Insight from PREFACE & AWA on Tropical Atlantic Tuna ecology and effects on western African fisheries economies

International audienc

Population dynamics in compressible flows

Organisms often grow, migrate and compete in liquid environments, as well as on solid surfaces. However, relatively little is known about what happens when competing species are mixed and compressed by fluid turbulence. In these lectures we review our recent work on population dynamics and population genetics in compressible velocity fields of one and two dimensions. We discuss why compressible turbulence is relevant for population dynamics in the ocean and we consider cases both where the velocity field is turbulent and when it is static. Furthermore, we investigate populations in terms of a continuos density field and when the populations are treated via discrete particles. In the last case we focus on the competition and fixation of one species compared to anotherComment: 16 pages, talk delivered at the Geilo Winter School 201

Vortex merger near a topographic slope in a homogeneous rotating fluid

This work is a contribution to the PHYSINDIEN research program. It was supported by CNRS-RFBR contract PRC 1069/16-55-150001.The effect of a bottom slope on the merger of two identical Rankine vortices is investigated in a two dimensional, quasi-geostrophic, incompressible fluid. When two cyclones initially lie parallel to the slope, and more than two vortex diameters away from the slope, the critical merger distance is unchanged. When the cyclones are closer to the slope, they can merge at larger distances, but they lose more mass into filaments, thus weakening the efficiency of merger. Several effects account for this: the topographic Rossby wave advects the cyclones, reduces their mutual distance and deforms them. This along shelf wave breaks into filaments and into secondary vortices which shear out the initial cyclones. The global motion of fluid towards the shallow domain and the erosion of the two cyclones are confirmed by the evolution of particles seeded both in the cyclone sand near the topographic slope. The addition of tracer to the flow indicates that diffusion is ballistic at early times. For two anticyclones, merger is also facilitated because one vortex is ejected offshore towards the other, via coupling with a topographic cyclone. Again two anticyclones can merge at large distance but they are eroded in the process. Finally, for taller topographies, the critical merger distance is again increased and the topographic influence can scatter or completely erode one of the two initial cyclones. Conclusions are drawn on possible improvements of the model configuration for an application to the ocean.PostprintPeer reviewe

Variability of Iberian upwelling implied by ERA-40 and ERA-Interim reanalyses

The Regional Ocean Modeling System ocean model is used to simulate the decadal evolution of the regional waters in offshore Iberia in response to atmospheric fields given by ECMWF ERA-40 (1961–2001) and ERA-Interim (1989–2008) reanalyses. The simulated sea surface temperature (SST) fields are verified against satellite AVHRR SST, and they are analysed to characterise the variability and trends of coastal upwelling in the region. Opposing trends in upwelling frequency are found at the northern limit, where upwelling has been decreasing in recent decades, and at its southern edge, where there is some evidence of increased upwelling. These results confirm previous observational studies and, more importantly, indicate that observed SST trends are not only due to changes in radiative or atmospheric heat fluxes alone but also due to changes in upwelling dynamics, suggesting that such a process may be relevant in climate change scenarios

An approach for the identification of exemplar sites for scaling up targeted field observations of benthic biogeochemistry in heterogeneous environments

Continental shelf sediments are globally important for biogeochemical activity. Quantification of shelf-scale stocks and fluxes of carbon and nutrients requires the extrapolation of observations made at limited points in space and time. The procedure for selecting exemplar sites to form the basis of this up-scaling is discussed in relation to a UK-funded research programme investigating biogeochemistry in shelf seas. A three-step selection process is proposed in which (1) a target area representative of UK shelf sediment heterogeneity is selected, (2) the target area is assessed for spatial heterogeneity in sediment and habitat type, bed and water column structure and hydrodynamic forcing, and (3) study sites are selected within this target area encompassing the range of spatial heterogeneity required to address key scientific questions regarding shelf scale biogeochemistry, and minimise confounding variables. This led to the selection of four sites within the Celtic Sea that are significantly different in terms of their sediment, bed structure, and macrofaunal, meiofaunal and microbial community structures and diversity, but have minimal variations in water depth, tidal and wave magnitudes and directions, temperature and salinity. They form the basis of a research cruise programme of observation, sampling and experimentation encompassing the spring bloom cycle. Typical variation in key biogeochemical, sediment, biological and hydrodynamic parameters over a pre to post bloom period are presented, with a discussion of anthropogenic influences in the region. This methodology ensures the best likelihood of site-specific work being useful for up-scaling activities, increasing our understanding of benthic biogeochemistry at the UK-shelf scale

International conference ICAWA 2016 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

Mesoscale physical processes can affect the fate and distribution of fish eggs and larvae by aggregating and retaining them in favorable habitats or spreading them away to inappropriate areas. Here we examined the effect of fine scale processes on fish larval retention through a biophysical model of the early life stages of Sardinella aurita taken as an example within the Senegalese upwelling region. A set of 3 ROMS configurations is used from coarser (~ 9km, 7.5km) to finer (~ 2km) resolution. The ICHTHYOP individual based model is then forced by the different model experiments. To take into account more realistic physical forcing, the finer resolution configuration was used to test the effect of synoptic winds and tides. We first show that a finer resolution of physical processes provides a retention twice stronger. In addition that experiment shows 2 contrasted results in the region which differ from previous modelling work : 1) the northern domain considered so far as a low retention area displays high retention values and 2) a better resolution of the hydrodynamic of the Senegalese shelf modifies significantly the retention spatial patterns in the southern area although the seasonality remains similar. Simulation using synoptic winds and tidal forcing suggest a reduction of retention induced by a higher larval mortality due to enhanced export transport and stronger lethal temperature exposure

Regional consequences of climate changes on the West African coastal environment : identifying knowledge gaps [résumé]

ICAWA : International Conference AWA, Dakar, SEN, 13-/12/2016 - 15/12/2016Climate change will impact the coastal socio-ecosystems in various ways. This will be particularly true along the West African shores where no specific regional projection exercise has been carried out so far. We propose to present 1) the type of approaches that have been implemented for other coastal regions, e.g. in other upwelling systems to gain insight into the regional/local consequences of climate change; 2) the needs of regional stakeholders; 3) the scientific programs that could address these needs

Submesoscale activity over the Argentinian shelf

Submesoscale activity over the Argentinian shelf is investigated by means of high resolution primitive equation numerical solutions. These reveal energetic turbulent activity (visually similar to the one occasionally seen in satellite images) at scales O(5 km) in fall and winter that is linked to mixed layer baroclinic instability. The air-sea heat flux responsible for (i) deepening the upper ocean boundary layer (at these seasons) and (ii) maintaining a cross-shelf background density gradient is the key environmental parameter controlling submesoscale activity. Implications of submesoscale turbulence are investigated. Its mixing efficiency estimated by computing a diffusivity coefficient is above 30 m(2) s(-1) away from the shallowest regions. Aggregation of surface buoyant material by submesoscale currents occurs within hours and is presumably important to the ecosystem.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP[06/04106-4]FAPESP[06/03968-2]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP[IAI CRN2076]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)NSFNSF[GEO-0452325]CNPq[grant 307785/2004-1]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Regional consequences of climate changes on the West African coastal environment : identifying knowledge gaps [résumé]

ICAWA : International Conference AWA, Dakar, SEN, 13-/12/2016 - 15/12/2016Climate change will impact the coastal socio-ecosystems in various ways. This will be particularly true along the West African shores where no specific regional projection exercise has been carried out so far. We propose to present 1) the type of approaches that have been implemented for other coastal regions, e.g. in other upwelling systems to gain insight into the regional/local consequences of climate change; 2) the needs of regional stakeholders; 3) the scientific programs that could address these needs