Lagrangian pathways in the southern Benguela upwelling system
Introduction
In the southern Benguela upwelling system (SBUS), anchovy spawn in austral spring over the Agulhas Bank, a 200 m deep large coastal plateau that extends south of the tip of the African continent (Hutchings, 1992). The nursery area1 is located off the west coast of southern Africa, 400 km north of the spawning area. This makes the SBUS distinctive from its northern Benguela counterpart and the other major eastern boundary upwelling systems (EBUS) for which spawning and nursery grounds overlap. In their early-stage, anchovy larvae have minimal swimming ability, which makes them behave as passive particles. Therefore, their dispersion and transport, from the Agulhas Bank to the nursery area, are directly dependant on the oceanic circulation (Pagès et al., 1991).
Hutchings (1992) presented a schematic picture of the major processes impacting the eggs and larvae journey from the western Agulhas Bank to the west coast upwelling region (Fig. 1). This scheme was based on knowledge of the coastal circulation and on numerous observations of eggs and larvae distribution (Armstrong et al., 1987; Bang and Andrews, 1974; Shannon, 1985; Shelton and Hutchings, 1982; Shelton and Hutchings, 1990). A coastal current flowing northward and following the shelf, namely the Benguela Jet, is considered as the major conveyor belt linking the western Agulhas Bank and the west coast upwelling area.
According to Nelson (1989), the Benguela Jet is a narrow current (∼20 km) that flows northward along the South African west coast between Cape Point (34.15° S, 18.4° E) and Cape Columbine (33° S, 17.5° E). The jet can sustain high velocities, ranging from 25 cm·s−1 to 75 cm·s−1 (Bang and Andrews, 1974; Gordon, 1985). It lies over a poleward undercurrent (Nelson and Hutchings, 1983), that flows over the shelf edge and occasionally reaches the surface. The jet varies in position and strength, and responds rapidly to changes in wind stress magnitude and direction. It follows the seasonal cycle of the upwelling (Armstrong et al., 1987) and intensifies in summer when coastal upwelling of cold waters contributes to reinforce the across-shore density gradient (Veitch et al., 2006; Veitch et al., 2017). In winter, when the wind direction reverses, this across-shore density gradient weakens, but persists due to the intrusions of warm Agulhas waters from the South (Twatwa Mhlongo et al., 2005).
Hutchings' (1992) scheme for Lagrangian transports of fish eggs and larvae has been widely used to set-up numerous Lagrangian modelling studies in the Southern Benguela (Mullon et al., 2003). Huggett et al. (2003), followed by Parada et al., 2003, Parada et al., 2008, carried out pioneering work demonstrating that the dominant anchovy spawning patterns in the Southern Benguela could be reproduced when combining a 3D hydrodynamic model with a Lagrangian particle-tracking tool. They also quantified the impact, on eggs and larvae transport success, of parameters such as the initial location, density and patchiness of eggs, as well as behavioral processes such as diurnal vertical migration, mortality due to lethal temperatures or predation. However, neither the local pathways nor the physical processes responsible for the transport of particles were studied in detail, despite the fact that the success of their experiments was obviously linked to the capacity of the hydrodynamical model to reproduce the main characteristics of the currents in the region.
The modelling works of Veitch et al. (2006) and Blanke et al. (2009) provided insights on the importance of the Benguela Jet for the connectivity between the western Agulhas Bank and the west coast upwelling area. More recently, in a detailed modelling study of the shelf edge currents, Veitch et al. (2018) described the bifurcation of this Jet between Cape Point and Cape Columbine into a weak narrow branch flowing over the shelf edge and a strong offshore branch flowing in the northwestward direction. A bifurcation of the jet, as well as other physical processes such as induced offshore Ekman transport and occasional entrainment by Agulhas rings, have been suggested as important physical processes impacting the alongshore transit of fish eggs and larvae (Hutchings et al., 1998; Skogen et al., 2003; Veitch et al., 2006; Garavelli et al., 2012).
This work builds upon these recent physical modelling studies to investigate in details how the horizontal and vertical structure of the shelf edge jets and the presence of mesoscale turbulence impact the along-shore trajectory of passive particles in the SBUS. The approach consists in a set of Lagrangian particles tracking experiments based on a robust hydrodynamical model capable of simulating the major physical processes influencing the along-shore transport of particles, i.e. a realistic seasonal coastal circulation, a realistic mesoscale eddy field, Agulhas leakage bringing warm Agulhas waters offshore of the Benguela shelf.
While our study remains motivated by questions related to the dynamics of anchovy reproductive patterns and to recruitment variability, it distinguishes from the others as it focuses on the physical transport processes that impact the survival of individuals at early stages, whereas previous studies mainly focused on the sensitivity to biological processes (Lett et al., 2015). The Lagrangian experiments are built upon the pioneering work of Huggett et al. (2003) to focus exclusively on the northward alongshore portion of the hypothesized trajectory of anchovy larvae from the Agulhas Bank to St Helena Bay, i.e. the portion between Cape Point and St Helena Bay. If the Benguela Jet is assumed to play a dominant role in the transport of fish larvae, the success of this route should be in theory more predictable than the cross-shelf transport in the vicinity of the turbulent Agulhas retroflection.
A first step towards elucidating the role of the jet consists of identifying the different physical processes that control the seasonal cycle of this northward alongshore connectivity between Cape Point and St Helena Bay, just north of Cape Columbine. The Lagrangian experiments aim at addressing the following: (1) the identification of the dominant pathways that ensure this connectivity; (2) their characteristics and robustness; (3) the ocean dynamics that link to the alongshore transport success seasonal variability. Unlike the climatologically forced model of (Veitch et al. (2018), our ocean model is forced by 6-hourly surface forcing (wind, heat and freshwater fluxes) over the 1989–2010 period. The use of a 20 year simulation forced with a realistic non filtered wind field that conserves its full interannual, seasonal and intraseasonal variability gives robustness to our conclusions regarding the role of each individual process on the transport success of particles.
The outline of the paper is as follows. The numerical model and the Lagrangian experiments designed for the purpose of this study are presented in Section 2. This section also includes an evaluation of the model's realism by comparing simulated and observed sea surface temperature (SST) and eddy kinetic energy (EKE). Section 3 characterizes the seasonal cycle of the alongshore connectivity in terms of duration of travel and initial positions. The Lagrangian pathways taken by particles are identified in Section 4 while Section 5 investigates the link between the observed Lagrangian dispersal patterns and the across-shore structure of the ocean circulation, mainly the alongshore currents and the mesoscale turbulent eddy field. Finally, a summary and discussion of our results highlight the complexity of the across-shore structure of alongshore jet currents in the region.
Section snippets
Numerics
This study uses the Regional Ocean Modelling System (ROMS; Shchepetkin and McWilliams, 2005) in its Coastal and Regional Ocean Community (CROCO) version (Debreu et al., 2012; Penven et al., 2006). ROMS is a free-surface, terrain-following σ-coordinate model with split-explicit time stepping and with Boussinesq and hydrostatic approximations. The advection scheme is third-order upstream biased, which reduces dispersion errors, essentially enhancing precision for a given grid resolution (
Characteristics of the alongshore connectivity
The following section intends to characterize the seasonal cycle of the alongshore connectivity between the SARP line and St Helena Bay in terms of: i) transport success, ii) time taken by successful particles to travel northward, iii) initial vertical and across-shore position of successful particles.
Fig. 7 plots the monthly climatology of the alongshore wind stress at the SARP line together with transport success TSm and its standard deviation.
Both monthly alongshore transport success and its
Alongshore Lagrangian pathways
In order to get a general view of the routes taken by Lagrangian particles, maps of particles' spatial distribution (plumes) are plotted according to their age τ (in days) on the horizontal grid of the “BENGR15” simulation. The age of particles is zero when they are released. For a series of release events, we count all particles aged τ found within the water column of the grid cell located at position (x, y) in Cartesian coordinates. After dividing this by the total number of particles released
Characteristics of the ocean surface circulation
The Lagrangian experiments show that a “fast” inshore route and a “slow” offshore route co-exist and bring particles from Cape Point to St Helena Bay. The following section investigates how the Eulerian oceanic circulation of the model relates to these identified pathways.
Maps of monthly climatological surface currents show a nearshore equatorward jet, namely the Benguela Jet (Fig. 13). This jet can be observed all-year round, with a marked seasonal variability. In October, it is intensified
Discussion and conclusions
Lagrangian experiments, based on a 22 years (1989–2010) interannual simulation of the oceanic circulation in the SBUS, are used to describe the alongshore connectivity between Cape Point and St Helena Bay. Hutchings (1992) had previously pointed out the existence of an alongshore corridor followed by fish eggs and larvae that were spawn on the Agulhas Bank. A series of Lagrangian studies had then linked this corridor with the existence of the Benguela Jet, a shelf edge equatorward current (
Acknowledgements
The authors acknowledge the funding of N. Ragoasha's PhD by the South-African National Research Foundation (NRF, South Africa) and the French Institute for Research and Sustainable Development (IRD, France). This work was also supported by the French national program LEFE/INSU under the project's name Benguela Upwelling Innershelf Circulation (BUIC). This work was granted access to the HPC resources of [TGCC/CINES/IDRIS] under the allocation 2017- [DARI n° A0020107443] attributed by GENCI
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