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.