the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
ISOM 1.0: A fully mesoscale-resolving idealized Southern Ocean model and the diversity of multiscale eddy interactions
Abstract. We describe an idealized Southern Ocean model (ISOM 1.0) that contains simplified iconic topographic features in the Southern Ocean and conduct a fully mesoscale-resolving (2 km) simulation based on the Massachusetts Institute of Technology general circulation model (MITgcm). The model obtains a fully developed and vigorous mesoscale eddying field with a k-3 eddy kinetic energy (EKE) spectrum and captures the topographic effect on stratification and large-scale flow. To make a more naturally conceptual introduction of large eddy simulation (LES) methods into ocean mesoscale parameterization, we propose the concept of mesoscale ocean direct numerical simulation (MODNS). A qualified MODNS dataset should resolve the first baroclinic deformation radius and ensure that the affected scales by the dissipation schemes are sufficiently smaller than the radius. Such datasets can serve as the benchmark for a priori and a posteriori tests of LES schemes or mesoscale ocean large eddy simulation (MOLES) methods into ocean general circulation models (OGCMs). The 2 km idealized simulation meets the requirement of MODNS and also captures part of the submesoscale processes. Therefore, its output can be a type of MODNS and provide reliable data support for relevant a priori and a posteriori tests. We also illustrate the diversity and high complexity of multiscale eddy interactions related to mesoscale processes. We emphasize the importance of submesoscale phenomena on the evolution of mesoscale processes when mesoscale activities are vigorous and of high eddy number density. In addition, we use the model to conduct multipassive tracer experiments and reveal guidelines for the initial settings of passive tracers to delay the homogenization process and ensure the mutual independence of tracers over a long period.
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RC1: 'Comment on gmd-2024-72', Anonymous Referee #1, 13 May 2024
In this manuscript, the authors present an Idealized Southern Ocean Model ("ISOM 1.0") which is essentially an idealized configuration of the MITgcm. They argue that with a lateral resolution of 2km, this represents a type of DNS for the mesoscale ocean. The authors present some basic diagnostics for the mesoscale eddy energy and some typical eddy interactions. However, the focus appears to be primarily on the model configuration and the concept of ISOM as a tool, rather than original scientific results.Â
Overall, I am struggling with a recommendation for this manuscript. I have no fundamental issues with the overall concept, even though I disagree with some of the parameter choices - see below. The question is whether the concept is really that new? For example, the configuration, while differing in certain respects, is not so different to that of Neverworld.
In the conclusions, the authors state: "The prominent feature of the model is the successful simulation of a fully developed and vigorous mesoscale eddying field. We reproduce the EKE spectrum of k−3 predicted by geostrophic turbulence theory. In addition, the simulated geographical distribution of eddy activities is qualitatively consistent with the realistic situation, and the model can describe the topographic effect on stratification and large-scale flow." However, I'm afraid that these results do not strike me as particularly novel.
On balance, I have suggested a major revision to give the authors the opportunity to respond. However, I would need to see original scientific results before I could recommend publication, or otherwise a really compelling case made for why the concept is fundamentally different to what has gone before and merits publication in its own right.Â
Specific comments:Â
Line 31: "as well as generating ..."
Line 35: A reference to Eden and Greatbatch (2008, doi: 10.1016/j.ocemod.2007.09.002) is also appropriate here.Â
Line 46: Can you explain what you mean by "its mathematical properties are not fully satisfied by the grid discretization of numerical models"? Does this imply something that is needed beyond the mathematical properties derived in Maddison and Marshall (2013, doi: 10.1017/jfm.2013.259), sections 2.3 and 3.3, and which hold for isopycnal thickness-weighted averaging?Â
Section 2.1: Please define all symbols used in the equations, including u, v, θ, etc. You also switch from θ in equation 4 to T in equation 7, and it's unclear why this is named potential temperature rather than simply temperature given the linear equation of state.Â
Section 2.2: Why is the reference ocean depth so shallow at 3km, and why does the model Drake Passage shallows to 1km (by eye, the model Drake Passage looks even shallower in Figure 1a, but this might be an illusion?)
The authors subsequently find that the ACC transport is relatively weak at 65Sv, which I don't find surprising given the overly shallow reference ocean depth and Drake Passage depth (despite the authors' comment to the contrary on line 178). A realistic ACC transport should be a prerequisite and I suggest that the authors considering deepening their ocean to at least 4km, and the model Drake Passage to around 3km since there is circumpolar connection at the latter depth.
It is also worth noting that with the surface temperature relaxed strongly to 0 degrees at the south, and the northern vertical temperature profile prescribed through the sponge layer, the thermal wind is effectively imposed in the model: Â what transport does this give, assuming no flow at 3km?Â
Line 147: Does "The topography within the passage has a piecewise linear depth" mean that the depth varies in a linear manner with partial cells, or that you are employing piecewise linear shaved cells? I think the former, but the text might be read as implying the latter.Â
General comment on Section 2 and Figure 1a: I'm not so sure the idealized topography is as novel as the authors imply. Many model studies have employed idealized Drake Paggase and/or ridge, e.g., the choices made here are not so different to those used in the two layer model of Tansley and Marshall (2001, doi: 10.1175/1520-0485(2001)031<3258:OTDOWD>2.0.CO;2).Â
Line 242: Why do you choose a 7th order advection scheme for the 8km simulation? This is indeed a very accurate scheme with minimal spurious diapycnal mixing, but I would not have expected that to be a major issue for such short integrations, unless you have a problem with undershoots, as described by Hecht (2010, doi:10.1016/j.ocemod.2010.07.005)? Â I understand that the 3rd order DST advection scheme is chosen for computational efficiency at higher resolution - does this introduce any issues and, if not, then why not use it for the 8km integrations for consistency?Â
Line 297: The Rossby number is usually defined as a non-dimensional parameter quantifying the relative importance of the inertial and Coriolis accelerations, or of relative and planetary vorticity. Here, however, it is used as non-dimensionalized relative vorticity, which gives it a fundamentally different meaning. Indeed, on line 300, the authors state that mesoscale processes dominates the flow when |Ro| ≪ O(1), but there are many regions (filaments) in Figure 6(d) where Ro=1 and yet submesoscale processes are prevalent. I therefore strongly advise avoiding this terminology and instead describing what is shown, i.e., ζ/f.Â
Line 398: Space missing after "tensor".Â
Section 5: This should be written in the past tense, "In this paper, we have introduced ...", etc.Â
General comment: The paper is full of acronyms which do not help with its readability. Can the authors please work hard to reduce the number of these?Â
Data availability: To be truly reproducible, the authors should provide the version number of MITgcm employed in this study.Â
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Citation: https://doi.org/10.5194/gmd-2024-72-RC1 -
RC2: 'Comment on gmd-2024-72', Anonymous Referee #2, 21 May 2024
Xie et al. develop a submesoscale permitting direct numerical simulation (DNS) of an idealized Southern Ocean. They show that by incorporating idealized topographic features and increasing the model horizontal resolution to 2km, they are able to represent some of the dynamic processes of eddy-eddy and eddy-jet interactions in the Southern Ocean. They argue that such DNS of oceanic mesocale processes will benefit the assessment of eddy parametrizations. While this manuscript does not present novel results in particular, I envision that the authors will use this as a stepping stone for future studies in developing and assessing eddy parametrizations.
I would like to have the following points addressed by the authors before recommending the manuscript for publication:
- Lines 88-89: The authors claim "We emphasize that the focus of the simulations should be on controlling the dynamics of the idealized model rather than on precise comparisons with observations or realistic model results." but without comparisons to direct observations, it would be difficult to assess the impact of advection schemes of momentum on the states realized by MODNS (e.g. Uchida et al., 2022, https://doi.org/10.5194/gmd-15-5829-2022); Thiry et al., 2024, https://doi.org/10.5194/gmd-17-1749-2024). Do the authors intend on testing different advection schemes as part of the ISOM database? It appears that even at 2km resolution, numerical simulations of the ocean are very sensitive to the advection schemes and/or closures used for surface convection.
- Line 135: The vertical resolution is quite low for a model with the horizontal resolution of 2km (e.g. Balwada et al., 2018, https://doi.org/10.1029/2018GL079244; Ajayi et al., 2020, https://doi.org/10.1029/2019JC015827). Does this hinder the development of baroclinic instability and/or flow-bathymetry interactions in ISOM? Furthermore, the depth of 3000m is quite shallower that the actual Southern Ocean. Given this fact, does it make sense to compare the ACC transport to observational estimates as in Fig. 3?
- Line 184: Regarding the intense meridional displacement of the eddies and idealized ACC jet, have the authors played with different meridional wind profiles? In order to acheive a meridionally narrower ACC, some studies employ a quadratic form in sinusoid (e.g. Balwada et al., 2018, https://doi.org/10.1029/2018GL079244).
- Section 3.2: The -3 slope is a prediction for the forward enstrophy cascade range but -5/3 is predicted for the inverse energy cascade range. Given that this is a modeling study, spectral cascades of enstrophy and energy should be also shown to convince the reader on the robustness of the spectral slopes.
- Section 4: Perhaps the authors have in mind to document the eddy transport tensor diagnosed from their passive tracers in another paper but I would like to see how the tensor changes in time as the homogenization of passive tracers takes place. If the tensor were to show strong time dependence, this would be an obstacle towards obtaining robust estimates of the tensor.
Citation: https://doi.org/10.5194/gmd-2024-72-RC2 -
RC3: 'Comment on gmd-2024-72', Anonymous Referee #3, 03 Jun 2024
This work introduces an idealized Southern Ocean (SO) prototype at eddy-resolving resolutions (2 to 8 km) within the MITgcm model. The new model could successfully capture some mesoscale/sub-mesoscale features, including -3 upscale energy transfer, eddy-jet stream interactions and small/moderate Rossby number ranges by resolving Rossby deformation radius. However, some other SO features, such as ACC transport is underestimated in this new model. Also, a framework for passive tracer is provided.Â
This is a nice piece of work that provides dataset for high-resolution ocean modeling in the SO region. The manuscript is also well written and provided detailed information about their MODNS setup. To improve the usefulness of MODNS dataset, I think a few points need to be illustrated before I can recommend the publication of this work at Geoscientific Model Development. Here, I have itemized my major comments along with minor corrections:
1. Figure 3: as it is explained in the manuscript the ACC transport is significantly underestimated in this setup. One possible reason for this performance is that the channel depth in the Drake Passage area is not realistic. I suggest the channel depth to be increased to be around 4 km and that the bottom topography can be set to be around 2.5 km height like the realistic Drake Passage region. I think this changes to the SO configuration setup would improve the poor estimate of ACC transport. I also suggest the author perform an investigation on changes of ACC transport with the surface wind stress and/or initial salinity profile.Â
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2. Lines 295-300: In this manuscript, Ro ~ 1 is introduced as an indication for submesoscale process. While moderate Ro might be a sign for smaller eddy-like features (in comparison with mesoscale range), but Ro~1 may not be considered as submesoscale "process". I suggest the authors find other metrics when they discuss submesoscale process (see e.g. McWilliams JC. 2016, Submesoscale currents in the ocean. Proc. R. Soc. A 472: 20160117, for more information).
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3. Equations (1) and (2): in actual LES/DNS setup for atmosphere and ocean, it is important to have a coupled (anisotropic) horizontal and vertical dissipation scheme in horizontal and vertical momentum equations (see e.g. Khani S, ML Waite 2020, An anisotropic subgrid-Scale parameterization for large-eddy simulations of stratified turbulence. Mon. Wea. Rev. 148, 4299-4311). I understand that if the authors might leave implementing this coupled/anisotropic setup for their future work, but I would like they add a short statement about this in their manuscript since their model setup is performed at very resolution 2 km.Â
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4. Figure 5: the horizontal axis in these plots is labelled as 'scale' which might be confusing. I suggest using horizontal wavenumber k_h [1/km] (the authors might consider labeling the top horizontal axis with the 'horizontal scale [km]'.
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Minor comments:
- Line 365: "submesoscale signals" ---> submesoscale filaments
- Lines 397 and 400: put a space before '('.
- I suggest a proof-reading before submitting the revision (there are a few grammatical typos in the text).Â
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Citation: https://doi.org/10.5194/gmd-2024-72-RC3
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