Numerical simulations of marine surface particle dispersal are a crucial tool for addressing many outstanding issues in physical oceanography of 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 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 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 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 of advecting 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 the dominant impact of Stokes drift and tends to decrease the mean Lagrangian surface drift speed in summer through the dominant impact of wave-driven Eulerian currents. Furthermore, simulations that approximate the surface wave impact by including Stokes drift (but ignoring wave-driven Eulerian currents) do not necessarily yield better estimates of surface particle dispersal patterns with explicit wave impact representation 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.@en

Marine plastic debris floating on the ocean surface is a major environmental problem. However, its distribution in the ocean is poorly mapped, and most of the plastic waste estimated to have entered the ocean from land is unaccounted for. Better understanding of how plastic debris is transported from coastal and marine sources is crucial to quantify and close the global inventory of marine plastics, which in turn represents critical information for mitigation or policy strategies. At the same time, plastic is a unique tracer that provides an opportunity to learn more about the physics and dynamics of our ocean across multiple scales, from the Ekman convergence in basin-scale gyres to individual waves in the surfzone. In this review, we comprehensively discuss what is known about the different processes that govern the transport of floating marine plastic debris in both the open ocean and the coastal zones, based on the published literature and referring to insights from neighbouring fields such as oil spill dispersion, marine safety recovery, plankton connectivity, and others. We discuss how measurements of marine plastics (both in situ and in the laboratory), remote sensing, and numerical simulations can elucidate these processes and their interactions across spatio-temporal scales.

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Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.

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