Browsing by Author "Herbette, Steven"
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- ItemOpen AccessInteraction of the antarctic circumpolar current with topography: impacts on the Southern Ocean eddy dynamics(2013) Kobo, Nomkwezane Sanny; Reason, Chris; Monteiro, Pedro; Herbette, Steven
- ItemOpen AccessThe influence of the land-sea breeze on coastal upwelling systems(2021) Fearon, Giles; Vichi, Marcello; Herbette, Steven; Veitch, JenniferThe 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 (<∼100 m) surface oscillations are dampened and shear-driven mixing at the thermocline is reduced. Convergence/ divergence of the forced surface oscillations drive evanescent internal waves which elevate local vertical mixing above that from the forced response alone. The internal wave response is dampened by a gradually sloping bottom topography. St Helena Bay (∼32.5° S), in the southern Benguela upwelling system, possesses a combination of physical characteristics which favour an enhanced response to the land-sea breeze, namely a near-critical latitude, a local enhancement of τ ac0 , and a tendency for the development of a shallow stratified surface layer. Here, land-sea breeze forcing contributes to large diurnal variability in sea surface temperatures (SST's). During relaxation events, mean SST's are notably reduced due to land-sea breeze-driven vertical mixing. During upwelling events, the land-sea breeze drives a net warming of inner shelf waters primarily due to enhanced retention of the deepened surface mixed layer. The deepened thermocline impacts geostrophically-driven alongshore currents within St Helena Bay, which are strengthened (weakened) during upwelling (relaxation) events. It appears likely that the land-sea breeze plays an important role in the productivity of the system.
- ItemOpen AccessThe variability of Lagrangian transport in the southern Benguela Current upwelling system(2020) Ragoasha, Moagabo Natalie; Reason, Christopher; Herbette, Steven; Veitch, Jenny; Cambon, Gildas; Roy, ClaudeThis 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. High-resolution 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 contributing to positive transport success anomalies. 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.