the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Transient Attracting Profiles in the Great Pacific Garbage Patch
Abstract. A major challenge for cleanup operations in the Great Pacific Garbage Patch is the daily prediction of plastic concentrations that allows to identify hotspots of marine debris. Lagrangian simulations of large particle ensembles are the method in use and effectively reproduce observed particle distributions at synoptic scales 𝒪 (1000 km). However, they lose accuracy at operational scales 𝒪 (1−10 km) and operators regularly experience differences between predicted and encountered debris accumulations within the garbage patch. Instead of asking Where do objects go as they follow the current? as in Lagrangian methods, we here take a different approach and question Which locations attract material?. The recently developed concept of Transient Attracting Profiles (TRAPs) provides answers to this since it allows to detect the most attractive regions of the flow. TRAPs are the attractive form of hyperbolic Objective Eulerian Coherent Structures and can be computed from the instantaneous strain field on the ocean surface. They describe flow features that attract drifting objects and could facilitate offshore cleanups which are currently taking place in the Great Pacific Garbage Patch. However, the concept remains unapplied since little is known about the persistence and attractive properties of these features in the Pacific. Therefore, we compute a 20-years dataset of daily TRAP detections from satellite-derived mesoscale velocities within the North Pacific subtropical gyre. We are the first to track these instantaneous flow features as they propagate through space and time. This allows us to study the life cycle of TRAPs which can range from days to seasons and on average lasts for Λ ≈ (6 ± 12) days. We show how long-living TRAPs with lifetimes of Λ > 30 days intensify and weaken over their life cycle and demonstrate that the evolution stage of TRAPs affects the motion of nearby surface drifters. Our findings indicate that operators in the Great Pacific Garbage Patch should search for long-living TRAPs that are at an advanced stage of their life cycle. These TRAPs are most likely to induce large-scale confluence of drifting objects and their streamlining into hyperbolic pathways. Such a streamlined bypass takes on average φ ≈ (5.3 ± 3.8) days and could be exploited to filter the flow around TRAPs. But we also find TRAPs that retain material over multiple weeks where we suspect material clustering at the submesoscale, prospective research could investigate this with soon available high-resolution observations of the flow. Eventually, our research may also benefit other challenges that are related to the search at sea, such as optimal drifter deployment, the identification of foraging hotspots or humanitarian search and rescue.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-1215', Anonymous Referee #1, 28 May 2024
The paper identifies TRAPs – objectively defined (frame-independent) analogs of hyperbolic points, in the eastern N. Pacific using a 20-year long record of mesoscale surface currents derived from SSH altimetry and wind-induced Ekman estimates. The authors present statistics of TRAPS – lifetime, propagation speed, strength of attraction, probability of occurrence etc., investigate the relative vorticity patterns around TRAPs, and look for the influence of TRAPs on real drifters passing nearby by constructing composite maps of drifter velocities in the vicinity of TRAPs.
The results suggest that TRAPs are most commonly located in regions between mesoscale eddies, have similar westward propagation speeds to mesoscale eddies, and are associated with the hyperbolic flow geometry in their vicinity. The most typical vorticity patterns around TRAPs correspond to a quadrupole – 2 cyclonic and 2 anticyclonic vortices, with a TRAP at the center between them.
The paper is easy to read and the statistics is carefully estimated and clearly presented.
My main comments are:
- The mesoscale currents used for the analysis are likely only marginally applicable for predicting the small-scale distribution of garbage on a day-to-day basis and thus would only marginally help in any real cleanup efforts, which is the main motivation that the authors give for this work. I am not convinced that TRAPs and their statistics would be unchanged if the submesoscale flow features were resolved. This should be clearly explained, so as not to overstate the usability of TRAPs for real garbage cleanup.
- There are several subjective choices that likely affect the statistical results. First, limiting traps to 1 deg arclength seems both arbitrary and unnecessary. I don’t think it is necessary to put any limit on TRAPs lengths. It would be better to identify the full extent of a TRAP line length and analyze its statistics. Second, a 75 km radius was used for the pairing of drifters and TRAPs. Again, this seems like an arbitrary and unnecessary choice. It would be better to define the area of influence for each TRAP around the local minimum, perform statistical analysis of the basin of influence, and use the basin of influence for pairing with drifters.
- The authors seem to suggest that drifters (and other floating objects like garbage) are more likely to be found near TRAPs than in other regions of the flow. However, I don’t think there is any confirmation of this claim in the paper. It would be interesting to compare the statistics for the drifters occurrences and retention times within TRAPs to that within the mesoscale eddies and maybe even for a random subset of regions of comparable size and spatial distribution.
Citation: https://doi.org/10.5194/egusphere-2024-1215-RC1 - AC1: 'Reply on RC1', Luca Kunz, 05 Jul 2024
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RC2: 'Comment on egusphere-2024-1215', Anonymous Referee #2, 30 May 2024
Please find my review in the attached document.
- AC2: 'Reply on RC2', Luca Kunz, 05 Jul 2024
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AC3: 'Reply on RC2', Luca Kunz, 05 Jul 2024
We highlight that our reply to minor comment 5 refers to Section 2 of our response to Referee #1. Similarly, our reply to minor comment 6 refers to Section 1 of our response to Referee #1. We would like to point Referee #2 to these sections in our reply to Referee #1.
Citation: https://doi.org/10.5194/egusphere-2024-1215-AC3 - AC1: 'Reply on RC1', Luca Kunz, 05 Jul 2024
Data sets
Datasets to Transient Attracting Profiles in the Great Pacific Garbage Patch Luca Kunz https://doi.org/10.5281/zenodo.10993736
Model code and software
Track and analyse Transient Attracting Profiles in the Great Pacific Garbage Patch Luca Kunz https://github.com/kunzluca/trapsgpgp
Video supplement
Supplementary Videos to Transient Attracting Profiles in the Great Pacific Garbage Patch Luca Kunz https://doi.org/10.5281/zenodo.10943728
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