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The physical and biogeochemical functioning of eastern boundary upwelling systems is generally understood within the context of the upwelling - relaxation cycle, driven by sub-diurnal wind variability (i.e. with a time-scale of greater than a day). Here, we employ a realistically configured and validated 3D model of the southern Benguela upwelling system to quantify the impact of super-diurnal winds associated with the land-sea breeze (LSB). The ocean response to the LSB is found to be particularly enhanced within St Helena Bay (SHB), a hotspot for productivity which is also prone to Harmful Algal Bloom (HAB) development. We attribute the enhanced response to a combination of near-critical latitude for diurnal-inertial resonance (~32.5°S), the local enhancement of the LSB, and the local development of a shallow stratified surface layer through bay retention. Pronounced advection of the surface layer by diurnal-inertial oscillations contributes to large differences in day- and night-time sea surface temperatures (SST’s) (more than 2°C on average in SHB). Event-scale diapycnal mixing is particularly enhanced within SHB, as highlighted by a numerical experiment initialised with a subsurface passive tracer. These super-diurnal processes are shown to influence sub-diurnal dynamics within SHB through their modulation of the vertical water column structure. A deeper thermocline retains the upwelling front closer to land during active upwelling, while geostrophically-driven alongshore flow is impacted through the modulation of cross-shore pressure gradients. The results suggest that the LSB is likely to play an important role in the productivity and therefore HAB development within SHB, and highlight potential challenges for observational systems and models aiming to improve our understanding of the physical and biological functioning of the system.

This study applied a previously used Lagrangian individual-based model (IBM) for sardine in the Southern Benguela to an improved and more robust hydrodynamic model to investigate whether a more representative spatial coverage, greater horizontal and vertical resolution, more realistic winds and improved representation of mesoscale features such as eddies and filaments would give different results for transport and retention of early life stages. Despite major differences between the old and new hydrodynamic models, overall the IBM results were quite similar to the previous southern Benguela sardine IBM study. This surprising result indicates that it is the macroscale circulation features resolved by the two hydrodynamic models that are controlling transport and retention of sardine early life stages. The contribution of transient mesoscale features such as eddies and filaments appears to be less important when transport patterns are averaged over the 21-year-long experiment. Another aim of this study was to better estimate the contribution of south coast spawning to west coast sardine recruitment. This was possible because of an eastward extension of the geographical domain of the new hydrodynamic model which provided a more realistic representation of the south coast spawning ground. Three main spawning and nursery area systems, similar to those identified in the previous sardine IBM, were identified: west coast and west coast (WC-WC), south coast and west coast (SC-WC), and south coast and south coast (SC-SC). Spawning area proved to be an important determinant of modelled retention and transport success, with spawning depth also playing an important role on the west coast. The main difference observed from the previous study was an increase in the average percentage of particles released on the south coast and transported to the west coast (P-0, 17.4%). This indicates more connectivity between the southern and western sardine stocks than previously thought and is therefore important for fishery management. Standardized anomalies from the modelled retention/transport were compared with recruitment estimates from stock assessment models but there was no correlation between the two sets of anomalies. However, a significant correlation was observed between the modelled retention/transport anomalies for the west coast and total cumulative upwelling anomalies for the Southern Benguela (r = -0.67, p < .001).

This study re-investigates the work of Lutjeharms et al. [2001,2003] who documented the properties of a Natal Pulse using isopycnal Lagrangian floats. We combined Lagrangian analyses and Eulerian maps derived from objective analysis to better describe the evolution of a Natal Pulse along three density surfaces referred to as the surface (satellite-observed), shallow (isopycnal-1026.8 kg.m−3) and deep (isopycnal-1027.2 kg.m−3) layer. Our observations show that this Natal Pulse extended to a depth of 1000m and was associated with cyclonic relative vorticity values of about 6.5 to 8.5x10−5.s−1 in the surface and shallow layer and 4x10−5.s−1 in the deep layer. This Natal Pulse contributed to cross-shelf exchange through the offshore advection of Eastern Agulhas Bank water near the surface, onshore advection of South Indian Central Water and/or Indian Equatorial Water in the shallow layer and Antarctic Intermediate Water in the deep layer. Sea Surface Temperature maps showed that the downstream progression of the Natal Pulse along the 3000m isobath was related to a readjustment of its rotation axis. This readjustment advected Eastern Agulhas Bank water into the Natal Pulse eddy and triggered a SST cooling of about 3°C in the cyclonic area. The importance of a warm recirculating Agulhas plume originating from the Natal Pulse was highlighted. This warm water plume extended to a depth of 700m and was associated with onshore velocities exceeding those experienced within the Natal Pulse eddy by a factor of two. Our observations indicate that the June/July 1998 Natal Pulse and its associated plumes enhanced cross-shelf exchanges. This article is protected by copyright. All rights reserved.

Colloque avec actes et comité de lecture. Internationale.; International audience; The instability of a shelf front, with the characteristics of the Ushant front (steep density gradient and narrow jet), is investigated on the f-plane within the framework of a two-layer shallow-water model. Linear stability analysis and nonlinear numerical simulations show that baroclinic waves, with 20 and 40~km wavelengths, are the most unstable ones, with growth rates on the order of 1/1 day, which is comparable to what is observed on SST images. The analysis of growing perturbations in a two-layer primitive equation model (MICOM) shows that the latter are vertically shifted, which corroborates that the governing mechanism for the generation asymmetric meanders and eddies in our simulations, is baroclinic instability. Then, in order to take into account tidal effects, we add a time-periodic barotropic mean flow. Although we succeed observing some characteristics of parametric resonance (frequency selection, stepwise growth), baroclinic instability remains the main mechanism to explain the growth of meanders.

Colloque avec actes et comité de lecture. Internationale.; International audience; Source flows in a rotating tank are carried out to investigate the behavior of boundary currents. In the laboratory experiments, there is a northern boundary current which thickness increase with source strenght and a thin western boundary current which shows meandering behavior as the flow rate is increasing. The comparison with numerical simulations using the Miami Isopycnal Coordinates Ocean Model shows similarities but also some discrepancies with lab experiments mainly near the source and in the western boundary current.

The Weddell Sea is of global importance in the formation of dense bottom waters associated with sea ice formation and ocean-ice sheet interaction occurring on the shelf areas. In this context, the Weddell Sea boundary current system (BCS) presents a major conduit for transporting relatively warm water to the Weddell Sea ice shelves and for exporting some modified form of Wedell Sea deep and bottom waters into the open ocean. This study investigates the downstream evolution of the structure and the seasonality of the BCS along the Weddell Sea continental slope, combining ocean data collected for the past two decades at three study locations. The interannual-mean geostrophic flow, which follows planetary potential vorticity contours, shifts from being surface intensified to bottom intensified along stream. The shift occurs due to the densification of water masses and the decreasing surface stress that occurs westward, toward the Antarctic Peninsula. A coherent along-slope seasonal acceleration of the barotropic flow exists, with maximum speed in austral autumn and minimum speed in austral summer. The barotropic flow significantly contributes to the seasonal variability in bottom velocity along the tip of the Antarctic Peninsula. Our analysis suggests that the winds on the eastern/northeastern side of the gyre determines the seasonal acceleration of the barotropic flow. In turn, they might control the export of Weddell Sea Bottom Water on seasonal time scales. The processes controlling the baroclinic seasonality of the flow need further investigation.

The land-sea breeze is resonant with the inertial response of the ocean at the critical latitude of 30° N/S, however its role in the physical and biogeochemical functioning of eastern boundary upwelling systems (EBUS) is often over-looked. Here, we present a series of 1D, 2D, and 3D numerical experiments which elucidate the drivers of diurnal-inertial variability and vertical mixing in EBUS due to land-sea breeze forcing near the critical latitude. The amplitude of the diurnal anticyclonic rotary component of the wind stress (τ ac0 ) is shown to be a good predictor of the locally forced response. The water depth plays an important role, where for shallow water depths (

This study analyses the physical mechanisms that impact Lagrangian pathways and transport in the southern Benguela upwelling system (SBUS), an environment in which currents are key components of many important ecological processes, including the dispersal of marine larvae. Physical advection by currents is an important mechanism for egg and larvae transport success in the SBUS since the spawning areas and recruitment areas are separated by a long distance. Numerical model simulations of the SBUS coupled with particle tracking experiments are used to investigate Lagrangian pathways between the Cape Peninsula (34°S) and St Helena Bay (32°S) and how they are linked to the oceanic circulation. Transport success, given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released, is used quantify the alongshore connectivity between the two regions. We have identified and quantified the following physical drivers: (i) Benguela Jet, (ii) offshore Ekman transport, (iii) inner shelf poleward current, (iv) mesoscale eddies to be responsible for the spatial and temporal variability of the alongshore connectivity. The Benguela Jet was found to be the dominant driver of the connectivity at both seasonal and interannual timescales. Moreover, the presence of anti-cyclonic eddies near the shelf-edge negatively impact transport success by advecting particles into the open ocean. The opposite occurs with shelf-edge eddies as they transport particles onshore onto the shelf and the Benguela Jet. These findings will provide a valuable information for the future studies on the role of the physical drivers that impact transport of larvae and eggs., Cette étude a pour cadre le système d'upwelling côtier du Benguela sud (SUBS) dans lequel l'advection par les courants marins influe sur de nombreux processus biologiques. Le rôle de ces courants est exacerbé par la séparation géographique existant entre les zones de ponte et de nurserie de plusieurs populations de poissons marins. Des simulations numériques basées sur un modèle hydrodynamique du SUBS sont exploitées à l'aide d'outils lagrangiens pour identifier les trajectoires suivies par des particules entre la péninsule du Cap (34°S) et la baie de Sainte-Hélène (32°S) et pour explorer les liens entre ces trajectoires et la circulation océanique. Le rapport entre le nombre de particules atteignant la baie de Sainte-Hélène et le nombre total de particules relâchées est utilisé pour quantifier la connectivité entre les deux régions. Plusieurs processus physiques importants sont identifiés : (i) le Benguela jet, (ii) le transport d'Ekman vers le large, (iii) le courant s'écoulant vers le sud dans la bordure littorale et (iv) les tourbillons à mésoéchelle. Le Benguela jet est le mécanisme dominant la connectivité aux échelles de temps saisonnières et interannuelles. Le rôle contrasté des tourbillons océaniques suivant leur localisation et leurs caractéristiques a également été documenté. L'identification de ces processus physiques et la quantification de leurs impacts sur la connectivité entre les zones de ponte et de nurserie dans le SUBS sont des résultats qui vont indéniablement contribuer dans de futurs travaux à cerner le rôle des processus d'advection sur la survie larvaire et sur la dynamique des populations marines dans cet écosystème.