On the Dynamics of the Southern Senegal Upwelling Center: Observed Variability from Synoptic to Superinertial Scales

Capet, Xavier; Estrade, Philippe; Machu, Éric; Ndoye, Seyni; Grelet, Jacques; Lazar, Alban; Marié, Louis; Dausse, Denis; Brehmer, Patrice

doi:10.1175/jpo-d-15-0247.1

Cited by 40 publications

(88 citation statements)

References 73 publications

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“…Note though that shelf surface waters are, on average, ∼ 1°C too warm in the model (supporting information Text S2). Because a bias of similar amplitude also seems to affect bottom waters over the outer and midshelf (by comparing the simulation with observations from the UPSEN2/ECOAO experiments) [see Capet et al [, Figure 5], we suspect that the model deficiency arises from a lack of offshore isopycnal tilt resulting in the 15°C isotherm being ∼ 20 m too deep at the shelf break. This type of bias is common in upwelling simulations [ Capet et al , ].…”

Section: Resultsmentioning

confidence: 99%

Dynamics of a “low‐enrichment high‐retention” upwelling center over the southern Senegal shelf

Ndoye

Capet

Estrade

et al. 2017

Geophysical Research Letters

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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.

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Section: Resultsmentioning

confidence: 99%

Dynamics of a “low‐enrichment high‐retention” upwelling center over the southern Senegal shelf

Ndoye

Capet

Estrade

et al. 2017

Geophysical Research Letters

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“…These observed differences might be due to their physiological characteristics which are relatively distinct: S. aurita is more sensitive to temperature and salinity fluctuation than S. maderensis , which tolerate higher change in salinity and temperature (Ba et al., ; Cury & Fontana, ). This is also explained by the fact that, even if both species occupy the same area in Senegal (Cury & Fontana, ), their relative abundance strongly differs between Northern Senegal (in connection with Mauritania) and Southern Senegal (Capet et al., ; Ndoye et al., ), warmer and less sensitive to the detrimental effects of a strong upwelling season.…”

Section: Discussionmentioning

confidence: 99%

Contrasted optimal environmental windows for both sardinella species in Senegalese waters

Diankha

Bâ

Brehmer

et al. 2018

Fisheries Oceanography

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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.

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“…Prior to this survey, usual upwelling (northerly) winds of 10-15 knots were blowing over the area (Ndoye et al, 2014). Winds weakened just before the survey started ( Figure 2) and blew gently from 7 to 10 March alternating from weak northeasterlies (NE) in the morning to northwesterlies (NW) in the evening (not shown), a usual pattern for this region strongly affected by thermal winds (see Figure 2e in Capet et al, 2017, for the similar diurnal cycle observed in 2013). The relaxation became more pronounced between 10 and 13 March with very weak NW winds punctuated by short periods of weak southerlies.…”

Section: Resultsmentioning

confidence: 93%

“…High-resolution sampling of the water column during UPSEN revealed a short episode of complete subsurface anoxia. Reaching the O2 levels observed at six stations situated within 1-km distance requires the consumption of~60 μmol/kg of oxygen, this value being approximately the background concentration for near-bottom upwelling waters over the southern Senegalese shelf (see Figure 4 in Capet et al, 2017). Ignoring processes other than local phytoplankton degradation and assuming a photosynthesis/respiration reaction with a typical mean carbon:chlorophyll ratio of 50, such reduction requires 10 mgChl-a/m 3 .…”

Section: Discussionmentioning

confidence: 97%

First Evidence of Anoxia and Nitrogen Loss in the Southern Canary Upwelling System

Machu

Capet

Estrade

et al. 2019

Geophysical Research Letters

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The northeastern Atlantic hosts the most ventilated subsurface waters of any eastern boundary upwelling system, while coastal upwelling source waters are slightly above hypoxic levels. Anoxic conditions have previously been found offshore inside mesoscale eddies whose core waters undergo oxygen consumption for many months. Based on circumstantial in situ observations, this study demonstrates that the Senegalese coastal ocean is subjected to episodic occurrence of zero dissolved oxygen concentration at depth along with elevated nitrite concentration (11 mmol/m3) and nitrate/nitrite deficit to phosphate, thereby indicating severe anoxia and intense nitrogen loss. The anoxic event was associated with a prolonged upwelling relaxation episode in March 2012 and a nearshore diatom bloom that underwent degradation while being advected offshore in stratified waters. This is consistent with scenarios observed in other upwelling systems (Benguela and California), and such conditions are presumably frequent in the southern part of the Canary system.

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On the Dynamics of the Southern Senegal Upwelling Center: Observed Variability from Synoptic to Superinertial Scales

Cited by 40 publications

References 73 publications

Dynamics of a “low‐enrichment high‐retention” upwelling center over the southern Senegal shelf

Dynamics of a “low‐enrichment high‐retention” upwelling center over the southern Senegal shelf

Contrasted optimal environmental windows for both sardinella species in Senegalese waters

First Evidence of Anoxia and Nitrogen Loss in the Southern Canary Upwelling System

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