Surface mixing and biological activity in the North-West African upwelling

International audienceNear-shore water along the NorthWest African margin is one of the world's major upwelling regions. It is associated with physical structures of oceanic fronts which influence the biological productivity. The study of these coherent structures in connection with chlorophyll concentration data is of fundamental importance for understanding the spatial distributions of the plankton. In this work, we study the horizontal stirring and mixing in different upwelling areas using Lagrangian coherent structures (LCSs). These LCSs are calculated using the recent geodesic theory of LCSs. We use these LCSs to study the link between the chlorophyll fronts concentrations and surface mixing, based on 10 years of satellite data. These LCSs move with the flow as material lines, thus the horizontal mixing is calculated from the intersection of these LCSs with the finite time Lyapunov exponents (FTLEs) maps. We compare our results with those of a recent study conducted over the same area, but based on Finite Size Lyapunov Exponents (FSLEs) whose output is a plot of scalar distributions. We discuss the differences between FSLE and geodesic theory of LCS. The latter yields analytical solutions of LCSs, while FSLEs can only provide LCSs for sharp enough ridges of nearly constant height

Coastal upwelling off North‐West Africa

11 pages, 13 figuresNorth of Cape Blanc, the north‐easterly winds cause offshore flow of surface waters that are replaced by subsurface inflow of relatively cold and nutrient‐rich waters, driving the vertical cell of coastal upwelling. This vertical circulation, together with surface heating and horizontal mixing, causes the coastal upwelling front (typically about 200 m deep) that separates cold onshore from warm offshore waters. A southward baroclinic coastal jet is associated to this front, which causes vertical shear and mixing that contribute to the intensity of the vertical cell. Very importantly, this jet feeds from upstream waters, resulting in an along‐slope coherent flow, or the horizontal cell of coastal upwelling - this is the Canary Upwelling Current (CUC) that connects all surface coastal African waters north of Cape Blanc. Further south, because of the northward offshore flow and the seasonality of the winds, the connection remains only during winter and spring, very close to shelf break and in the top 100 m. North of Cape Blanc, a Poleward Undercurrent (PUC) flows in the relatively homogenous upwelled waters that found over the continental slope. South of Cape Blanc the PUC appears as a nearshore expression of the Mauritania Current. Both the southward CUC and the northward PUC constitute the true skeleton of the Canary Current Large Marine EcosystemThis review synthesizes work on coastal upwelling in the CCLME linked to several projects carried out during the last two decades, particularly projects CANOA (CTM2005-00444/MAR), MOC2 (CTM2008-06438-C02-01) and TIC-MOC (CTM2011–28867) funded by the Spanish government. Aïssa Benazzouz was partially supported by the 50th Anniversary Young African fellowship program of the Intergovernmental Oceanographic Commission of UNESCOPeer Reviewe

Wind-driven surface circulation in the Cape Blanc region

17 pages, 14 figures, 2 tablesWe analyze the short-term transition, on time scales of the order of days and weeks, of the surface fields in the coastal transition zone off Northwest Africa, between 15°N and 25°N, during winter and spring 2005 and 2006. This is a complex region characterized by the baroclinic coastal jet north of Cape Blanc, along-shore convergence and water export at the Cape Blanc giant filament, and substantial mesoscale variability between Capes Verde and Blanc. We use the anomalies of the wind impulse and two different coastal upwelling indexes, evaluated off 17°N, 20°N and 23°N, in order to assess the importance of wind forcing in this short-term variability. We also employ daily and weekly surface maps of wind, temperature, surface height and currents to investigate which are the mechanisms that lead to the relatively fast changes in the flow patterns. The coastal baroclinic jet and the Cape Blanc giant filament are ubiquitous features, the two being intrinsically related through the intensity of upwelling off Cape Blanc. Therefore, the strength of both features responds, to a large extent, to the fluctuations of the northeasterly winds; their intensity is greater in spring than winter but during both seasons they experience relatively fast oscillations related to the intermittency of the wind field. The mesoscalar features are visible in the entire domain, with time scales typically of the order of two weeks, but become prominent in the southern region during spring, apparently related to an intense northward coastal jet south of Cape Blanc. © 2013 Elsevier Ltd.This research has been funded by the Spanish Government through Projects MOC2 (ref. no. CTM2008-06438-C02-01) and TIC-MOC (ref. no. CTM2011-28867)Peer Reviewe

Potential Roles Of Eddy Kenetic Energy And Turbulence In Controlling The Bio-optical Ocean Proprieties

In the Canary Current System (CCS), coherent structures and concurrent movements of surface waters such as meanders, filaments and eddies strongly control the ocean bio-optical proprieties response to the coastal upwelling process. One of the outstanding problems is to understand the mechanisms of the bio-optical proprieties transfer and the connection mechanism between the coastal band and the ocean interior. We use a combination of satellite data and derived mesoscale indicators to provide a comprehensive view of the relationship between the physical and bio-optical proprieties off Moroccan upwelling region (part of the CCS) in terms of wind impulse responsible of sea turbulence, sea surface temperature (SST) response of the wind stress and ocean color proprieties considered as bio-optical ocean proxy response. To optimize the predicted ranges of these parameters, Generalized Additive Model (GAM) was applied. We conclude that the energetic mesoscales structures as seen from the satellite climatology observations can provide insight into dominant transport pathways controlling the bio-optical exchange from the coastal area to the ocean interior structured as an oceanic corridor connecting the Moroccan area to the Canary archipelagos

Offshore Wind Energy Resource in the Kingdom of Morocco: Assessment of the Seasonal Potential Variability Based on Satellite Data

This study provides a first estimate of the offshore wind power potential along the Moroccan Atlantic shelf based on remotely sensed data. An in-depth knowledge of wind potential characteristics allows assessment of the offshore wind energy project. Based on consistent daily satellite data retrieved from the Advanced Scatterometer (ASCAT) spanning the period from 2008 to 2017, the seasonal wind characteristics were statistically analyzed using the climatological Weibull distribution functions and an assessment of the Moroccan potential coastal wind energy resources was qualitatively analyzed across a range of sites likely to be suitable for possible exploitation. Also, an atlas of wind power density (WPD) at a height of 80 m was provided for the whole Moroccan coast. An examination of the bathymetrical conditions of the study area was carried out since bathymetry is among the primary factors that need to be examined with the wind potential during offshore wind project planning. The results were presented based on the average wind intensity and the prevailing direction, and also the wind power density was shown at monthly, seasonal and interannual time scale. The analysis indicated that the coastal wind regime of the southern area of Morocco has the greatest energy potential, with an average power density which can reach in some places a value around 450 W/m2 at heights of 10 m and 80 m above sea level (a.s.l) (wind turbine hub height) more particularly in the south of the country

On the temporal memory of coastal upwelling off NW Africa

25 pages, 16 figures, supporting Information http://onlinelibrary.wiley.com/doi/10.1002/2013JC009559/suppinfoWe use a combination of satellite, in situ, and numerical data to provide a comprehensive view of the seasonal coastal upwelling cycle off NW Africa in terms of both wind forcing and sea surface temperature (SST) response. Wind forcing is expressed in terms of both instantaneous (local) and time-integrated (nonlocal) indices, and the ocean response is expressed as the SST difference between coastal and offshore waters. The classical local index, the cross-shore Ekman transport, reproduces reasonably well the time-latitude distribution of SST differences but with significant time lags at latitudes higher than Cape Blanc. Two nonlocal indices are examined. One of them, a cumulative index calculated as the backward averaged Ekman transport that provides the highest correlation with SST differences, reproduces well the timing of the SST differences at all latitudes (except near Cape Blanc). The corresponding time lags are close to zero south of Cape Blanc and range between 2 and 4 months at latitudes between Cape Blanc and the southern Gulf of Cadiz. The results are interpreted based on calculations of spatial and temporal auto and cross correlations for wind forcing and SST differences. At temporal scales of 2–3 weeks, the alongshore advection of alongshore momentum compensates for interfacial friction, allowing the upwelling jet and associated frontal system to remain active. We conclude that the coastal jet plays a key role in maintaining the structure of coastal upwelling, even at times of relaxed winds, by introducing a seasonal memory to the system in accordance with the atmospheric-forcing annual cycleThis work has been supported by project TIC-MOC (CTM2011–28867) of the Spanish Ministerio de Ciencia e Innovación. Partial funding from the Institut de Recherche pour le Développement, UMR EME 212, is also gratefully acknowledged. Aïssa Benazzouz was partially supported by the 50th Anniversary Young African fellowship program of the Intergovernmental Oceanographic Commission. Evan Mason was supported by a Spanish government JAE-Doc grant (CSIC), cofinanced by FSE. Jesus Peña-Izquierdo was funded through a FPI doctoral grant linked to project MOC2 (CTM2008-06438-C02- 01) of the Spanish Ministerio de Ciencia e Innovación. The SST data were provided by GHRSST and the U.S. National Oceanographic Data Center, in a project partly supported by the NOAA Climate Data Record (CDR). QuikSCAT and CCMP global wind were obtained from the NASA Physical Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory. ROMS development at UCLA is supported by the Office of Naval Research (currently grant N00014-08-1–0597), the applications here shown were partially supported by the National Center for Supercomputing Applications under grant OCE030007 and utilized the ABE systemPeer reviewe

Caractérisation spatio-temporelle de la production primaire dans la côte Nord-Ouest Africaine par imageries satellitaires MODISA

Une série de quatorze-ans d’images de la couleur de l’océan du capteur MODIS Aqua (MODISA) de 2002 à 2015 a été utilisée pour caractériser la variabilité spatiale et temporelle de la production primaire (PP), à l’échelle saisonnière et interannuelle le long de la côte Nord-Ouest Africaine (entre 15°N et 36°N). Les données de la couleur de l’océan sont mises en œuvre par des modèles mathématiques en faisant une intercomparaison entre leurs produits. La dynamique de la PP est interprétée par la variabilité de la concentration en chlorophylle, la lumière disponible ainsi que celle de l’indice d’upwelling côtier (IUC). On a identifié cinq régions, chacune avec un comportement différent: la région entre 32°N et 36°N caractérisée par une forte saisonnalité de la production et de l’upwelling. La zone entre 28°N et 32°N marque une productivité élevée comparée à la première zone qui s’accompagne par une présence de filaments riches en chlorophylle près du Cap Ghir (31°N) et Cap Juby (28°N). La bande entre 25°N et 28°N présente une activité ascendante de la PP soutenue par l’aspect permanent de l’activité d’upwelling. Entre 21°N et 25°N, cette zone est très perturbée, avec une production intense de part et d’autre du Cap Blanc (21°N). Enfin, la zone au sud de 21°N, la PP est majoritairement abondante en hiver qui coïncide avec la saison d’upwelling dans cette zone. Le modèle qui a été envisagé comme le plus adéquat pour la zone d’étude est VGPM de Behrenfeld et Falkowski (1997) avec intégration du coefficient d’atténuation diffuse Kd490. Le modèle VGPM (Kd) corrigé montre une relation plus affinée avec l’IUC, mais à une échelle plus petite, le modèle de production change en fonction de la zone d’upwelling. A une échelle interannuelle, les différents modèles affichent un pattern d’évolution temporelle similaire mais avec des différences saisonnières et spatiales notables. Mots clés: Production primaire, Côte Nord-Ouest Africaine, Modèles mathématiques, Variabilité saisonnière et interannuelle, MODIS Aqua, Indice d’Upwelling Côtier.  Fourteen years of MODIS Aqua (MODISA) ocean color data are used to characterize the spatial and temporal variability of primary production from 2002 to 2015 in seasonal and interannual timescales along the northwest African coast (between 15°N and 36°N). The ocean color data are implemented into mathematical models in order to compare between their different products. Five regions with different behaviors are identified: the area between 32°N-36°N is characterized by a strong seasonality of productivity and upwelling activity. The region between 28°N and 32°N with high productivity in comparison with the first zone accompanied by presence of chlorophyll filaments in coastal zone near Cape Ghir (31°N) and Cape Juby (28°N). The area between 25°N and 28°N shows an ascending primary production activity supported by the permanent aspect of the activity of upwelling in this area. Then, the area between 21°N-25°N, which is disturbed, is characterized by intense production in Cap Blanc (21°N). Finally, the area south of 21°N, where we notice that primary production is mostly abundant in winter season coinciding with the upwelling activity (November to April). VGPM by Behrenfeld and Falkowski in 1997 with backscattering coefficient Kd490, seems to be the most appropriate model for our region of interest. This dynamic is interpreted in regards of remotely sensed chlorophyll, radiation, and coastal upwelling index. Several attempts to correlate between in-situ measurements and satellite observations have been conducted to output a corrected equation that allows us to establish a regional model. Corrected VGPM (Kd) Model indicate a more refined relation with CUI, but on smaller scale, the suitable production model change according to upwelling zone. At interannual timescale, the models show a similar pattern of temporal evolution but with significant seasonal and spatial differences. The dynamics of biomass and primary production study and its relationship with the CUI provides an interesting framework for future studies in NWA region.  Keywords: Primary production, Remote sensing, North-West Africa (NWA), Modeling, seasonal and interannual variability, Upwelling, MODIS-Aqua, CUI

On the temporal and spatial coherence of coastal upwelling

IV Congress of Marine Sciences, Encuentro de la Oceanografía Física Española (EOF 2014), 11-13 June 2014, Las Palmas de Gran Canaria.-- 1 pageIn a two-dimensional world, with depth and cross-shore distance as the only spatial axes, coastal upwelling responds quite fast to the along-shore sea-surface winds, on time scales of the order of hours to days. This is the reason why coastal upwelling indices, used to predict the intensity of upwelling at some coastal location, are often calculated in terms of the instantaneous along-shore local winds, i.e. the Bakun (1973) index and some of its variations simply assess the intensity of cross-shore Ekman transport. This approach ignores along-shore pressure gradients, internal friction and, most important for our analysis, the advection of upstream momentum. In this work we first examine under which circumstances, and how much, the upstream momentum can play a role similar to the along-shore wind stress in the sustainment of coastal upwelling. The key condition for the maintenance of upwelling beyond the wind intermittency turns out to be the advective temporal scale (related to the spatial coherence of the wind), which has to be longer than the local temporal scale, i.e. the spatial coherence of the along-shore coastal jet is capable of replacing the temporal intermittency of the cross-shore Ekman transport. Second, we apply these ideas to the coastal upwelling region off NW Africa. For this purpose we use time series of sea-surface temperature (SST) differences (between the coast and offshore regions) and coastal winds. We calculate the temporal and spatial auto-correlations for the wind and SST differences, and the cross-correlations between both variables. It turns out that off NW Africa the advective time scale is often longer than the local temporal scale, which brings a characteristic temporal memory of the order of 1-2 months. Finally, we define a cumulative upwelling index as the integration of past cross-shore Ekman transports over a time scale which brings the maximum correlation with the SST differences. This time scale, consistent with the time scale for maximum cross-correlations between SST differences and local Ekman transport, is the temporal memory of the upwelling systemPeer Reviewe

An improved coastal upwelling index from sea surface temperature using satellite-based approach - The case of the Canary Current upwelling system

17 pages, 16 figures, 3 tablesA new methodology to derive an SST-based upwelling index was based on a rigorous spatial analysis of satellite SST fields and their variability, by referring to previous works, from Wooster et al. (1976) to Santos et al. (2011). The data was precautiously processed by considering data quality aspects (including cloud cover) and the best way to derive accurate coastal SST and its offshore reference. The relevance of the developed index was evaluated by comparing its spatial and seasonal consistency against two wind-based indices as well as with the previous SST-based indices, largely superseding these later ones in term of overall quality and spatio-temporal dynamic. Our index adequately describes the spatio-temporal variability of the coastal upwelling intensity in the Canary Current upwelling system and has the advantage of describing complementary aspects of the coastal dynamics of the region that were not covered by Ekman-based indices.The proposed methodology is generic and can be easily applicable to various coastal upwelling systems, especially the four major eastern boundary upwelling ecosystems. © 2014 Elsevier Ltd.We thank the 50th Anniversary Young African fellowship programme of IOC (Intergovernmental Oceanographic Commission) as well as the French Institute of Research for Development (IRD) for partially supporting this workPeer Reviewe