the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2025
Modeling Indian Ocean circulation to study marine debris dispersion: insights into high-resolution and Stokes drift effects with Symphonie 3.6.6
Abstract. The Indian Ocean basin faces significant anthropogenic pressure due to its connection to over 2.2 billion people through river basins. Indian Ocean dynamics are characterized by strong regional and seasonal variability driven by the monsoon system and intense eddy activity. To address the issue of land-sea transfers and marine debris dispersion in this complex ocean, we developed a new circulation modeling configuration using the hydrodynamic model SYMPHONIE. Our configuration introduces a unique telescopic grid covering the entire basin, enabling the study of sub-basin connectivity while resolving meso and submesoscale processes in the coastal region, from the Mozambique Channel to the Bay of Bengal, at a resolution of 1 to 3 km. Additionally, we integrate the recently released high-resolution GloFAS river discharge dataset to force the physical simulations with daily freshwater inputs. Three annual experiments are conducted, alternatively considering Stokes drift forcing and different grid resolutions. Comparisons of temperature, salinity and sea level with in situ and satellite data show the good performance of the simulations and the ability of the high resolution model to accurately capture the spatial and temporal variability of surface dynamics and water masses over the Indian Ocean. We further analyze energy budgets and perform Lagrangian experiments to illustrate the critical role of resolution and Stokes drift in shaping the circulation and the resulting marine debris dispersion patterns. The effect of energy levels is particularly significant on trajectory statistics such as average travel distances and preferred spread direction. Notably, Stokes drift has a significant seasonal effect in the Arabian Sea during the southwest monsoon, while current field resolution strongly influences trajectories in the Mozambique Channel. Our results provide a robust modeling framework for studying Indian Ocean dynamics and exploring their effect on marine connectivity and the transport of matter, including pollutants, larvae or organic matter.
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RC1: 'Comment on egusphere-2025-1918', Anonymous Referee #1, 24 Jun 2025
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AC1: 'Reply on RC1', Lisa Weiss, 02 Sep 2025
We sincerely thank the editor and both reviewers for their time and careful consideration of our manuscript. We have significantly improved the overall quality of the manuscript by responding to their helpful and constructive comments. In the following, we address the comments raised and propose some modifications, including mainly :
- the description and implementation of wave effects,
- the model-observation comparisons,
- addressing land-sea transfers not supported by the Lagrangian analyses of the study.
Our response to each point made by the reviewer 1 is presented attached in the pdf file (in blue). Our corrections and additions to the manuscript text are underlined here in the responses.
Best regards,
Lisa Weiss and co-authors
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AC1: 'Reply on RC1', Lisa Weiss, 02 Sep 2025
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RC2: 'Comment on egusphere-2025-1918', Anonymous Referee #2, 11 Jul 2025
Comments on Weiss et al.: “Modeling Indian Ocean circulation to study marine debris dispersion: insights into high-resolution and Stokes drift effects with Symphonie 3.6.6”
To gain a better understanding of the pathways and accumulation of marine debris, significant developments have occurred over the past decade. The main three components which govern the quality of a dispersal model are the marine debris sources (initial conditions), the marine debris transport mechanisms (in other words the influence of the different forcing components namely circulation, wind and waves on the marine debris displacement) and the quality and reliability of the forcing models themselves. In that context, this paper aims to cover the latter two by studying the impact of wave-induced transport and higher resolution, which could presumably give a better description of the circulation, especially close to the coastline.
The paper is well structured and written with a good quality of English. The results are well presented with precise and good-quality figures. It, however, suffers from some unclarities in the modeling used to describe the wave-induced transport. As highlighted by the first reviewer, it is hard to understand whether we are looking at a one-way coupled circulation - wave model, or that the Stokes drift (which is a Lagrangian « thing ») is « simply » added to the circulation. These unclarities lead to troubling the appraisal of the interpretation made by the authors of such a phenomenon. Also, as stressed by the first reviewer, looking at the dispersal of marine debris only from a « synthetic » offshore release scenario feels a bit frustrating from a reviewer’s standpoint where all the effort put into having a more precise representation of river discharges and using higher resolution close to the coastline becomes, in turn, irrelevant. By solving those two points (clarifying the narrative and explanations around wave-induced transport and managing expectations on the marine debris dispersal relevance in the objectives) this work should become suitable for publication.
Comments (in addition to those given by Reviewer 1):
l.44: HYCOM has a meridional resolution of 0.08° and 0.04° at higher latitudes from the equator
l. 46: maybe worth mentionning the existence of a global LLC4320 (see Forget et al. 2015 and Rocha et al. 2016 e.g.) at a much higher resolution (1/48°) for 2011
l. 57-60: the explanation on the roles in the impact of wave-induced drift on Eulerian currents, wave-induced drift on Lagrangian particles, and possibly windage which could be added to the latter processes is pretty unclear. An interesting paper on the influence of the differents processes at a global scale (including a split between Ekman currents / geostrophy / tides etc…) is Onink et al. 2019.
l. 88: from the context it is clear that « tracers » correspond to temperature and salinity but why not explicitly mention them, in a dispersal / Lagrangian paper this can be confusing.
Figure 1: every 50 gridlines instead of meshes, maybe worth adding a red circle to materialize the release of Lagrangian particles
l. 161: rising velocity (singular) - because only one rising velocity is considered for all particles, the value of 1mm/s seems pretty low compared to experienced rising velocities for e.g. mesoscale plastics (see Lebreton et al. 2018, Supplementary Material)
l. 201: 47 rivers vs 46 rivers on line 193, am I missing something - see also Figure A4
paragraph starting l. 215: the discussion there « contradicts » the objective of having a better representation of river discharges given than GLOFAS seems to systematically overestimate the measurements.
Figure 7: the superimposition of daily tracer profiles is hard to read (dark blue and light blue)
Figure 8: it would be interesting to split between the different regions and not only focus on the whole domain - in relation with the analysis made after (figure 9) and before…
Figure 12: consider bigger fonts in the blue / red / yellow boxes
l. 622: «its implications for marine debris dispersion in the region », in light of what was said before, marine debris dispersion cannot be viewed independently from release locations. Demonstrating the impact of an improved circulation and transport modeling based on a (very) hypothetical release scenario (which cannot be validated by design) weakens the demonstration. Especially given that so much effort has been put into the river discharges (which could, combined with other socio-economic information, provide useful proxies for marine debris release, see e.g. Meijer et al. 2022)
l. 667: it is now understandable why the coastal releases were not considered so far as it is meant for future studies, so why not state clearly that the implications of such improved modeling on the marine debris dispersal in the region will only be tangible once realistic debris sources will be considered.
Figure A4: back to 46 rivers?
Figure A9/A10/A11: a slight increase of the font size in the roses could be beneficial
References:
Forget, G., Campin, J.-M., Heimbach, P., Hill, C. N., Ponte, R. M., and Wunsch, C.: ECCO version 4: an integrated framework for non-linear inverse modeling and global ocean state estimation, Geosci. Model Dev., 8, 3071–3104, https://doi.org/10.5194/gmd-8-3071-2015.
Rocha, C. B., Chereskin, T. K., Gille, S. T., and Menemenlis, D.: Mesoscale to Submesoscale Wavenumber Spectra in Drake Passage, J. Phys. Oceanogr., 46, 601–620, https://doi.org/10.1175/JPO-D-15-0087.1, 2016.
Onink, V., Wichmann, D., Delandmeter, P., van Sebille, E., 2019. The role of Ekman currents, geostrophy and stokes drift in the accumulation of floating microplastics. J. Geophys. Res. Oceans 124. https://doi.org/10.1029/2018JC014547.
Lebreton, L., et al. (2018). Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic. Sci. Rep., 8. https://doi.org/10.1038/s41598-018-22939-w
Citation: https://doi.org/10.5194/egusphere-2025-1918-RC2 -
AC2: 'Reply on RC2', Lisa Weiss, 02 Sep 2025
We sincerely thank the editor and both reviewers for their time and careful consideration of our manuscript. We have significantly improved the overall quality of the manuscript by responding to their helpful and constructive comments. In the following, we address the comments raised and propose some modifications, including mainly :
- the description and implementation of wave effects,
- the model-observation comparisons,
- addressing land-sea transfers not supported by the Lagrangian analyses of the study.
Our response to each point made by the reviewer 2 is presented attached in the pdf file (in blue). Our corrections and additions to the manuscript text are underlined here in the responses.
Best regards,
Lisa Weiss and co-authors
-
AC2: 'Reply on RC2', Lisa Weiss, 02 Sep 2025
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Comments on Weiss et al.: “Modeling Indian Ocean circulation to study marine debris dispersion: insights into high-resolution and Stokes drift effects with Symphonie 3.6.6”
The manuscript addresses an outstanding issue in the field of marine (pollution) transport modelling: providing coherent ocean velocity output from the coast to the open ocean that resolves the dominant transport processes from the submesoscale to the basin scale. It does so by introducing a new ocean model configuration for the Indian Ocean with grid refinement towards key coastal regions, inclusion of wave forcing, and a more realistic representation of river discharge. While the grid refinement and inclusion of wave forcing are not based on novel concepts, their combination in this context yields a potentially valuable new tool for marine pollution modelling that may support both sensitivity studies and improved estimates of pollution patterns compared to standard approaches.
The overall structure and presentation of the results is clear, including useful visualizations. However, the writing includes imprecise terminology, and several methodological concepts are insufficiently or inaccurately described. This limits a thorough assessment of the approach and specifically concerns the description and implementation of wave effects (see General Comment 1) and model-observation comparisons (General Comment 2). Additionally, the stated goal of addressing land-sea transfers is not clearly supported by the Lagrangian analyses presented (General Comment 3).
To conclude, while I see the potential of the manuscript to become a relevant addition to the field, I recommend major revisions to clarify key concepts, improve the terminology, and better align overall content and objectives.
General Comments:
1. Representation of wave effects
The description of wave-related processes is vague, and terminology is inconsistently used. In theory, waves affect Lagrangian transport both (i) directly via Stokes drift and (ii) indirectly through wave-induced modifications of Eulerian currents, including (anti-)Stokes forces such as the Stokes–Coriolis force. After rereading the methods, I was left with the impression that:
If this is correct, the implementation is basic and not fully aligned with the current state of the art (e.g., Couvelard et al., 2020). It also contrasts with recent findings suggesting that both Stokes drift and wave-induced modifications of Eulerian currents are important for Lagrangian transport (e.g., Röhrs et al., 2022; Cunningham et al., 2022; Rühs et al., 2025). I recommend that the authors:
2. Model-observation comparisons
The approach to model validation needs clarification:
Further detail on the nudging (location, depth, timescale) is needed as well, see also specific comment below.
3. Land-sea transfers
The abstract sets the goal to address land–sea transfers, but no direct analysis of this is presented. Lagrangian experiments are based on offshore releases, and sensitivity tests focus on grid resolution and wave forcing. The influence of more realistic river discharge, while implemented, is not tested. This feels like a missed opportunity. For example, exploring how coastal retention changes with the new configuration could strengthen the manuscript’s relevance considerably. If the authors choose not to pursue additional analyses, I suggest reformulating the manuscript’s goals to avoid overstating its scope.
Specific comments:
Technical corrections:
References: