The physical oceanography of the transport of floating marine debris

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

Toward the integrated marine debris observing system

Plastics and other artificial materials pose new risks to the health of the ocean. Anthropogenic debris travels across large distances and is ubiquitous in the water and on shorelines, yet, observations of its sources, composition, pathways, and distributions in the ocean are very sparse and inaccurate. Total amounts of plastics and other man-made debris in the ocean and on the shore, temporal trends in these amounts under exponentially increasing production, as well as degradation processes, vertical fluxes, and time scales are largely unknown. Present ocean circulation models are not able to accurately simulate drift of debris because of its complex hydrodynamics. In this paper we discuss the structure of the future integrated marine debris observing system (IMDOS) that is required to provide long-term monitoring of the state of this anthropogenic pollution and support operational activities to mitigate impacts on the ecosystem and on the safety of maritime activity. The proposed observing system integrates remote sensing and in situ observations. Also, models are used to optimize the design of the system and, in turn, they will be gradually improved using the products of the system. Remote sensing technologies will provide spatially coherent coverage and consistent surveying time series at local to global scale. Optical sensors, including high-resolution imaging, multi- and hyperspectral, fluorescence, and Raman technologies, as well as SAR will be used to measure different types of debris. They will be implemented in a variety of platforms, from hand-held tools to ship-, buoy-, aircraft-, and satellite-based sensors. A network of in situ observations, including reports from volunteers, citizen scientists and ships of opportunity, will be developed to provide data for calibration/validation of remote sensors and to monitor the spread of plastic pollution and other marine debris. IMDOS will interact with other observing systems monitoring physical, chemical, and biological processes in the ocean and on shorelines as well as the state of the ecosystem, maritime activities and safety, drift of sea ice, etc. The synthesized data will support innovative multi-disciplinary research and serve a diverse community of users

Barotropic wind-driven circulation patterns in a closed rectangular basin of variable depth influenced by a peninsula or an island

We study how a coastal obstruction (peninsula or coastal island) affects the three-dimensional barotropic currents in an oblong rectangular basin with variable bathymetry across the basin width. The transverse depth profiele is asymmetric and the peninsula or island lies in the middle of the long side of the rectangle. A semi-spectral model for the Boussinesq-approximated shallow water equations, developed in Haidvogel et al. and altered for semi-implicit numerical integration in time in Wang and Hutter, is used to find the steady barotropic state circulation pattern to external winds. The structural (qualitative) rearrangements and quantitative features of the current pattern are studied under four principal wind is inclination relative to the shore. The essentially non-linear relationships of the water flux between the two sub-basins (formed by the obstructing penisula) and the corresponding crosssectional area left open are found and analysed. It is further analysed whather the depth- integrated model, usually adopted by others, is meaningful when applied to the water exchange problems. The flow through the challel narrowing is quantitatively estimmated and compared with the three- dimensional results. The dynamics of the vortex structure and the indentification of the up-welling/down-welling zones around the obsrruction are discussed in detail. The influence of the transformation of the penisula into a coastal islang on the gloabal basin circulation is considered as are the currents in the channel. The geometric and physical reasons for the anisotropy of the current structure which prevail through all obtained solutions are also discussed

Wind-driven current simulations around the Island Mainau (Lake Constance)

Using three-dimensional numerical modelling for the shallow water equations on the rotating Earth in the Boussinesq approximation, we study the steady barotropic motion around the Island Mainau in Lake überlingen, forced by uniformly distributed winds of different directions. The method of substructuring is used to resolve the flow pattern near the Island Mainau with greater accuracy and thus to identify the peculiarities that are induced by the island as an obstruction to the current field within the lake basin. The barotropic response is analysed in detail for 16 different wind directions. It is shown to what extent these winds determine the distribution of the horizontal current and the up- and down-welling zones in the vicinity of the island. Current peculiarities, such as diverging and converging elements, locations of maximum current speeds and, in particular, the flow through the Mainau channel are identified. They provide hints to an optimal design of a flow measuring campaign under homogeneous conditions. It is further demonstrated that the island acts as an obstructing entity that effectively influences the flow within Lake Überlingen. For wind blowing along Lake Überlingen the baroclinic motion was also studied. The flow in the upper-layer and the lower-layer-return flow are modified over the lake

Down-slope cascading modulated by day/night variations of solar heating

Sloping sides of natural basins favour the formation of cross-shore temperature gradients (differential coastal heating/cooling), which cause significant littoral-pelagial water exchange. Autumnal denser water cascading along a sloping lake boundary, modulated by day/night variations of solar heating is considered numerically, in order to reveal the development of the cascading process in time, spatial structure of the exchange flows, and diurnal variations of volumetric flow-rate of littoral-pelagial exchange flow, as well as to compare its daily maxima at different depths/cross-sections, with known quasi-steady state predictions under constant buoyancy flux. The development of exchange flows progress through two phases: i) appearance and adjustment to day/night buoyancy flux variations; and ii) quasi-steady exchange, when variations of the flow rate in every next diurnal cycle are more or less the same as the previous day. The duration of the first phase depends on local depth (~1 day for depths of about 10 m, ~2 days for depths 15-25 m, and ~5 days down to 30 m for the considered initial linear vertical temperature stratification). Maximum horizontal exchange takes place in the cross-section where the thermocline meets the slope, and the cold down-slope currents detach from the bottom. The location of this cross-section advances off-shore with time, in accordance with the deepening of the upper mixed layer. The existence of a specific coastal circulation cell, with different water dynamics from those above the main part of the slope, is a characteristic feature of horizontal convective exchange. The mean value of the specific volumetric flow rate of the convective exchange, driven by day/night oscillations in its fully developed quasi-steady phase increases almost linearly with local depth, and is about twice as large as the quasi-steady exchange values, predicted by formula Q=0.0013·d1.37 (Q is measured in m2 s-1, and local depth above the slope d in m), suggesting that the thermal siphon, energized by oscillating day/night buoyancy fluxes, flushes coastal regions twice as efficiently as the cascading, developing under (more or less) uniform external conditions in field observations and laboratory experiments, which lie behind the given formula. Flushing time in the considered case has an order of 10-60 hours for a littoral zone of 6-30 m depth. Application of convective phase diagrams (e.g., Q vs ΔT) is suggested as a convenient way to describe the day/night convective exchange, allowing for visualization of the flow development process, its coherency and the time lag of the development at different depths