the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2025
Dense shelf-water and associated sediment transport in the Cap de Creus Canyon and adjacent shelf under mild winter regimes: insights from the 2021–2022 winter
Abstract. This study examines dense shelf water cascading (DSWC) and estimates the dense shelf-water and associated sediment transport in the Cap de Creus Canyon (northwestern Mediterranean) during the mild winter of 2021–2022. The FARDWO-CCC1 multiplatform survey in March 2022 revealed dense shelf waters on the continental shelf, which were transported to the canyon head. These cold, dense, and turbid waters, rich in dissolved oxygen and chlorophyll-a, downwelled along the canyon’s southern flank to depths around 350 m. During the observed downwelling event, estimated water and suspended sediment transport within the dense water vein were 0.3 Sv and 105 metric tons, respectively, mainly confined to upper canyon reaches. These transports were low compared to extreme winters, likely due to the influence of freshwater inputs and moderate meteorological winter conditions. Transport magnitudes were higher in the upper canyon section than in the mid-canyon section, where transport was estimated at 0.05 Sv, including around 104 metric tons of sediment. This observation suggests that during mild winters, while most of the dense water either remains on the shelf or the shelf-edge area, or flows southward along the coast, the Cap de Creus Canyon acts only as a partial sink for cascading waters. Mediterranean Sea Physics reanalysis data showed that the cascading season lasted approximately three months, from January to early April 2022, with several cascading pulses within the canyon. The highest dense shelf water transport occurred in mid-March, associated with easterly/south-easterly windstorms. This study confirms that remarkable dense shelf water and sediment transport occurs in the Cap de Creus Canyon, particularly along its southern flank, even during mild winters in absence of deep cascading and limited external forcing. Nevertheless, this phenomenon appears to make a significant contribution to the formation of Western Intermediate Water (WIW) in the region.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1310', Anonymous Referee #1, 24 Apr 2025
Referee Comment on:
“Dense shelf-water and associated sediment transport in the Cap de Creus Canyon and adjacent shelf under mild winter regimes: insights from the 2021–2022 winter” by Arjona-Camas et al.
General Comments:
This manuscript presents a well-written and carefully conducted observational study of dense shelf water cascading (DSWC) and associated sediment transport in the Cap de Creus Canyon during a mild winter (2021–2022). The authors employ a multi-platform approach—including gliders, moorings, ship-based CTD profiles, and reanalysis data—to describe the cascading evolution and to estimate transport of water masses and suspended sediments.
The manuscript is well structured and clearly written, with high-quality figures and solid data processing. However, the conceptual novelty is limited, as the key findings align closely with what is already established in the DSWC literature. Specifically, prior studies—including Mahjabin et al. (2019, Continental Shelf Research; 2019, JMSE; 2020, Scientific Reports)—have demonstrated:
- That DSWC can occur under mild to moderate wind forcing;
- That wind direction is a key modulator of cascading strength;
- That such events result in substantial sediment and biogeochemical transport.
Moreover, these studies introduced predictive frameworks such as the Simpson number and energy balance models, and examined canyon-free shelf settings under similar climatic regimes. These works are not cited in the current manuscript.
While the present study is geographically focused on the Cap de Creus Canyon, the manuscript could benefit from a deeper exploration of canyon-specific dynamics—such as flow steering, internal hydraulics, or sediment redistribution mechanisms—which are only briefly mentioned. Additionally, while the observations are carefully described, the broader significance of this mild-winter case for global DSWC understanding is not yet fully articulated. A more explicit discussion of the study’s unique contribution—particularly in terms of sediment asymmetry, constrained cascade depth, and implications for WIW formation—would significantly enhance the manuscript’s impact.
Specific Comments:
- On Novelty and Contextualisation
The Gulf of Lions is among the most studied regions globally for DSWC, with numerous works documenting both mild and extreme cascading events. While the present manuscript focuses on a specific mild winter (2021–2022), the authors should more clearly state what new understanding this adds. For example: Is the sediment asymmetry across the canyon novel? Is the observed upper canyon confinement unusual for mild winters? More detailed differentiation from earlier work is encouraged. - Wind Direction and Episodic Forcing
The manuscript appropriately links SE wind events to episodic downwelling and DSWC initiation. However, this connection is largely descriptive. Including wind stress time series or Ekman transport estimates would strengthen the argument and provide a more quantitative link to the observed cascading pulses. - Canyon-Specific Dynamics
While the Cap de Creus Canyon is central to the title and framing, the manuscript does not deeply examine its dynamic role beyond being a conduit. Consider discussing whether canyon morphology contributes to observed sediment asymmetries or restricts flow depth. Alternatively, consider softening the canyon emphasis if the goal is to document a shelf-wide mild DSWC event. - Citation Inclusion
Please cite the following prior studies if relevant:
- Mahjabin, T., Pattiaratchi, C., & Hetzel, Y. (2019). Wind effects on dense shelf water cascades in south-west Australia. Continental Shelf Research, 189, 103975.
- Mahjabin, T., Hetzel, Y., & Pattiaratchi, C. (2019). Spatial and temporal variability of dense shelf water cascades along the Rottnest continental shelf in southwest Australia. JMSE, 7(1), 30.
- Mahjabin, T., Pattiaratchi, C., & Hetzel, Y. (2020). Dense shelf water cascading around the Australian continent. Scientific Reports, 10, 9930.
These studies support the notion that DSWCs can occur under non-extreme conditions and offer theoretical and methodological insights that are directly relevant here.
Technical Corrections
- Abstract: The opening sentence “This study examines…” is generic and does not effectively convey the study’s context or significance. I recommend replacing it with a more engaging and informative sentence that introduces DSWC and the knowledge gap being addressed. For example:
“Dense shelf water cascading (DSWC) is a key process in transferring water masses and sediments from continental shelves to deep basins, yet its dynamics under mild winter regimes remain poorly characterized.” - Introduction: While DSWC is mentioned early, it is not clearly defined. I recommend including a short, reader-friendly definition in the introduction, such as:
“DSWC refers to the downslope flow of cold, dense water formed on continental shelves due to surface cooling and/or evaporation, which descends under gravity into deeper ocean basins.”
- Line 236: Typo — "metter" should be corrected to "meter".
Citation: https://doi.org/10.5194/egusphere-2025-1310-RC1 - AC2: 'Reply on RC1', Marta Arjona-Camas, 04 Jul 2025
- AC1: 'Reply on RC2', Marta Arjona-Camas, 04 Jul 2025
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RC2: 'Comment on egusphere-2025-1310', Anonymous Referee #2, 05 May 2025
Revision of Arjona-Camas et al.
This study addresses the dense-shelf water and associated sediment transport in the Cap de Creus
Canyon during the mild winter of 2021-2022. This canyon has been identified as a main pathway for the transfer of dense shelf water and sediments from the shelf to the slope and deep margin. The study bases on combination of data from gliders, ship-based CTD transects, instrumented mooring lines, and a reanalysis product.
The article is very clearly written and organized. The results are supported by a set of observations covering different spatio-temporal scales, which is an asset. I do no have any problem with the manuscript other than it is a bit hard to follow because of its very descriptive nature given the different datasets involved. In contrast, I think that the relevance of the study is not very clearly stated. However I do not know the region very well, so I ignore the state of the scientific knowledge and the reach of the relevance or novelty of this study, so I prefer not to evaluate that point.
Overall, it is a good paper. Mi main criticism is about the possibilities that the use of the reanalysis product offers, and which I feel it’s not exploited. I wonder why not to (really, with numbers) validate this reanalysis with your observations, and make the same computations for several years, separating mild and intense winter conditions. This would greatly strengthen the paper’s conclusions. So far, the article is a very nice compilation of observations from different datasets, but it is very descriptive and the cause-effect of the findings is often weakly sustained. I really think there is potential for more robust conclusions if further analysis were carried out by adding a longer time series from the reanalysis to put this winter, and other mild winters in context. This would allow to generalize your conclusions.
Due to this, I think that the paper can be accepted after minor revisions, but it would be a better paper with major revisions.
General comments
Abstract :
I didn’t really understand if the Cap de Creus Canyos is “only a partial sink of cascading waters” or if “remarkable dense shelf water and sediment transport occurs in the Cap de Creus Canyon,..., even during mild winters ”. Isn’t this a bit contradictory? Or maybe I’m missing the difference between these transports. In any case, please clarify. This is a question that remained even after reading the full manuscript.
Methods:
The interpolation method used in the sections should be stated. The figures look a bit weird and I think it might be an interpolation issue.
Particular comments
L51. What “it” makes reference to?
L74-75. More prevalent than the extreme ones, thus, reducing overall DSWC over time? Or more prevalent than the “no DSWC scenario”, thus, increasing overall DSWC over time?
L99-101. I’d remove: "which was monitored during the FARDWO-CCC1 cruise, and simultaneous measurements as its adjacent shelf acquired survey as part of the MELANGE-DUNES experiment" from here as it’s too much detail for the introduction
L118. Export of what? Just precise
L129. What do you mean with “the concentration of water”? Are you refering to the residence time? Please rewrite, the term is awkward.
L.136. The full water column gets mixed? It would be surprising.
L.149. 300-400 m is the upper limit I guess, above which stratification prevents the full mixing of the water column? In that case that would rather be a re-stratification, because DSW forms from the surface forcing, and the a light water layer develops in the surface. Is that it?
L.151. Gain
L.164-165. However, all the point of TEOS10 is to promote the use of the more adequate conservative temperature and absolute salinity instead.
L.193-194. But what’s the range of the bottom depth?
L.216. Data is a plural noun: “Data were..”
L.226-228. What type of data were used? Is it discharge volume?
L.286. Low compared to what? Give a reference please.
L.287. That’s kind of surprising the existence of a storm that is not cause by strong winds, isn’t it? Can you provide an explanation?
L292. This is also surprising!
L.293. Low compared to which reference value?
Fig 3. It would be better to inverse the y-axis for density, so the densest water corresponds to the bottom layers.
L319 and throughout the manuscript. It would be better to refer to the Moose stations by their location instead of LDC or CCC, which is complicated to remember.
L.336. Compared to what reference values? (please provide references whenever you state that XX values are low or high).
Fig 5. Please avoid the used of divergent color maps for non-divergent fields as in the left column. This is misleading. Also, I’d personally prefer to see latitude instead of distance in the x-axis. I think it helps the readers to know where they are.
L341. This information belongs to methods. I actually missed it when I read it.
L.340-350. I suggest to better indicate what is from glider and what from cruise. It took me a moment to understand.
Fig 6: The color bars for panels f and i are not the same, even if they have the same limits and correspond to the same variables, which is misleading and makes comparison difficult.
L430. However, the discharge was low this winter, and dense water forms other years. This makes me think that this is not a reason to justify the low density.
L.432-435. I can’t really see a decrease in density, which makes me think that river discharge is not a key factor.
Fig 8. Wouldn’t it be better to plot bottom density in order to identify dense water? Also, please change the color map for a non-divergent one. This one is misleading.
L.446-447. As I said above, we cannot judge if the values are low or high if we don’t have references.
L479. Suggest.
L489. Flows.
L.500-510. This paragraph should definitely go to Methods and not in the discussion.
L.513. 0.05 Sv is practically zero, taking into account the strong variability. I actually would say the mean is negative? Have the authors double checked this mean? In any case, given the difference in the T1 and T2 value, I would not define the Cap de Creus Canyon as a partial sink, it is rather not at sink during mild winters. Wether or not this canyon is a sink, or export occurs through it remains confusing to me throughout the manuscript.
L519-520. You state you used the reanalysis “to assess the variability of dense shelf water export in the Cap de Creus Canyon during the mild winter of 2021-2022.” but the computation spans the October-May period, so, beyond winter.
L.525. I miss having some numbers to compare the reanalysis with the observations and quantify how well they match. You should plot the same variable for the T1 and T2 transects, integrated over the same depths. You could event add a line for the value of each variable in your observations. This would provide robustness to the reanalysis results.
L.546. “relatively weak wind forcing”.
L.560-562. How was this percentage estimated? I’m a bit confused. When we say export, I think about the water transport down-canyon to reach deeper depths, if water doesn’t get to leave the shelf I wouldn’t call it export. Throughout the manuscript the authors state (and the transport numbers suggest) that the actual export is very weak. I would like to know how these percentage were computed and, as asked before, what are the reference values in Sv (for instance a climatological mean, or the typical values in strong winters) for transport.
- AC1: 'Reply on RC2', Marta Arjona-Camas, 04 Jul 2025
- AC2: 'Reply on RC1', Marta Arjona-Camas, 04 Jul 2025
Data sets
CC1000 observatory data A. Sanchez-Vidal et al. https://doi.org/10.17882/104746
BILLION observatory data X. Durrieu de Madron et al. https://doi.org/10.17882/45980
CTD and ADCP data collected during the cruise FARDWO CC1 A. Sanchez-Vidal et al. https://doi.org/10.17882/105499
Glider data F. Bourrin https://data-selection.odatis-ocean.fr/coriolis/uri/p83112098
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