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

An elastic-viscous-plastic sea ice model formulated on Arakawa B and C grids

The elastic-viscous-plastic (EVP) sea ice rheology has been introduced in the large-scale Louvain-la-Neuve sea-Ice Model, version 2 (LIM2), and its performance has been evaluated. Centred difference versions of the rheology have been implemented on both an Arakawa B grid and a C grid, and their performance have been intercompared in coupled simulations with the Nucleus for European Modelling of the Ocean (NEMO) model. Integrations with both implementations lead to fairly similar results which compare well with observations and with previous LIM simulations. The C grid version, however, offers a number of advantages: (a) easier ice coupling with NEMO, which is itself defined on a C grid; (b) possibility of representing ice transport across one-cell-wide straits and passages; (c) better representation of inertial-plastic compressive waves. For these reasons, we recommend the use of the C grid EVP formulation of the ice dynamics in future LIM applications. (C) 2009 Elsevier Ltd. All rights reserved

On the role of tides and strong wind events in promoting summer primary production in the Barents Sea

Tides and wind-driven mixing play a major role in promoting post-bloom productivity in subarctic shelf seas. Whether this is also true in the high Arctic remains unknown. This question is particularly relevant in a context of increasing Arctic Ocean stratification in response to global climatic change. We have used a three-dimensional ocean-sea ice-plankton ecosystem model to assess the contribution of tides and strong wind events to summer (June–August 2001) primary production in the Barents Sea. Tides are responsible for 20% (60% locally) of the post-bloom primary production above Svalbard Bank and east of the Kola Peninsula. By contrast, more than 9% of the primary production is due to winds faster than 8ms−1 in the central Barents Sea. Locally, this contribution reaches 25%. In the marginal ice zone, both tides and wind events have only a limited effect on primary production (<2%). Removing tides or winds faster than 8ms−1 promotes a regime more sustained by regenerated production with a f-ratio (i.e. the proportion of nitrate-based “new” primary production in the total primary production) that decreases by up to 26% (east of the Kola Peninsula) or 35% (central Barents Sea), respectively. When integrated over all Barents Sea sub-regions, tides and strong wind events account, respectively, for 6.8% (1.55TgC; 1TgC=1012gC) and 4.1% (0.93TgC) of the post-bloom primary production (22.6TgC). To put this in context, this contribution to summer primary production is equivalent to the spring bloom integrated over the Svalbard area. Tides and winds are significant drivers of summer plankton productivity in the Barents Se

Evaluation of a polynya flux model by means of thermal infrared satellite estimates

We test the ability of a two-dimensional flux model to simulate polynya events with narrow open-water zones by comparing model results to ice-thickness and ice-production estimates derived from thermal infrared Moderate Resolution Imaging Spectroradiometer (MODIS) observations in conjunction with an atmospheric dataset. Given a polynya boundary and an atmospheric dataset, the model correctly reproduces the shape of an 11 day long event, using only a few simple conservation laws. Ice production is slightly overestimated by the model, owing to an underestimated ice thickness. We achieved best model results with the consolidation thickness parameterization developed by Biggs and others (2000). Observed regional discrepancies between model and satellite estimates might be a consequence of the missing representation of the dynamic of the thin-ice thickening (e.g. rafting). We conclude that this simplified polynya model is a valuable tool for studying polynya dynamics and estimating associated fluxes of single polynya event

Simulating the mass balance and salinity of Arctic and Antarctic sea ice. 1. Model description and validation

This paper is the first part of a twofold contribution dedicated to the new version of the Louvain-la-Neuve sea ice model LIM3. In this part, LIM3 is described and its results are compared with observations. LIM3 is a C-grid dynamic-thermodynamic model, including the representation of the subgrid-scale distributions of ice thickness, enthalpy, salinity and age. Brine entrapment and drainage as well as brine impact on ice thermodynamics are explicitly included. LIM3 is embedded into the ocean modelling system NEMO, using OPA9, a hydrostatic, primitive equation, finite difference ocean model in the 2° × 2°cos[phi] configuration ORCA2. Model performance is evaluated by performing a hindcast of the Arctic and Antarctic sea ice packs, forced by a combination of daily NCEP/NCAR reanalysis data and various climatologies. The annual cycle of sea ice growth and decay is very realistically captured with ice area, thickness, drift and snow depth in good agreement with observations. In the Arctic, the simulated geographical distributions of ice thickness and concentration are significantly improved when compared with earlier versions of LIM. Model deficiencies feature an overestimation (underestimation) of ice thickness in the Beaufort gyre (around the North Pole) as well as an underestimation of ice thickness in the Southern Ocean. The simulated first year/multiyear sea ice limit agrees with observations. The values and distribution of sea ice age in the perennial ice zone are different from satellite-derived values, which is attributed to the different definitions of ice age. In conclusion, in light of the exhaustive sea ice analysis presented here, LIM3 is found to be an appropriate tool for large-scale sea ice and climate simulations

LIM3, an advanced sea ice model for climate simulation and operational oceanography

In this contribution, we briefly present the version 3 of the Louvain-la-Neuve Ice Model (LIM). The results of two 1970-2007 hindcasts performed with the ocean modelling system NEMO (Nucleus for European Modelling of the Ocean) -one using LIM2 and the other using LIM3 - are compared to available observations of ice concentration, thickness and mixed layer depth. LIM3 is found to significantly improve the simulation of sea ice characteristics compared to the earlier LIM2 version, making it a more appropriate and accurate tool not only in ice-ocean and climate simulations but also presumably for operational oceanography

Geostrophic Adjustment Problems in a Polar Basin

The geostrophic adjustment of a homogeneous fluid in a circular basin with idealized topography is addressed using a numerical ocean circulation model and analytical process models. When the basin is rotating uniformly, the adjustment takes place via excitation of boundary propagating waves and when topography is present, via topographic Rossby waves. In the numerically derived solution, the waves are damped because of bottom friction, and a quasi-steady geostrophically balanced state emerges that subsequently spins-down on a long time scale. On the f-plane, numerical quasi-steady state solutions are attained well before the system's mechanical energy is entirely dissipated by friction. It is demonstrated that the adjusted states emerging in a circular basin with a step escarpment or a top hat ridge, centred on a line of symmetry, are equivalent to that in a uniform depth semicircular basin, for a given initial condition. These quasi-steady solutions agree well with linear analytical solutions for the latter case in the inviscid limit. On the polar plane, the high latitude equivalent to the β-plane, no quasi-steady adjusted state emerges from the adjustment process. At intermediate time scales, after the fast Poincaré and Kelvin waves are damped by friction, the solutions take the form of steady-state adjusted solutions on the f-plane. At longer time scales, planetary waves control the flow evolution. An interesting property of planetary waves on a polar plane is a nearly zero eastward group velocity for the waves with a radial mode higher than two and the resulting formation of eddy-like small-scale barotropic structures that remain trapped near the western side of topographic features

The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model

We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic