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![Low‐pass (7 days) filtered time series for (a) surface wind stress (WS) (meridional [black] and zonal [gray] components, in N m⁻²); (b) meridional (v; black) and zonal (u; gray) velocities (m s⁻¹); (c) temperature (blue, in °C) and salinity (orange, in psu) measured at 28 m depth (7–8 m above sea floor) at Melax station from February 11, 2015 to August 1, 2016; (d) mixed layer depth (MLD; blue, in meters) and stratification index in the bottom layer (temperature gradient between 25 and 28 m, orange, in °C m⁻¹); (e) daily (dots) and monthly (line) MODIS SOM‐NVA chlorophyll concentration (in mgChl m⁻³) from February 11, 2015 to August 1, 2016, averaged between 14° and 14.75°N and from the coast to the 100 m isobath, standard deviation is represented by the vertical line; (f) dissolved oxygen (DO, in µmol kg⁻¹) measured at 28 m depth. Pink shadings represent upwelling events defined by upwelling criterion (southward wind stronger than 5.5 m s⁻¹, see Section 2.3). The adjusted ASCAT wind stress and modeled current (see Section 2.3) at Melax (see pixels in Figure 1) are plotted and overlaid with green shading in (a and b), respectively. The adjusted model salinity (see Section 2.3) is plotted in (c) between February 2016 and the end of April 2016 (thick orange line). MLD is computed as T(z = 1 m)−T(z = MLD) = 0.2 °C (Montégut et al., 2004). The dotted blue line in (f) represents the hypoxia threshold set at 60 µmol kg⁻¹. Black arrows (b and f) indicate periods of strong northward currents and DO concentration increase.](publication/351543609/figure/fig2/AS:11431281250851735@1718060344912/Low-pass-7days-filtered-time-series-for-a-surface-wind-stress-WS-meridional.png)
Low‐pass (7 days) filtered time series for (a) surface wind stress (WS) (meridional [black] and zonal [gray] components, in N m⁻²); (b) meridional (v; black) and zonal (u; gray) velocities (m s⁻¹); (c) temperature (blue, in °C) and salinity (orange, in psu) measured at 28 m depth (7–8 m above sea floor) at Melax station from February 11, 2015 to August 1, 2016; (d) mixed layer depth (MLD; blue, in meters) and stratification index in the bottom layer (temperature gradient between 25 and 28 m, orange, in °C m⁻¹); (e) daily (dots) and monthly (line) MODIS SOM‐NVA chlorophyll concentration (in mgChl m⁻³) from February 11, 2015 to August 1, 2016, averaged between 14° and 14.75°N and from the coast to the 100 m isobath, standard deviation is represented by the vertical line; (f) dissolved oxygen (DO, in µmol kg⁻¹) measured at 28 m depth. Pink shadings represent upwelling events defined by upwelling criterion (southward wind stronger than 5.5 m s⁻¹, see Section 2.3). The adjusted ASCAT wind stress and modeled current (see Section 2.3) at Melax (see pixels in Figure 1) are plotted and overlaid with green shading in (a and b), respectively. The adjusted model salinity (see Section 2.3) is plotted in (c) between February 2016 and the end of April 2016 (thick orange line). MLD is computed as T(z = 1 m)−T(z = MLD) = 0.2 °C (Montégut et al., 2004). The dotted blue line in (f) represents the hypoxia threshold set at 60 µmol kg⁻¹. Black arrows (b and f) indicate periods of strong northward currents and DO concentration increase.
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The observation station “Melax” was deployed in 2015 on the wide and shallow south Senegalese shelf to study the ocean dynamics, air‐sea interactions, and dissolved oxygen (DO) cycle. Data from February 2015 to August 2016 were used to study the main physical processes affecting the variability of DO in the bottom layer (∼30 m depth) on time scales...
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The impacts of El Niño on sea surface salinity (SSS) in the South China Sea (SCS) are investigated using satellite observations, in-situ data, and reanalysis products. Here, we show that positive SSS anomalies cover most of the SCS during the mature phase of El Niño. The physical processes controlling these positive SSS anomalies are different from...
Citations
... The west African coastal ocean is bordered by an oxygen minimum zone (OMZ) composed of a shallow (∼100 m) and deep (∼400 m) DO minimum (Brandt et al., 2015;Thomsen et al., 2019). While waters from the deep OMZ hardly reach the surface (Glessmer et al., 2009), the Senegalese shelf is directly connected to the relatively shallow OMZ, which supplies low DO waters to the shelf during the upwelling season (Tall et al., 2021;Capet et al., 2017). On the other hand, the west African boundary current (WABC; Barton, 1998;Kounta et al., 2018) flowing northward over the west African shelf and slope transports relatively DO-rich waters over the SSUS shelf during upwelling relaxation periods (Tall et al., 2021). ...
... While waters from the deep OMZ hardly reach the surface (Glessmer et al., 2009), the Senegalese shelf is directly connected to the relatively shallow OMZ, which supplies low DO waters to the shelf during the upwelling season (Tall et al., 2021;Capet et al., 2017). On the other hand, the west African boundary current (WABC; Barton, 1998;Kounta et al., 2018) flowing northward over the west African shelf and slope transports relatively DO-rich waters over the SSUS shelf during upwelling relaxation periods (Tall et al., 2021). Recent observations suggest that the SSUS shelf has already suffered episodes of anoxia: a short-lived (2 day-long) anoxic event (associated with nitrogen loss through denitrification) was uncovered in March 2012 (Machu et al., 2019), likely a consequence of diatom degradation during a prolonged wind relaxation. ...
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Synoptic intensification or relaxation of upwelling favorable winds are major sources of variability in eastern boundary upwelling systems. This study aims to investigate their impact on the planktonic ecosystem of the South Senegalese Upwelling Sector (SSUS), located south of the Cape Verde peninsula over a wide and shallow continental shelf. Numerical experiments using a three-dimensional coupled physical–biogeochemical model with four plankton functional types simulated the response of the coastal planktonic ecosystem to idealized synoptic (∼10 days) wind intensification and relaxation of the same amplitude. We find that these perturbations induce spatio-temporal oscillations of plankton concentrations. Zooplankton response occurred with a time lag that manifests itself in space as an equatorward/downstream shift in distribution relative to phytoplankton. Overall, the transmission of the synoptic perturbation from the physics to zooplankton is characterized by a damping in relative anomalies. All these elements and the weakness of the asymmetries in the biogeochemical/planktonic ecosystem responses between intensification and relaxation events support the hypothesis that synoptic variability has limited impact on the climatological state of low–latitude upwelling systems such as the SSUS.
PLAIN LANGUAGE SUMMARY
In some coastal regions of the world such as off Senegal, winds preferentially blow alongshore and induce subsurface, cold and nutrient rich waters to rise to the surface layer and favor the development of plankton blooms. These so-called upwelling favorable winds are not steady. Their fluctuations produce dynamical and biogeochemical variability over a broad range of scales. Here, we studied the biogeochemical effect of 10-day (i.e. weather or synoptic) wind fluctuations over the southern Senegal continental shelf. We used a numerical model with a simplified planktonic ecosystem consisting of two phytoplankton and two zooplankton size classes. The wind perturbations modulate ocean physics, the enrichment of the sun-lit surface layer in nutrients and the planktonic ecosystem. The plankton’s response to wind fluctuations exhibited oscillations more complex and relatively less intense than those of the wind. The modest effect of the studied short-term wind fluctuations on plankton found in this study may be specific to low-latitude coastal oceans with wide continental shelves.
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 days 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 mid-shelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification and ocean currents, despite a moderate but systematic bias in current intensity. Modeled synoptic wind and heat flux fluctuations produce clear modulations of all dynamical variables with robust SSUS-scale and mesoscale spatial patterns. A mixed-layer heat budget analysis is performed over the continental shelf to uncover the dominant processes involved in SSUS synoptic variability. Modulations of horizontal advection and atmospheric forcing are the leading-order drivers of heat changes during either wind intensification or relaxation while vertical dynamics is of primary importance only in a very localized area. Also, modest asymmetries in the oceanic responses to upwelling intensification and relaxation are only identified for meridional velocities. This brings partial support to the hypothesis that synoptic variability has a modest net effect on the climatological state and functioning of upwelling systems dynamics.
The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen depleted environments in both coastal and open ocean systems since the middle of the 20th century. This paper provides a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. Case studies were selected to achieve a balance in system diversity and global coverage. Each case study describes system attributes, including the present-day oxygen
environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. Spatial distributions of oxygen and time series of oxygen data provide the
opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others
subtractive, makes attribution to any particular driver challenging. System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.
