Clustering of Marine‐Debris‐ and Sargassum‐Like Drifters Explained by Inertial Particle Dynamics

Abstract

Drifters designed to mimic floating marine debris and small patches of pelagic Sargassum were satellite tracked in four regions across the North Atlantic. Though subjected to the same initial conditions at each site, the tracks of different drifters quickly diverged after deployment. We explain the clustering of drifter types using a recent Maxey-Riley theory for surface ocean inertial particle dynamics applied on multidata-based mesoscale ocean currents and winds from reanalysis. Simulated trajectories of objects at the air-sea interface are significantly improved when represented as inertial (accounting for buoyancy and size), rather than as perfectly Lagrangian (fluid following) particles. Separation distances between simulated and observed trajectories were substantially smaller for debris-like drifters than for Sargassum-like drifters, suggesting that additional consideration of its physical properties relative to fluid velocities may be useful. Our findings can be applied to model variability in movements and distribution of diverse objects floating at the ocean surface. Plain Language Summary Predicting the fate of floating matter requires one to recognize that they respond differently than Lagrangian (i.e., infinitesimally small, neutrally buoyant) particles to the action of surface currents and winds. Indeed, the Maxey-Riley equation of fluid mechanics-a Newton second-law-type ordinary differential equation-shows that even the motion of seemingly small neutrally buoyant particles immersed in a fluid in motion can substantively deviate from that of Lagrangian particles. The Maxey-Riley equation has been recently extended to account for the combined effects of ocean current and wind drag on finite-size particles floating at the ocean surface. We show here that the paths of drifters that mimic marine debris and small Sargassum patches cluster according to their inertial characteristics consistent with the Maxey-Riley theory.

Monitoring pelagic Sargassum inundation potential for coastal communities

“…The present implementation of SIR assumes inundation potential based on the FA density value within 50 km of coastal regions. The amounts of Sargassum likely to wash ashore depend upon the complex interaction among ocean currents, surface winds, waves, tides, and the physical properties of the Sargassum aggregation in ways that are not yet fully understood (Beron-Vera and Miron 2020; Miron et al 2020;Olascoaga et al 2020;Putman et al 2020). Thus, at this stage, SIR assumes that coastal areas flanked by large amounts of Sargassum have greater inundation potential than areas where less Sargassum is nearby.…”

“…Thus, increased spatiotemporal resolution of SIR needs to be further enhanced by accounting for the role of ocean currents, winds, and waves on the movement of Sargassum. Initial work to model trajectories of pelagic Sargassum (Putman et al 2020) and drifters designed to mimic small patches of Sargassum (Miron et al 2020;Olascoaga et al 2020) has already been conducted.…”

“…Locations of coastal areas are shown in Figure 1. winds, waves, and currents and estimate how likely it is for Sargassum to wash ashore given the prevailing conditions. Given the positive buoyancy of Sargassum accounting for winds may be especially important to predict its movement into and within coastal areas (Miron et al 2020;Olascoaga et al 2020;Putman et al 2020). However, near coastal areas, the spatial heterogeneity and temporal variability of wind effects are higher and often affected by bathymetry, tides, local currents among other processes.…”

“…Changes in wind patterns, such as related to the North Atlantic Oscillation, and nutrient availability at the ocean surface likewise play important roles in the basin-scale distribution and abundance of Sargassum (Brooks et al 2018;Wang et al 2019;Berline et al 2020;Johns et al 2020). At meso-spatial scales and synoptic temporal scales, recent experiments tracking mats of Sargassum and Sargassum-like drifters demonstrate that winds contribute additional velocity to floating Sargassum, altering its trajectory relative to water currents (Miron et al 2020;Olascoaga et al 2020;Putman et al 2020). Accounting for wind and other inertial effects produces better predictions of Sargassum movement (Brooks et al 2019;Putman et al 2020).…”

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“…Varias hipótesis se han propuesto para explicar tales eventos, incluyendo un exceso de cargas de nutrientes, el cambio en las corrientes y temperaturas de la superficie del mar, inusualmente elevadas (Djakouré et al, 2017;Miron et al, 2020). Muchos de estos estudios sugieren que: 1)-Los florecimientos de Sargassum han sido producto de las descargas anómalas de nutrientes a través del río Amazonas y están relacionadas con la deforestación río arriba; 2)-Los cambios en los florecimientos del noroeste de África, causados por el aumento de la aridez en el continente y los vientos alisios más fuertes del NE indujeron un afloramiento o surgencia costera estacional intensiva; 3)-Los cambios en las surgencias, en mar abierto las que se relacionan con los cambios en las corrientes oceánicas anormales y los patrones de viento relacionados con el cambio climático; 4)-Cambios en la frecuencia o alteración de los patrones de deposición de polvo africano, son los responsables de este fenómeno o bien la combinación de los factores anteriormente mencionados (Sissini et al, 2017;Oviatt et al, 2019;.…”