the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Apr 2024
Non-negligible impact of Stokes drift and wave-driven Eulerian currents on simulated surface particle dispersal in the Mediterranean Sea
Abstract. Numerical simulations of marine surface particle dispersal are a crucial tool for addressing many outstanding issues in physical oceanography with societal relevance, such as marine plastic pollution. However, the quality of these Lagrangian simulations depends on the ability of the underlying numerical model to represent the prevailing ocean circulation features. Here, we investigate how simulated marine surface particle dispersal changes, if the – often omitted or only approximated – impact of wind-generated surface waves on the upper ocean circulation is considered. We use velocity fields from a high-resolution coupled ocean-wave model simulation and a complementary stand-alone ocean model simulation for the Mediterranean Sea to answer the following questions: 1) How does the explicit representation of waves impact the simulated surface particle dispersal, and what is the relative impact of Stokes drift and wave-driven Eulerian currents? 2) How accurately can the wave impact be approximated by the commonly applied approach to advect particles with non wave-driven Eulerian currents and Stokes drift from stand-alone ocean and wave models? We find that the representation of surface waves tends to increase the simulated mean Lagrangian surface drift speed in winter through a dominant impact of Stokes drift, and to decrease the mean Lagrangian surface drift speed in summer through a dominant impact by wave-driven Eulerian currents. Furthermore, simulations that approximate the surface wave impact by including the Stokes drift (but ignoring the wave-driven Eulerian currents) do not necessarily yield a better estimate of the surface particle dispersal patterns with explicit representation of the wave impact than simulations that do not include any surface wave impact. Our results imply that – whenever possible – velocity fields from a coupled ocean-wave model should be used for surface particle dispersal simulations.
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RC1: 'Comment on egusphere-2024-1002', Tamay Ozgokmen, 02 May 2024
I agree with the main premise & conclusion of this manuscript that Stoke's drift, very near surface currents via coupled modeling are critical to dispersion of surface material (such as oil spills, and microplastics). The manuscript is solid, well written, includes insightful test cases. The authors are informed scientists in this field.
The main deficiency of the manuscript is that it is modeling only and lacks real-data verification... which is very very hard, so understandable.
Also, some relevant work is not cited:
Curcic, M., S.S. Chen and T. M. Özgökmen, 2016: Hurricane-induced ocean surface transport and dispersion in the Gulf of Mexico. Geophys. Res. Lett., 43, 2773-2781.
Laxague, N.J.M., T.M. Özgökmen, B. K. Haus, G. Novelli, A. Shcherbina, P. Sutherland, C. Guigand, B. Lund, S. Mehta, M. Alday, and J. Molemaker, 2018: Observations of near-surface current shear help describe oceanic oil and plastic transport. GRL, 45(1), 245-249.
The first paper involves testing with a truly coupled ocean-wave-atmosphere model with using real data and the second involves current measurements going all the way to top 1 cm near the air-sea surface (which was very hard to pull off); could be interest to the authors. (I had to declare my identity since I am posting these papers.)
Citation: https://doi.org/10.5194/egusphere-2024-1002-RC1 -
AC1: 'Reply on RC1', Siren Rühs, 22 May 2024
We greatly appreciate the positive feedback on our manuscript and the pointers to additional references.
We agree that data verification of our results would be desirable. However, deciphering and separating the individual wave-induced processes that alter Lagrangian surface velocities from observations is indeed beyond the scope of this study, as it poses a scientific challenge on its own (which we discuss in section 5.1, lns 630-649, of the preprint). We specifically designed this study as conceptual model work to be able to not only derive the Stokes drift but also distinguish wave-driven Eulerian currents from non-wave driven Eulerian currents.
We will incorporate the results of Laxague et al. (2018) into our discussion in a revised version of our manuscript, as these results raise an important limitation of all larger-scale applications that typically make use of Lagrangian velocities estimated from Eulerian currents averaged over the upper meter(s) of the water column. They show that Lagrangian velocities in the upper few centimeters of the water column may be significantly stronger compared to those averaged over the upper meter(s), due to strong vertical shear of the Eulerian currents and Stokes drift, as well as microscale wave breaking and skin friction. Hence, our results are - strictly speaking - not directly applicable for particles bound to the upper few centimeters of the ocean, such as non-emulsified and emulsifying oil as well as macro- and mesoplastic, but more representative for slightly submerged particles at 1m depth, representing for example microplastics. However, considering that Stokes drift as well as Eulerian currents both experience a strong vertical shear, the main conclusion of our study that Stokes drift as well as wave-driven Eulerian currents can have a non-negligible impact of surface-particle dispersal is expected to remain valid.
While we read Curcic et al. (2016) with great interest, we find their results slightly less relevant for our study. Curcic et al. (2016) provide an impressive example of how Stokes drift can have a substantial impact surface dispersal during extreme events, based on the combined evaluation of data from a drifter release experiment and a coupled atmopshere-wave-ocean model. However, extreme events are – as mentioned in section 3.1.3, ln. 264, of the preprint – not the focus of our study. Moreover, we are particularly interested in the relative impact of Stokes drift versus wave-driven Eulerian currents; but the model employed by Curcic et al. (2016) does not incorporate the Stokes-Coriolis and vortex forces and hence does not allow for distinguishing the effect of wave-driven Eulerian currents. Having said all this, given the results of Curcic et al. (2016), it may be interesting to conduct follow-up studies investigating the relative importance of Stokes drift versus wave-driven Eulerian currents during extreme events. We will add a respective statement in a revised version of the manuscript, including the reference to Curcic et al. (2016).
We would like to thank the reviewer for their feedback and comments that have helped to improve our manuscript!
Citation: https://doi.org/10.5194/egusphere-2024-1002-AC1
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AC1: 'Reply on RC1', Siren Rühs, 22 May 2024
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RC2: 'Comment on egusphere-2024-1002', Brandon Reichl, 20 May 2024
Review of “Non-negligible impact of Stokes drift and wave-driven Eulerian currents on simulated surface particle dispersal in the Mediterranean Sea” by S. Rühs et al.
This manuscript analyzes the impact of including ocean surface gravity waves when simulating ocean currents that are used for Lagrangian advection. The conventional approach is to consider the effect of surface waves on Lagrangian particles through their phase-averaged Stokes drift. Previous studies have therefore considered adding the Stokes drift to the output of ocean models that did not account for wave driven processes. This study employs an ocean circulation model that is coupled to an ocean surface wave model, and thus considers the impacts of surface waves on the ocean circulation model physics, including through the impact of waves on vertical mixing, surface fluxes, and on the resolved scale currents/advection in the ocean model. The results show that Lagrangian parcel simulations using the ocean circulation model with full wave coupling yields a different result from simulations using the ocean model without wave coupling. This difference can not be accounted for by adding the Stokes drift to the output of the non-coupled model. The study therefore suggests that the feedbacks of ocean waves to modify the background current should be properly accounted for when using output of numerical ocean circulation models to drive Lagrangian particle simulations.
I found this study to be well-written, thoughtful, and important for the ocean modeling community. I have a few comments on the presentation that I think can better clarify the result and its place within the scope of similar literature.
General Comments
- A general comment for definitions of the decomposition in the equation at L50. The Lagrangian current is routinely separated into an Eulerian component and a Stokes drift component that is attributed to the surface wave field. In this study the Eulerian current is further separated into a “non-wave” component U_Enw, and a wave-driven component U_Ew. The non-wave component is later defined from the non-coupled simulation and the wave component is defined from the residual of the coupled simulation minus the non-coupled simulation. This is useful conceptually to decompose the Eulerian current and explain the results. The approach here is pragmatic, but I do think it is important to make the choice of this definition clear early in the paper (e.g., as is conveyed later in Table 2). There are non-linear terms when you back substitute U_Ew and U_Enw into the momentum equations, such that one could consider more refined ways to decompose the wave-driven part, not just from this residual approach.
- It is important to clarify that for the Wagner et al. (2022, also disclosing that I am a coauthor on that work) work we conducted our experiments on a 25 km ocean-wave model and resolved wave-current interactions at that scale. There are other differences between this work and our simulations (e.g., more wave physics impacts than just resolved-scale wave-current interactions are considered in this work), but I personally did not find it surprising or controversial that a different result may be found at 4km resolution with wave-current interactions resolved at smaller scales. The results may in fact be compatible, perhaps yielding insight into the scales where the resolved scale impacts are important.
Specific Comments
L53: I suggest using different language, "explicitly resolved" to me implies simulating the surface phases of waves directly by the ocean model. But I think it is meant that they are sometimes coupled to spectral wave models.
L115: I don’t think Craig and Banner (1994) is the best reference for Langmuir turbulence. Perhaps McWilliams et al. (1997, doi:10.1017/S0022112096004375) and other more recent reviews (e.g., Belcher et al., 2012 doi:10.1029/2012GL052932, D’Asaro, 2014 doi:10.1146/annurev-marine-010213-135138)?
L148: But it does neglect the feedbacks of U_Ew on U_Enw. This is a benefit of the approach here, could that be tested here?
L152: “controversy” seems like an overly strong word to me.
L166: WaveWatch -> WAVEWATCH.
L169: despite -> except
L175: What is the first cell thickness? I can imagine the results could be sensitive if the first cell is particularly thin or coarse.
L200: Cell horizontal interfaces or vertical interfaces or both?
L220: Are there any citations for this? Which specific “TKE” scheme is it? A k-l type, a Mellor-Yamada, GLS, etc.? Is the momentum flux directed only down the Eulerian vertical current shear or also down the Stokes gradient (e.g., Harcourt 2013, doi: 10.1175/JPO-D-12-0105.1)? This detail could be important since down-Stokes mixing can be an additional source of “anti-Stokes” current.
L225: Is there a citation for this? Otherwise, it may be better to say the parameterized Langmuir turbulence is expected to be more realistic with the simulated, sea-state dependent Stokes drift than the wind speed based Stokes drift.
L3.1.2: Some discussion of the wave model performance in this configuration would be helpful. Are there obs comparisons in a previous study that can be cited?
L229: Is there a spectral tail added for the Stokes drift computation?
Table 1: It would be inconsistent to include some of these Stokes drift impacts in NEMO without others, so I suggest not splitting into three subcolumns when intermediate experiments aren’t attempted. I didn’t catch the distinction between the modified TKE scheme [note typo in manuscript] and Langmuir turbulence parameterization, if more details are given as noted by comment at L220 it would help here.
L244: Why not spin-up the coupled version from rest? Is it possible that analysis in early 2019 is contaminated from the initial adjustment?
L252: I don’t expect these results to be particularly sensitive to this detail, but I’m surprised that the atmospheric fields are only updated every six hours. This seems fairly coarse in time at ~10km spatial resolution. Are the fields interpolated in time to force NEMO and WAVEWATCH?
L266: Suggest to clarify if Stokes drift is similarly averaged over 1m.
L295: Specify horizontal grid, vertical grid, or both
L302: This seems like a missed opportunity in this study, otherwise it leaves an open question if ocean circulation models need to include full wave physics or the effect can still be partially accounted for via intermediate approaches. It seems very relevant to me to answer the second question. Can the authors offer some comments on its potential utility in Section 5.1?
Table 2: I find this table quite useful interpreting the definitions, it would be useful to refer to this table when defining the u_Enw and u_Ew components.
L333: This is a practical approach, but I think this is an important point to make earlier (e.g., when discussing the decomposition). It is important to know that it is defined as a residual and includes all the feedback that would be missed in the intermediate approach.
Figure 4: I find the bar plot (panel a) busy and difficult to understand. You may consider if the maps are sufficient on their own to make less effort for a reader to understand the figure.
Figure 6: The differences between the panels are often subtle. I wonder if showing the difference from the 1st experiment instead of the value for the 2nd and 3rd experiment would make a clearer indication of the changes?
L609: Suggest to clarify what is meant by intrinsic variability in this context. It is an eddying model, so I did wonder if 2 years is sufficient experiment length for all the statistics?
L625: This may be true and is a worthwhile point, but I don't know that this has really been tested in this work. This study shows differences in the Eulerian (gridded mean) fields between coupled and non-coupled, which weren’t found in Wagner et al. (2022). As mentioned in the general comments there are other differences between these works, particularly the horizontal grid-spacing, which could explain the different conclusions.
Review by: Brandon Reichl
Citation: https://doi.org/10.5194/egusphere-2024-1002-RC2 - AC2: 'Reply on RC2', Siren Rühs, 27 Jul 2024
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