Phenology and mechanisms of the early upwelling formation in the southern coast of Senegal (original) (raw)
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Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981–2012
Deep Sea Research Part I: Oceanographic Research Papers, 2014
Seasonal coastal upwelling was analyzed along the NW African coastline (11-351N) from 1981 to 2012. Upwelling magnitudes are calculated by wind speed indices, sea-surface temperature indices and inferred from meteorological station, sea-surface height and vertical water column transport data. A permanent annual upwelling regime is documented across 21-351N and a seasonal regime across 12-191N, in accordance with the climatology of previous studies. Upwelling regions were split into three zones: (1) the Mauritania-Senegalese upwelling zone (12-191N), (2) the strong permanent annual upwelling zone (21-261N) and (3) the weak permanent upwelling zone (26-351N). We find compelling evidence in our various indices for the Bakun upwelling intensification hypothesis due to a significant coastal summer wind speed increase, resulting in an increase in upwelling-favorable wind speeds north of 201N and an increase in downwelling-favorable winds south of 201N. The North Atlantic Oscillation plays a leading role in modifying interannual variability during the other seasons (autumn-spring), with its influence dominating in winter. The East Atlantic pattern shows a strong correlation with upwelling during spring, while El Niño Southern Oscillation and Atlantic Multi-decadal Oscillation teleconnections were not found. A disagreement between observationally-based wind speed products and reanalysisderived data is explored. A modification to the Bakun upwelling intensification hypothesis for NW Africa is presented, which accounts for the latitudinal divide in summer wind regimes. Fig. 3. The monthly climatology (1981-2010) for the Mauritania-Senegalese upwelling zone (12-191N, blue line), permanent upwelling zone (21-261N, red line) and the weak permanent upwelling zone (26-351N, green line) for the seven UI W (units: m 3 s À 1 100 m À 1 ) and three UI ΔSST (units: 1C) indices. Error bars on the individual months highlight the (1s) monthly variability. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Journal of Physical Oceanography
In addition to their well-known seasonal cycle, eastern boundary upwelling systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling-favorable winds with 5–10-day typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynamical effects of wind-forced synoptic fluctuations on the South Senegalese Upwelling Sector (SSUS) are characterized. The region geomorphology is unique with its wide continental shelf and a major coastline discontinuity at its northern edge. The ocean response to synoptic events is explored using a modeling framework that involves applying idealized synoptic wind intensification or relaxation to a five-member climatological SSUS ensemble run. Model evaluation against sparse midshelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification, and ocean currents, despite a moderate but ...
Spatial and temporal dynamics of the upwelling off Senegal and Mauritania : local change and trend
L'élaboration d'une chaîne de traitement spécifique aux données infrarouge thermique des satellites de la série METEOSAT a été réalisée. L'abondance des données satellitales liées à la répétitivité des observations permet d'accéder à une résolution de 5 jours et 6 km, soit très en deçà des méthodes d'investigation utilisables en routine en océanographie côtière. La description spatio-temporelle précise et régulière de l'upwelling côtier de la Mauritanie à la Guinée est ainsi accessible depuis 1984. La zone étudiée est soumise à la très forte saisonnalité de l'upwelling côtier, lequel présente aussi de fortes anomalies interannuelles. La connaissance de la structure des champs thermiques superficiels permet de relier la position moyenne des zones de remontée à la topographie du plateau continental. La détermination de la TSM sur une bande continue centrée sur le maximum de résurgence permet le calcul d'indices d'upwelling spatialisés et de caractér...
Weakening of the Senegalo–Mauritanian upwelling system under climate change
Climate Dynamics
Upwelling processes bring nutrient-rich waters from the deep ocean to the surface. Areas of upwelling are often associated with high productivity, offering great economic value in terms of fisheries. The sensitivity of spring/summer-time coastal upwelling systems to climate change has recently received a lot of attention. Several studies have suggested that their intensity may increase in the future while other authors have shown decreasing intensity in their equatorward portions. Yet, recent observations do not show robust evidence of this intensification. The senegalo-mauritanian upwelling system (SMUS) located at the southern edge of the north Atlantic system (12°N-20°N) and most active in winter/spring has been largely excluded from these studies. Here, the seasonal cycle of the SMUS and its response to climate change is investigated in the database of the Coupled Models Inter comparison Project Phase 5 (CMIP5). Upwelling magnitude and surface signature are characterized by several sea surface temperature and wind stress indices. We highlight the ability of the climate models to reproduce the system, as well as their biases. The simulations suggest that the intensity of the SMUS winter/spring upwelling will moderately decrease in the future, primarily because of a reduction of the wind forcing linked to a northward shift of Azores anticyclone and a more regional modulation of the low pressures found over Northwest Africa. The implications of such an upwelling reduction on the ecosystems and local communities exploiting them remains very uncertain.
Dynamics of a "low-enrichment high-retention" upwelling center over the southern Senegal shelf
Senegal is the southern tip of the Canary upwelling system. Its coastal ocean hosts an upwelling center which shapes sea surface temperatures between latitudes 12 ∘ and 15 ∘ N. Near this latter latitude, the Cape Verde headland and a sudden change in shelf cross-shore profile are major sources of heterogeneity in the southern Senegal upwelling sector (SSUS). SSUS dynamics is investigated by means of Regional Ocean Modeling System simulations. Configuration realism and resolution (Δx ≈ 2 km) are sufficient to reproduce the SSUS frontal system. Our main focus is on the 3-D upwelling circulation which turns out to be profoundly different from 2-D theory: cold water injection onto the shelf and upwelling are strongly concentrated within a few tens of kilometers south of Cape Verde and largely arise from flow divergence in the alongshore direction; a significant fraction of the upwelled waters are retained nearshore over long distances while travelling southward under the influence of northerly winds. Another source of complexity, regional-scale alongshore pressure gradients, also contributes to the overall retention of upwelled waters over the shelf. Varying the degree of realism of atmospheric and oceanic forcings does not appreciably change these conclusions. This study sheds light on the dynamics and circulation underlying the recurrent sea surface temperature pattern observed during the upwelling season and offers new perspectives on the connections between the SSUS physical environment and its ecosystems. It also casts doubt on the validity of upwelling intensity estimations based on simple Ekman upwelling indices at such local scales.
Upwelling off southern Senegal and Gambia takes place over a wide shelf with a large area where depths are shallower than 20 m. This results in typical upwelling patterns that are distinct (e.g., more persistent in time and aligned alongshore) from those of other better known systems, including Oregon and Peru where inner shelves are comparatively narrow. Synoptic to superinertial variability of this upwelling center is captured through a 4-week intensive field campaign, representing the most comprehensive measurements of this region to date. The influence of mesoscale activity extends across the shelf break and far over the shelf where it impacts the midshelf upwelling (e.g., strength of the upwelling front and circulation), possibly in concert with wind fluctuations. Internal tides and solitary waves of large amplitude are ubiquitous over the shelf. The observations suggest that these and possibly other sources of mixing play a major role in the overall system dynamics through their impact upon the general shelf thermohaline structure, in particular in the vicinity of the upwelling zone. Systematic alongshore variability in thermohaline properties highlights important limitations of the 2D idealization framework that is frequently used in coastal upwelling studies.
SST patterns and dynamics of the southern Senegal-Gambia upwelling center
The 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 functioning (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.
Upwelling dynamics and cold-water filaments off the Senegal and Mauritania coasts [résumé]
2015
ISRA/Centre de Recherche Océanographique de Dakar-Thiaroye (CRODT), BP 2221 Dakar, Senegal Istituto di Oceanografia e Geofisica Sperimentale OGS, Borgo Grotta Gigante 42/c, 34010 Sgonico (TS), Italy UPMC Univ. Paris 6)-MNHN, LOCEAN/IPSL Laboratory, 4 Place Jussieu, Paris 75252, France Laboratoire de Physique de l’Atmosphère et de l’Océan Siméon Fongang (LPAO-SF/ESP-UCAD), Dakar, Sénégal Laboratoire d'Océanographie, des Sciences de l'Environnement et du Climat (LOSEC) Département de Physique UFR Sciences & Technologies, Université de Ziguinchor BP 523 Ziguinchor, Sénégal Institut de Recherche pour le Développement (IRD) US IMAGO BP 7
Journal of Geophysical Research: Oceans, 2014
1] Satellite scatterometers provide continuously valuable surface wind speed and direction estimates over the global ocean on a regular grid both in space and time. The Level 3 data derived from the Advanced Scatterometer (ASCAT), available at 1/4 spatial resolution (hereafter AS25), and Quick Scatterometer (QuikSCAT), available on 1/2 and 1/4 horizontal grids (QS50 and QS25, respectively), are studied at regional scales in both the Benguela and Canary upwelling systems. They are compared to the European Center for Medium-Range Weather Forecast surface wind analysis, with insight into their intrinsic and actual spatial resolutions. In the coastal band, the finest spatial patterns are found in the QS25 winds and are O(75 km). This demonstrates the sensitivity of the high-resolution satellite-derived winds to coastal processes related to sea surface temperature (SST) perturbations and land-sea transition. Next, short-lived upwelling episodes (SUEs) calculated from SST anomalies are defined consistently with the QS25 actual resolution. These cold events refer to local, short-lived perturbations that add to seasonal upwelling variability. We characterize concomitant atmospheric synoptic conditions for SUEs identified at chosen latitudes and highlight two subregions in both upwelling systems, with contrasted patterns for the alongshore wind stress component and curl. The complexity of the latter patterns is closely linked to local, short-term SST variability. Closer to the shore, numerical sensitivity experiments show that the imbalance between Ekman transport and Ekman pumping has an impact on ocean dynamics: wind reduction in the coastal QS25 forcing, partially induced by orography, tends to reduce coastal SST cooling.