[1] The space-time structure of long-period ocean swell fields is investigated, with particular attention given to features in the direction orthogonal to the propagation direction. This study combines space-borne synthetic aperture radar (SAR) data with numerical model hindcasts and time series recorded by in situ instruments. In each data set the swell field is defined by a common storm source. The correlation of swell height time series is very high along a single great circle path with a time shift given by the deep water dispersion relation of the dominant swells. This correlation is also high for locations situated on different great circles in entire ocean basins. Given the Earth radius R, we define the distance from the source Ra and the transversal angle b so that a and b would be equal the colatitude and longitude for a storm centered on the North Pole. Outside of land influence, the swell height field at time t, H ss (a, b,t) is well approximated by a function H ss,0 (t -Ra/C g )/ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sin ð Þ ð Þ p times another function r 2 (b), where C g is a representative group speed. Here r 2 (b) derived from SAR data is very broad, with a width at half the maximum that is larger than 70°, and varies significantly from storm to storm. Land shadows introduce further modifications so that in general r 2 is a function of b and a. This separation of variables and the smoothness of the H ss field, allows the estimation of the full field of H ss from sparse measurements, such as wave mode SAR data, combined with one time series, such as that provided by a single buoy. A first crude estimation of a synthetic H ss field based on this principle already shows that swell hindcasts and forecasts can be improved by assimilating such synthetic observations.

[1] A three-dimensional wave-current model is used to investigate wave-induced circulations in a small estuarine bay and its impact on freshwater exchanges with the inner shelf, related to stratified river plume dispersion. Modeled salinity fields exhibit a lower salinity surface layer due to river outflows, with typical depth of 1 m inside the bay. The asymmetric wave forcing on the bay circulation, related to the local bathymetry, significantly impacts the river plumes. It is found that the transport initiated in the surf zone by the longshore current can oppose and thus reduce the primary outflow of freshwater through the bay inlets. Using the model to examine a high river runoff event occurring during a high-energy wave episode, waves are found to induce a 24 h delay in freshwater evacuation. At the end of the runoff event, waves have reduced the freshwater flux to the ocean by a factor 5, and the total freshwater volume inside the bay is increased by 40%. According to the model, and for this event, the effect of the surf zone current on the bay flushing is larger than that of the wind. The freshwater balance is sensitive to incident wave conditions. Maximum freshwater retention is found for intermediate offshore wave heights 1 m < H s < 2 m. For higher-energy waves, the increase in the longshore current reduces the retention, which is two times lower for H s 5 4 m than for H s 5 2 m.

No abstract

Marine litter is identified as the major ocean pollution today, with a significant impact on the environment. The transport of litter by ocean circulation is still poorly understood, and the management of marine litter remains a major challenge for local authorities. This work investigates the transport of floating marine litter (FML) in the coastal area of the south-eastern Bay of Biscay. It provides a first assessment of the use of operational ocean observations and models for the support of FML management in this region. The study is based on a Lagrangian modelling approach, using ocean surface velocity forcing by (i) measurements with High-Frequency Radars and (ii) predictions from the IBI Copernicus model. The IBI model is first assessed in a Lagrangian perspective using different observations of surface ocean currents and transport. Then, analyzing a multiyear hindcast of the Lagrangian model, high retention is shown in the southeast during spring and summer. Northward dispersion is obtained along the French coast during autumn and winter. In winter, large river inflows are not compensated by the inner shelf transport, resulting in the highest material densities along the coast. The results also suggest that the Iberian Poleward Current may act as a cross-shore transport barrier in the area.

Large scale convergence regions of floating marine litter are commonly observed in semi-enclosed seas as the Bay of Biscay. However, clean-up activities on such accumulation regions are limited by the spread of the large-size floating litter on the sea surface. Data gathered by a small-scale fishing vessel devoted to active fishing for floating litter activities during the spring and summer of 2018 reveals that the linear streaks of high concentration of floating litter (so-called litter "windrows") are common accumulation structures in the south-east coast of the Bay of Biscay. The random search of litter windrows for their collection through surface tows of macro-nets was proved to be an effective action for floating litter mitigation. A total of 196 tows collected 16.2 tons of floating marine litter in 68 working days. Most of the litter windrows were around 1 km length and, on average, accumulated 77.75 kg of floating marine litter. Fishing, shipping and aquaculture sectors were the source of 35% of the 4,130 litter items analyzed (55% in weight of the sourced items), and plastic was the most common type of material (96% in terms of items). A better understanding of the phenomenon of the litter windrows, capable to guide clean-up efforts in space and time, would provide a considerable improvement in the efficiency of mitigation actions to reduce the marine litter pollution. The observations of litter windrows in the coastal area of the south-east of the Bay of Biscay demonstrate the key role of submesoscale processes in the distribution of FML. The present work provides a thorough description of floating litter windrows in nature, which it was non-existent to date. The results are the kind of proof necessary to boost the research addressed on the submesoscale aggregations of FML. Coupling litter windrows observations with remote-sensing technology and high-resolution modeling techniques offer great opportunities for the mitigation actions against marine litter.