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The fate of plastics entering the 3D ocean circulation from rivers discharge is examined through the Lagrangian analysis of neutrally buoyant particles. Particles are released continuously over 1991–2010 at the surface along the coasts according to monthly estimates of rivers plastic waste input. They are advected by daily currents from a state-of-the-art global ocean model at 1/12° resolution. At the end of the simulation (year 2010), particles remaining in the surface layer of 1 m thickness represent less than 2% of the total particles released. These are concentrated in the center of subtropical gyres, mostly in the South Indian Ocean, and the North Pacific, in relation with the large sources from Asia, and in good agreement with previous 2D numerical experiments in the surface layer. These patterns remain similar down to about 30 m depth, this upper layer strongly influenced by Ekman currents trapping about 20% of the total released particles. About 50% of the total released particles remain in the upper 100 m, and up to 90% are found in the upper 400 m at the end of the experiment. Below the mixed layer, they are more widely dispersed horizontally and follow the main global pathways of ocean ventilation of mode and deep water masses. Plastic particles, neutrally buoyant because of their small size or biofouling, are thus expected to be strongly dispersed in the global ocean thermocline following mode waters patterns, and reach the deeper layers following the North Atlantic Deep Water formation path. Two major source regions have a global impact. Particles from the western North Pacific spread over the whole Pacific Ocean poleward of 20°S, whereas particles from Indonesia spread over the whole latitude band from 60°S to 20°S. Also see: https://micro2022.sciencesconf.org/427804/document, In MICRO 2022, Online Atlas Edition: Plastic Pollution from MACRO to nano

Satellite altimetry has revealed the sea surface signature of a wide range of processes and scales, from tides to multidecadal trends. While ocean warming and continental ice melt are the main drivers of global mean sea level changes, regional sea level variability also responds to fluctuations in salinity, ocean circulation, air-sea fluxes. Temperature has been identified as the main driver of regional sea level trends, but less attention has been paid to salinity impacts mainly because of the lack of historical in situ data. Recent improvements have been made in estimating continental freshwater discharges especially from Greenland and rivers, and in particular their interannual variability. Thus, it is now possible to investigate the impacts of fully-varying freshwater discharges on regional sea level and ocean circulation/dynamics. Realistic, fully-variable freshwater discharges data have been implemented in a global 1/4° ocean/sea-ice/icebergs general circulation model based on NEMO. Global sensitivity runs have been produced over 1980-2018 using 2 sources of freshwater runoffs: (i) solid/liquid discharges from Greenland, (ii) river runoffs. These 2 sources are independently set to climatological or fully variable forcings in successive simulations. In this presentation, we will present recent results (from the IMHOTEP OST/ST project) on Greenland freshwater discharge on regional sea level trends. We find that Greenland discharge may impact regional sea level trends in the Beaufort gyre over 1998-2011 with values larger than 10 mm/yr. We find that the halosteric contribution drives these trend differences located in the the upper 300m. Salinity budgets will be investigated to understand the sources and origin of such a freshening and whether the Greenland is directly responsible for such regional sea level changes.

The main goal of the IMHOTEP OST-ST project is to investigate the response of the global ocean to fully-varying freshwater discharges from rivers and from the Greenland ice sheet between 1980 and 2018. This response concerns sea surface salinity (SSS), regional sea surface height (SSH) and its manometric/steric components, heat and freshwater contents, water mass properties, regional to basin-scale circulation, etc. We are currently analyzing satellite/in-situ observations and the outputs from a recent series of NEMO-based global ocean/sea-ice/iceberg 1/4° simulations where runoffs are individually set to climatological and fully-variable in various source regions. The JRA55 fully-variable atmospheric forcing is applied in all simulations without any SSS relaxation, which is replaced by a precipitation correction term that lets salinity free to evolve. Daily river discharge forcing data were derived from the ISBA-CTRIP hydrological reanalysis, and monthly (solid and liquid) freshwater discharges from Greenland are derived from satellite altimetry data (SAR mode) combined with regional climate simulations (Mouginot et al., 2019). Special attention was dedicated to the bathymetry and the partitionning between solid and liquid discharges all around Greenland, in order to realistically feed icebergs and freshwater plumes. This presentation is focused on the interannual variability and 40-year trends of SSS in response to fluctuating runoffs at global scale, based on 5 simulations and a comparison to an ESA L4 product merging SMOS, Aquarius and SMAP SSS data (Boutin et al., 2021). Results show that the simulated SSS interannual variability compares well with observations. River runoffs directly force interannual SSS variability which can reach 0.5 to 0.75 g/kg in specific regions (Amazon plume, Siberian coast, Maritime Continent, China river mouths, etc) and about 0.2 g/kg along many coasts, with substantial penetration into the open ocean in several regions. Substantial (positive or negative) 40-year trends in the runoffs of key rivers also have a direct impact on regional SSS trends, which exhibit substantial freshening trends off Siberian coasts and throughout the Maritime Continent, and salinization off the equatorial African west coast in particular. These first results also suggest that SSS interannual variability is primarily driven by atmospheric and runoff fluctuations; this is quite different for the interannual variability of SSH and of many other oceanic climate indices, which are strongly impacted by chaotic intrinsic variability (CIV) in addition to the former two external drivers. Assessing the interannual response of these latter oceanic fields to fluctuating runoffs thus require an attenuation of this low-frequency CIV, which in turn requires ensemble ocean simulations with slightly different initial states (as done during the OCCIPUT project, https://meom-group.github.io/projects/occiput/). The IMHOTEP project team will perform in 2022 two 10-member ensembles of global ocean/sea-ice 1/4° simulations driven by the same JRA55 forcing, one ensemble with climatological and one without interannually-varying runoffs: comparing the fluctuations of ensemble means of key variables will allow a more complete analysis of the multivariate and multi-scale response of the ocean to fluctuating runoffs, regardless of CIV. These analyses will provide direct insight for the interpretation of signals observed by satellite and in situ platforms, and their attribution to distinct drivers.

Trabajo presentado en la Ocean Sciences Meeting, celebrada en San Diego del 16 al 21 de febrero de 2020., One of the aims of the SWOT satellite mission is to resolve sea surface height (SSH) at wavelengths down to 15km in the global oceans. In practice, whether SSH signals can be observed at a given wavelength will depend on the relative amplitude of the level of SSH variance and the level of instrumental and geophysical noise at that scale. Several methods have been proposed for mitigating the impact of instrumental noise in the SWOT swath but large uncertainties remain as to what will be the finest scales that SWOT SSH data will resolve in different regions and seasons. This is particularly due to the uncertainty as to how fast unbalanced motions (as for instance internal tides and inertia gravity waves) will affect SSH signals at different scales and times, and therefore affect the performance of existing denoising algorithms. Strong seasonal variations of the finest resolvable scale are also expected because of the strong but opposite seasonality of balanced submesoscale dynamics and fast unbalanced motions. In this study we use a collection of ocean model simulations and advanced denoising algorithms to document what scales SWOT SSH data will resolve in the global oceans. We use pseudo-SWOT SSH data drawn from hourly output of three high-resolution, tide-resolving, basin-to-global scale simulations based respectively on the MITgcm, HYCOM and NEMO ocean models. Our study focuses on different regions and seasons in order to document the variety of regimes that SWOT will actually sample. Our results show that existing denoising algorithms perform well even in the presence of intense unbalanced motions. We show that using denoising algorithms therefore systematically increase the largest resolvable wavenumber. Advanced denoising algorithms also allow to reliably reconstruct SSH gradients (related to geostrophic velocities) and second order derivatives (related to geostrophic vorticity) on the SWOT swath down to the finest resolvable scale. Our results also show that a significant uncertainty remains as to what will be SWOT’s finest resolved scale in a given region and season because of the large spread in the level of variance predicted among our high-resolution ocean model simulations.