In Eastern boundary upwelling systems (EBUS), winds blow alongshore during the upwelling season, causing nutrient-replete deep waters to come to the euphotic zone through divergence of Ekman transport at the coast, stimulating elevated net primary production (NPP) and inorganic carbon uptake. This primary production induces secondary production, the recycling of nutrients, and export of organic carbon into the aphotic zone. EBUSs are strongly advective systems: Ekman currents, as well as intense geostrophic mesoscale eddies and filaments, transport nearshore surface water offshore. The transport of organic matter (OM) from formation sites to export sites, occurs in three-dimensional space: the location of carbon export may be offset spatially from the region of elevated PP (

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.

<p>In addition to the wind seasonal cycle, Eastern Boundary Upwelling Systems undergo intraseasonal fluctuations. These synoptic fluctuations are characterized by an intensification or a relaxation of upwelling favorable winds of a period of about 10 days and are believed to have a major impact on the upwelling dynamics. Here we focus on the South Senegalese Upwelling System (SSUS) which is located south of the sharp Cape Verde peninsula which acts as an abrupt coastline break and has a particularly shallow continental shelf. Previous studies described not only the SSUS climatological dynamics but also the importance of synoptic events that play a major role in the observed variability. However, their precise impacts on the 3D dynamics on the shelf remain unclear and consequences on biogeochemistry are unknown. We identify the key dynamical and biogeochemical processes of the coastal ocean in its response to synoptic events. This is done using a modeling experiment that consists in applying idealized synoptic wind intensification and relaxation to climatological SSUS states (with CROCO-PISCES). We find that synoptic fluctuations affect the regional circulation and shape robust anomalies of temperature, boundary layer depth, sea surface height,<span>  </span>surface and subsurface currents<em>.</em> Nutrients supply in the euphotic layer is significantly affected by synoptic fluctuations (+-30%). We find asymmetrical responses in nitrate, iron and silicate concentrations both between intensification and relaxation and between the inner and outer shelf regions. Persistent nitrate depletion is observed over the inner shelf. Phytoplanktonic ecosystem response to synoptic wind intensification thus differs spatially, with enhanced development of diatoms over the outer shelf and of nanophytoplankton over the inner shelf. Consequences on the zooplanktonic ecosystem are observed with a time delay and space shift, consistent with typical prey - predator relationships. Processes at play in the nutrients supply and planktonic ecosystem structure in response to synoptic fluctuations are discussed.<span> </span></p>