subtropical and Subpolar gyres, or the Antarctic Circumpolar Current (ACC) are maintained. We compare key metrics across eddy-resolving (1/16°), eddy-permitting (1/4°) and eddy parameterizing (1°) simulations to showcase the evaluation of eddy parameterizations in two ways: by testing their impact on the mean state and by directly diagnosing the missing eddy fluxes from coarse-grained high-resolution experiments.
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Status: open (until 21 May 2025)
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RC1: 'Comment on egusphere-2025-1100', Anonymous Referee #1, 25 Apr 2025
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Summary and recommendation
This manuscript proposes a modeling hierarchy, DINO, intended to act as a testbed for eddy parameterizations. The new hierarchy extends NeverWorld2, a previous such testbed, by including both temperature and salinity, an (idealized) nonlinear equation of state, an inter hemispheric overturning circulation, and diabatic processes. All of these either influence or are influence by mesoscale eddies, so DINO would provide a more stringent and comprehensive test of eddy parameterizations. The ocean modeling community is in desperate need of such standardized testbeds and DINO stands to be a very useful contribution. The manuscript is generally well-written and the hierarchy carefully documented, with a few exceptions detailed under my specific comments. The comments primarily ask for clarification, although I do have concerns about the design of the freshwater forcing and the shortness of the period analyzed. I support publication if the authors can address these comments and concerns.
Specific comments
- Two of the design decisions seem unusual or arbitrary. While they are unlikely to impact the ability of DINO to serve as a testbed for parameterizations, they deserve a few lines of additional justification
- The reentrant part of the domain spans 20º. This is significantly wider than than the width of Drake Passage, which is about 8º wide. What is the rationale for choosing this width? To match NeverWorld2?
- Why is a minimum depth 2000 m? Neverworld2 has a minimum depth of 200 m, which is a reasonable (if deep) value for continental shelves. In nature, the upper part of the North Atlantic deep western boundary current (associated with Labrador Sea Water) is found around 1000 m depth (Bower and Hunt 2000) and the interactions of the DWBC with the slope are thought to impact the Gulf Stream (Zhang and Vallis 2007). It would thus seem desirable to have the DWBC flow along the sloping topography rather than against the free-slip wall. However, Figure 5 shows that most of the southward flow of the mid-depth overturning cell is found at densities of 27 kg m–3 or lighter, which figure 6 shows is shallower than 2000 m.
- It should be clarified that the equation of state (equation 6) is not an approximation to the in situ density, but the potential density (apparently referenced to the surface). The in situ density has a pressure dependence that leads to a nearly linear increase in density of about 4.5 kg m–3 per km of depth. This, if the density at the surface is about 1026 kg m–3, the density at 2000 m should be about 1035 kg m–3. The potential density referenced to 2000 m (used in figures 5 and 6) should therefore be in the 30s rather than the 20s. It might be simpler to use potential density referenced to the surface in these figures—the numerical values are unlikely to change much, but they’d be closer to what people would expect for potential density.
- Lines 112–114: Note that AABW and NADW have essentially the same density at the surface, but AABW is denser than NADW at depth due to the thermobaric effect (Nycander et al. 2015). Since DINO’s equation of state supports the thermobaric effect, surface forcing that produces AABW that is denser than NADW at the surface may result in AABW that is excessively dense at depth.
- Lines 189–190: It is not clear how starting from rest ensures conservation or what is being conserved.
- The approach to freshwater forcing does not seem adequate. Salinity resorting is indeed unrealistic, but five years is unlikely to be sufficient to produce a stable climatology of moisture fluxes and four years is not long enough for the circulation to adjust to the change in the boundary conditions. Since the procedure for producing the freshwater forcing is repeated independently for each model resolution, this leads to each resolution being subjected to different freshwater forcing. This is undesirable for a model hierarchy that is supposed to only differ by resolution and subgridscale parameterizations. In lieu of devising a new freshwater forcing scheme (which would require expensive recomputations), it would be more straightforward and clarifying to simply forgo freshwater forcing and analyze the cases with salinity restoring.
- Similarly, four years does not seem sufficient to characterize the mean state of higher resolution models.
- Page 12: The rationale for the approach to separating the mean and eddy heat fluxes is not clear. A three month average doesn’t seem sufficient to separate mesoscale eddy timescales from the mean—why not use an average over the full four years available? Also, considering that resolved eddies still play a role in the R1 simulation, why are the effect of these not also diagnosed and added to the GM contribution?
Technical corrections
- Remove indent on line following equation (5).
References
Bower, A. S., and H. D. Hunt, 2000: Lagrangian observations of the deep western boundary current in the North Atlantic Ocean. Part I: Large-scale pathways and spreading rates. J. Phys. Oceanogr., 30 (5), 764–783.
Nycander, J., M. Hieronymus, and F. Roquet, 2015: The nonlinear equation of state of sea water and the global water mass distributions. Geophys. Res. Lett., 42, 7714–7721.
Zhang, R., and G. K. Vallis, 2007: The role of bottom vortex stretching on the path of the North Atlantic western boundary current and on the Northern Recirculation Gyre. J. Phys. Oceanogr., 37 (8), 2053–2080.
Citation: https://doi.org/10.5194/egusphere-2025-1100-RC1
Model code and software
DINO configuration David Kamm https://zenodo.org/records/15016824
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