the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The Mixed Layer Depth in the Ocean Model Intercomparison Project (OMIP): Impact of Resolving Mesoscale Eddies
Anne Marie Treguier
Clement de Boyer Montégut
Alexandra Bozec
Eric P. Chassignet
Baylor Fox-Kemper
Andy McC. Hogg
Doroteacino Iovino
Andrew E. Kiss
Julien Le Sommer
Pengfei Lin
Camille Lique
Hailong Liu
Guillaume Serazin
Dmitry Sidorenko
Qiang Wang
Xiaobio Xu
Steve Yeager
Abstract. The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed layer depth (MLD) in six pairs of non-eddying (1° resolution) and eddy-rich (up to 1/16°) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model validation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
ocean eddies, which are the largest source of ocean variability and modulate the mixed-layer properties. We find that the mixed-layer depth is better represented in eddy-rich models but, unfortunately, not uniformly across the globe and not in all models.
Anne Marie Treguier et al.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-310', Stephen M. Griffies, 05 Mar 2023
Review of "The Mixed Layer Depth in the Ocean Model Intercomparison
Project (OMIP): Impact of Resolving Mesoscale Eddies" by Treguier et
al.This is a well-written summary of the comparison of mixed layers in a
suite of OMIP2 simulations, including both coarse (1degree) and fine
grids. The results are provocative and provide
a benchmark and motivation for future work. I support publication and
only have a few comments.--line 26: We never really "validate" climate models. Instead we
"evaluate" models.--line 74: "period period"
--line 188: "of course" is subjective; suggest removing
--In many many places in the manuscript, the word "resolution" is used
when you really mean "grid spacing". Resolution is a non-dimensional
number whereas grid spacing measures the distance between grid points, typiically in degrees or km.
In most places this quibble is not so important since we "know what is
meant" even if we do not say it clearly. But on line 425 one reads the
rather confusing sort of statement that can result when "resolution"
and "grid spacing" are interchanged: "with a horizontal resolution of
less than 1km". Does that phrase mean the grid spacing is coarser than
1km or finer than 1km?? This is the sort of confusion that a novice
(and experienced) reader can come across when "resolution" is used
when "grid spacing" is meant. I suggest clarifying all uses of
"resolution".--line 575: The authors observe that the MLD is more widely spread
among the OMIP simulations than SST. This is a very important
statement that perhaps has been noted before but is worth emphasizing
more. It offers an important counter-point to those who discount
OMIP simulations for using a prescribed atmosphere and so "have the
answer given to them". There is a lot more physics going into the MLD
than just that provided by the SST and SSS.--Now for my slightly nontrivial and somewhat self-serving comment.
Namely, the authors point to the sensitivity of the MLD to the density
threshold (Figure 2), the upper layer depth, and time sampling. In the end, they
propose a 10m upper layer starting point rather than the "top grid
cell" advised by Griffies et al (2016). I think this is a good
suggestion. Yet they also support the density threshold approach, even
though it suffers from the problems they note in their Figure 2, as
well as those problems noted by BM04 whereby the density threshold
should be a function of the SST/SSS given the nonlinear equation of
state.These limitations and hyper-sensititivities motivated Reichl et al (2022) to
propose a potential energy-based threshold.
That approach also can use monthly mean T/S to directly compare model
to obs, and it can be implemented online. So it is a practical
approach and simpler than some methods like Holte and Talley, though
more complex than the density threshold method.I do not ask the authors to add the energy approach to their analysis,
as that would be more work than I can reasonably request. But I do
suggest they be somewhat more circumspect about the density threshold
for future MIPs. I might be wrong, but at this point I think a
potential energy approach ala Reichl et al is a physically compelling
and practical approach that avoids many of the problems with the
density approach.END OF REVIEW
Citation: https://doi.org/10.5194/egusphere-2023-310-RC1 -
AC2: 'Reply on RC1', Anne Marie Treguier, 23 Apr 2023
Many thanks for your review of our manuscript and your encouraging comments.
Your remark about line 575 is very interesting. We will try to emphasize our statement better in the revised version of our manuscript. We will also take into account all your minor comments, and carefully check the manuscript throughout for misuses of "resolution" and "grid spacing".
We agree that we could mention in more detail the new approach presented by Reichl et al., and we will do so in our revised manuscript. As you point out, more work is necessary to fully implement the potential energy method at the global scale, and this is beyond the scope of our paper. Reichl et al. suggest that further studies will explore the choice of the potential energy anomaly that must be prescribed for the computation of the mixed layer depth. The diurnal cycle, which is still not well resolved by ARGO observations, could be investigated from the point of view of potential energy. One could also consider the interplay between the potential energy in the mixed layer and three dimensional dynamics such as submesoscale structures which are found to play a significant role in setting the stratification in some regions (Yu et al, 2019).
Reference: Yu et al., JPO 2019: DOI: 10.1175/JPO-D-18-0253.1
Citation: https://doi.org/10.5194/egusphere-2023-310-AC2
-
AC2: 'Reply on RC1', Anne Marie Treguier, 23 Apr 2023
-
RC2: 'Comment on egusphere-2023-310', A.J. George Nurser, 17 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-310/egusphere-2023-310-RC2-supplement.pdf
- AC1: 'Reply on RC2', Anne Marie Treguier, 23 Apr 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-310', Stephen M. Griffies, 05 Mar 2023
Review of "The Mixed Layer Depth in the Ocean Model Intercomparison
Project (OMIP): Impact of Resolving Mesoscale Eddies" by Treguier et
al.This is a well-written summary of the comparison of mixed layers in a
suite of OMIP2 simulations, including both coarse (1degree) and fine
grids. The results are provocative and provide
a benchmark and motivation for future work. I support publication and
only have a few comments.--line 26: We never really "validate" climate models. Instead we
"evaluate" models.--line 74: "period period"
--line 188: "of course" is subjective; suggest removing
--In many many places in the manuscript, the word "resolution" is used
when you really mean "grid spacing". Resolution is a non-dimensional
number whereas grid spacing measures the distance between grid points, typiically in degrees or km.
In most places this quibble is not so important since we "know what is
meant" even if we do not say it clearly. But on line 425 one reads the
rather confusing sort of statement that can result when "resolution"
and "grid spacing" are interchanged: "with a horizontal resolution of
less than 1km". Does that phrase mean the grid spacing is coarser than
1km or finer than 1km?? This is the sort of confusion that a novice
(and experienced) reader can come across when "resolution" is used
when "grid spacing" is meant. I suggest clarifying all uses of
"resolution".--line 575: The authors observe that the MLD is more widely spread
among the OMIP simulations than SST. This is a very important
statement that perhaps has been noted before but is worth emphasizing
more. It offers an important counter-point to those who discount
OMIP simulations for using a prescribed atmosphere and so "have the
answer given to them". There is a lot more physics going into the MLD
than just that provided by the SST and SSS.--Now for my slightly nontrivial and somewhat self-serving comment.
Namely, the authors point to the sensitivity of the MLD to the density
threshold (Figure 2), the upper layer depth, and time sampling. In the end, they
propose a 10m upper layer starting point rather than the "top grid
cell" advised by Griffies et al (2016). I think this is a good
suggestion. Yet they also support the density threshold approach, even
though it suffers from the problems they note in their Figure 2, as
well as those problems noted by BM04 whereby the density threshold
should be a function of the SST/SSS given the nonlinear equation of
state.These limitations and hyper-sensititivities motivated Reichl et al (2022) to
propose a potential energy-based threshold.
That approach also can use monthly mean T/S to directly compare model
to obs, and it can be implemented online. So it is a practical
approach and simpler than some methods like Holte and Talley, though
more complex than the density threshold method.I do not ask the authors to add the energy approach to their analysis,
as that would be more work than I can reasonably request. But I do
suggest they be somewhat more circumspect about the density threshold
for future MIPs. I might be wrong, but at this point I think a
potential energy approach ala Reichl et al is a physically compelling
and practical approach that avoids many of the problems with the
density approach.END OF REVIEW
Citation: https://doi.org/10.5194/egusphere-2023-310-RC1 -
AC2: 'Reply on RC1', Anne Marie Treguier, 23 Apr 2023
Many thanks for your review of our manuscript and your encouraging comments.
Your remark about line 575 is very interesting. We will try to emphasize our statement better in the revised version of our manuscript. We will also take into account all your minor comments, and carefully check the manuscript throughout for misuses of "resolution" and "grid spacing".
We agree that we could mention in more detail the new approach presented by Reichl et al., and we will do so in our revised manuscript. As you point out, more work is necessary to fully implement the potential energy method at the global scale, and this is beyond the scope of our paper. Reichl et al. suggest that further studies will explore the choice of the potential energy anomaly that must be prescribed for the computation of the mixed layer depth. The diurnal cycle, which is still not well resolved by ARGO observations, could be investigated from the point of view of potential energy. One could also consider the interplay between the potential energy in the mixed layer and three dimensional dynamics such as submesoscale structures which are found to play a significant role in setting the stratification in some regions (Yu et al, 2019).
Reference: Yu et al., JPO 2019: DOI: 10.1175/JPO-D-18-0253.1
Citation: https://doi.org/10.5194/egusphere-2023-310-AC2
-
AC2: 'Reply on RC1', Anne Marie Treguier, 23 Apr 2023
-
RC2: 'Comment on egusphere-2023-310', A.J. George Nurser, 17 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-310/egusphere-2023-310-RC2-supplement.pdf
- AC1: 'Reply on RC2', Anne Marie Treguier, 23 Apr 2023
Peer review completion
Journal article(s) based on this preprint
ocean eddies, which are the largest source of ocean variability and modulate the mixed-layer properties. We find that the mixed-layer depth is better represented in eddy-rich models but, unfortunately, not uniformly across the globe and not in all models.
Anne Marie Treguier et al.
Anne Marie Treguier et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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