Coupled effects of tide and swell on water renewal in a meso-tidal channel lagoon: Case of the Toliara Lagoon (Madagascar)

International audienceIn this article, based on the example of the Toliara lagoon in Madagascar, we identified the major impacts of the two main drivers of the water renewal in a typical channel lagoon: the tide and the swell breaking on the reef. For this purpose, we applied the numerical model Croco which is based on ROMS-AGRIF and uses a realistic and high-resolution configuration. The validation was performed with in situ ADCP data, then, realistic and sensitivity simulations were run to infer tides and waves role in ocean-lagoon exchanges and to estimate water age and water origin. The modelling approach adopted enabled an understanding of the effects of swell and tide on the water renewal. During low wave conditions, tide and waves counteract each other on cross-reef flow: the cross-reef inflow tends to increase with waves breaking on the reef, whereas cross-reef flow tends to decrease or to be outflowing with tidal action. Hence, the lagoon-ocean exchange is reduced and is at a minimum when wave height reaches 0.85 m and the tide has minimum amplitude. During high wave conditions, both tide and waves increase ocean-lagoon exchanges by increasing the cross-reef inflow, whereas during low wave conditions, the cross-reef flow is mainly outward with increase with tidal amplitude and decrease with wave height until the threshold of 0.85 m

A model study of the seasonality of sea surface temperature and circulation in the Atlantic North-eastern Tropical Upwelling System

International audienceThe climatological seasonal cycle of the sea surface temperature (SST) in the north-eastern tropical Atlantic (7–25°N, 26–12°W) is studied using a mixed layer heat budget in a regional ocean general circulation model. The region, which experiences one of the larger SST cycle in the tropics, forms the main part of the Guinea Gyre. It is characterized by a seasonally varying open ocean and coastal upwelling system, driven by the movements of the intertropical convergence zone (ITCZ). The model annual mean heat budget has two regimes schematically. South of roughly 12°N, advection of equatorial waters, mostly warm, and warming by vertical mixing, is balanced by net air-sea flux. In the rest of the domain, a cooling by vertical mixing, reinforced by advection at the coast, is balanced by the air-sea fluxes. Regarding the seasonal cycle, within a narrow continental band, in zonal mean, the SST early decrease (from September, depending on latitude, until December) is driven by upwelling dynamics off Senegal and Mauritania (15–20°N), and instead by air-sea fluxes north and south of these latitudes. Paradoxically, the later peaks of upwelling intensity (from March to July, with increasing latitude) essentially damp the warming phase, driven by air-sea fluxes. The open ocean cycle to the west, is entirely driven by the seasonal net air-sea fluxes. The oceanic processes significantly oppose it, but for winter north of ~18°N. Vertical mixing in summer-autumn tends to cool (warm) the surface north (south) of the ITCZ, and advective cooling or warming by the geostrophic Guinea Gyre currents and the Ekman drift. This analysis supports previous findings on the importance of air-sea fluxes offshore. It mainly offers quantitative elements on the modulation of the SST seasonal cycle by the ocean circulation, and particularly by the upwelling dynamics

Effect of climatic variability on horse mackerel abundance in Senegalese waters

7-11<span style="font-size:9.0pt;font-family: " times="" new="" roman";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-gb;mso-fareast-language:en-us;mso-bidi-language:="" hi"="" lang="EN-GB">The investigation of climate variables impacts on small pelagic resources has increased rapidly in recent years. An observational sea surface temperature from the Advanced Very High Resolution Radiometer (AVHRR Pathfinder V5.0) satellite and the North Atlantic Oscillation index from 1999 to 2009 were used to investigate the abundance of horse mackerel in Senegalese waters using simple linear model and generalized additive models. The results show that the variability of horse mackerel abundance seems to be controlled by the sea surface temperature through the North Atlantic driven effect. The generalized additive model seems to explain better the link between climatic terms and horse mackerel abundance than the simple linear model.</span

SST patterns and dynamics of the southern Senegal-Gambia upwelling center

International audienceThe southern end of the Canary current system comprises of an original upwelling center that has so far received little attention, the Southern Senegal-Gambia Upwelling Center (SSUC). We investigate its dynamical functioning by taking advantage of favorable conditions in terms of limited cloud coverage. Analyses and careful examinations of over 1500 satellite images of sea surface temperature scenes contex-tualized with respect to wind conditions confirm the regularity and stability of the SSUC dynamical function-ing (as manifested by the recurrence and persistence of particular SST patterns). The analyses also reveal subtle aspects of its upwelling structure: shelf break cooling of surface waters consistent with internal tide breaking/mixing; complex interplay between local upwelling and the Mauritanian current off the Cape Verde headland; complexity of the inner-shelf/mid shelf frontal transition. The amplitude of the diurnal cycle suggests that large uncertainties exist in the SSUC heat budget. The studies limitations underscore the need for continuous in situ measurement in the SSUC, particularly of winds

Studying the contribution of different fishing gears to the Sardinella small-scale fishery in Senegalese waters

International audienceThis study investigated variations of landings of two key species, Sardinella aurita and Sardinella maderensis, in Senegalese waters over a ten-year period (2004-2013). Using generalized additive models, it was found that fishing gear played a major role in explaining differences in monthly landings for both species (51-71% deviance explained). Its effect was more significant in the southern part of Senegal. Fishing effort (number of trips) accounted only for 4-18% of variability in landings. Purse seine (PS) fishing was the most important contributor to the landings of both species. In addition, in the southern area, surrounding gillnet fishing was also important for S. maderensis. Modeling results showed that the relationship between monthly effort and landings was generally positive and leveling off, while it was dome shaped for PSs and surrounding gillnets. Thus, when estimating fishing effort indices for management in Senegal, it is necessary to account for differences in fishing gears and the non-linear relationship between fishing effort and landings

Contrasted optimal environmental windows for both sardinella species in Senegalese waters

We investigate Sardinella aurita and Sardinella maderensis recruitment success relative to the variability of oceanographic conditions in Senegalese waters using generalized additive models (GAM). Results show that recruitment of both species is marked by a strong intra-annual (seasonal) variation with minimum and maximum in winter and summer, respectively. Their interannual variations are synchronous until 2006 (recruitment decreasing), while from 2007 there is no synchrony. The model developed shows that sardinella recruitment variability is closely related to the tested environmental variables in the study area. However, the key environmental variables influencing the recruitment success are different for both species: the Coastal Upwelling Index and the sea surface temperature for S.aurita and S.maderensis, respectively. We report that recruitment success of S.aurita and S.maderensis are associated with distinct ranges of sea surface temperature, upwelling intensity, wind-induced turbulence, concentration of chlorophyll-a and north Atlantic oscillation index. Considering food security and socio-economic importance of both stocks, we recommend that consideration is given to the environmental variability in the small pelagic fish national management plans, particularly in the context of climate change

A small-scale oceanic eddy off the coast of West Africa studied by multi-sensor satellite and surface drifter data, and by a numerical model

International audienceHigh-resolution satellite images and oceanographic field measurements have revealed that oceanic eddies with diameters ranging from 1 to several hundred km are ubiquitous phenomena in the World's ocean. While eddies with horizontal scales above 100 km have been studied extensively using altimeter data, only few papers exist dealing with observations of eddies with horizontal scales below 50 km. These small-scale eddies cannot be resolved by conventional altimeters, but they can be observed from space by high-resolution optical/infrared sensors and by synthetic aperture radars (SARs). In this paper we report about a single small-scale cyclonic (cold) eddy which was generated at the headland of Cap-Vert off the coast of Senegal following a sudden freshening of the trade winds. Due to favorable cloud conditions, we were able to track the time evolution of the eddy for 31 days by satellite images acquired in the visible/ infrared band. Furthermore, the eddy was also imaged during this period by a space-borne SAR. Cold eddies become visible on SAR images via the change in the small-scale sea surface roughness caused by the damping of short surface waves by biogenic surface films or/and by the change of the stability of the air-sea interface. Biogenic surface films consist of surface-active material secreted by biota in the cold eddy. The satellite data we are using are from the MODIS sensor onboard the American Aqua satellite, the AVHHR sensor onboard the European MetOp satellite, and the Advanced SAR (ASAR) onboard the European Envisat satellite. The sea surface temperature (SST) and chlorophyll-a (CHL) maps derived from MODIS data show that the eddy propagated from its birth place at Cap-Vert in the Senegal upwelling region westward into the open North Atlantic. During the 31 days of satellite observations, the eddy moved 200 km westward thereby carrying nutrients from the upwelling region into the oligotrophic North Atlantic, where it caused enhanced CHL concentration. Maximum CHL concentration was encountered few days after the eddy generation, which is consistent with a delayed plankton growth following nutrient supply into the euphotic zone within the eddy. Furthermore, we recorded the movement of the eddy also by a satellite-tracked surface drifter. To our knowledge, this is the first time that a small-scale eddy has been tracked over such a long time period by high-resolution satellite images and simultaneously by a surface drifter. Model calculations carried out with the "Mercator" ocean circulation model show that the generation of the small-scale eddy was linked to a sudden increase of the trade winds. This wind event caused enhanced southward flow and upwelling at the coast of Senegal. The model calculations show further that the eddy was generated by flow separation at the headland of Cap-Vert

Extreme Coastal Water Levels Evolution at Dakar (Senegal, West Africa)

Increasingly, it is reported that the coastline of the Dakar region is affected by coastal flooding due to extreme water levels during wave events. Here, we quantify the extreme coastal water levels as well as the different factors contributing to coastal flooding during the period 1994–2015. Severe water levels reach values of 1.78 m and increase by 8.4 mm/year. The time spent above this threshold has already increased by 1.7 over the study period and will increase by 2100 to 8 times with 0.4 m mean sea level rise and up to 20 times with 0.8 m in the IPCC low and high greenhouse gas emission scenarios, respectively. Tide is the main contributor to the extremes when combined with large wave runup, due to wave breaking which contributes to 38% of the increase in extreme events while sea level rises to 44%. Our results show that because of its prominent location, Dakar region is affected by waves coming from the Northern and Southern Hemispheres with contrasted evolutions: wave runup events increase faster (7 mm/year) during austral winter due to a maximum of the South Atlantic storm activity, and have a decreasing trend (−3 mm/year) during boreal winter (December, January, February) driven by the evolution of corresponding climate modes

Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

International audienceIn West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968-1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20 th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1-10 km downwards from the upstream estuary entry, during the dry season, to a location 40-70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 Water 2020, 12, 647 2 of 25 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary