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Abstract. Although the movement and aggregation of microplastics at the ocean surface have been well studied, less is known about the subsurface. Within the Maxey–Riley framework governing the movement of small, rigid spheres with high drag in fluid, the aggregation of buoyant particles is encouraged in vorticity-dominated regions. We explore this process in an idealized model that is qualitatively reminiscent of a 3D eddy with an azimuthal and overturning circulation. In the axially symmetric state, buoyant spherical particles that do not accumulate at the top boundary are attracted to a loop consisting of periodic orbits. Such a loop exists when drag on the particle is sufficiently strong. For small, slightly buoyant particles, this loop is located close to the periodic fluid parcel trajectory. If the symmetric flow is perturbed by a symmetry-breaking disturbance, additional attractors for small, rigid, slightly buoyant particles may arise near periodic orbits of fluid parcels within the resonance zones created by the disturbance. Disturbances with periodic or quasiperiodic time dependence may produce even more attractors, with a shape and location that recurs periodically. However, not all such loops attract, and rigid particles released in the vicinity of one loop may instead be attracted to a nearby attractor. Examples are presented along with mappings of the respective basins of attraction.

 

Abstract The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993–2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 10 6 m 3 s −1 ), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill. 

Motivated by recent evidence of Atlantic bluefin tuna spawning in the Slope Sea, we investigated the spatio-temporal distribution of oceanographic conditions that are conducive to successful spawning by bluefin in this region. Specifically, we considered advection patterns and water temperatures based on a new high-resolution ocean circulation model. After validating model velocities and temperatures using observations, three criteria were used to evaluate the success of simulated bluefin spawning during 2013: water temperature at spawning locations, mean water temperature along larval trajectories, and larval residence time within the Slope Sea. Analyses of satellite-based, decade-long (2008–2017) datasets suggest that conditions, specifically water temperatures and advection patterns, in the Slope Sea in 2013 were representative of typical years. The temperature criteria are more frequently satisfied in the southern and southwestern parts of the domain, whereas the residence time criterion favors more northern areas further from the Gulf Stream. The probability map of successful spawning locations shows a maximum near the northwestern bight of the Slope Sea. Spawning success is near-zero through most of June, increases in July, and peaks in early-to-mid August. Overall, water temperatures and retentive capabilities suggest that the Slope Sea provided suitable conditions for successful spawning of bluefin during 2013.