Abstract: It has been suggested that the Subtropical Shelf Front (STSF) could be a preferential site for the detrainment of Subantarctic Shelf Waters (SASW) and related planktonic shelf species onto the open SW Atlantic Ocean. The offshore detrainment of SASW and planktonic shelf species might be an exportation mechanism, affecting the population abundances of fishing resources in Argentina, Uruguay and Southern Brazil. In this study, we characterize for the first time the 3-D structure of the STSF and the main routes of offshore export of SASW from the Patagonian shelf during austral summer (summer and early fall) and winter (winter and early spring) by using numerical hydrodynamical model results and Lagrangian tracking simulations of neutrally buoyant floats. The transport of SASW towards the open ocean is 1 Sv (1 Sv = 106 m3.s-1) during summer and 0.8 Sv during winter. SASW are exported offshore mainly near the Brazil–Malvinas Confluence (BMC) region during both seasons. The STSF appears to act as an important retention mechanism for the plankton over the inner and middle shelf mainly during late summer and early fall. Our findings could explain the life cycle of distinct fish species which are distributed in the region, as well as the population abundance variability of such species.
Abstract:The California Current System (CCS) is an eastern boundary upwelling system characterized by strong eddies that are often generated at the coast. These eddies contribute to intense, long-distance cross-shelf transport of upwelled water with enhanced biological activity. However, the mechanisms of formation of such coastal eddies, and more importantly their capacity to trap and transport tracers, are poorly understood. Their unpredictability and strong dynamics leave us with an incomplete picture of the physical and biological processes at work, their effects on coastal export, lateral water exchange among eddies and their surrounding waters, and how long and how far these eddies remain coherent structures. Focusing our analysis on the southern part of the CCS, we find a predominance of cyclonic eddies, with a 25-km radius and a SSH amplitude of 6 cm. They are formed near shore and travel slightly northwest offshore for ~190 days at ~2 km day−1. We then study one particular, representative cyclonic eddy using a combined Lagrangian and Eulerian numerical approach to characterize its kinematics. Formed near shore, this eddy trapped a core made up of ~67% California Current waters and ~33% California Undercurrent waters. This core was surrounded by other waters while the eddy detached from the coast, leaving the oldest waters at the eddy’s core and the younger waters toward the edge. The eddy traveled several months as a coherent structure, with only limited lateral exchange within the eddy.
Abstract:Juvenile jack mackerel were found in 2009 in the Challenger break and the East Pacific ridge (CHAEPR). This seamount region is ∼3500 km from the coastal historic jack mackerel nursery grounds off Chile (north of 30°S). We reviewed historic evidence of juveniles around this seamount and data on several local environmental conditions: sea surface temperature, chlorophyll-a, wind, turbulence levels, and Eddy kinetic energy (EKE). A Lagrangian model for the early life stages of jack mackerel in the eastern South Pacific was used to assess the potential of the seamount region as a permanent nursery ground. Transport/retention mechanisms were assessed by releasing virtual particles coupled to a growth model into the flow simulated by an eddy-resolving ocean model. Model simulations showed high inter-annual variability for particle retention in the seamount region; high retention levels were associated with low EKE such that the particles were retained for several months. Satellite altimetry has shown a local minimum in eddy activity in the region where the juveniles were observed; this minimum was consistent with the above temporal relationship. The inclusion of the CHAEPR oceanic seamount region as a potential nursery ground for jack mackerel expands the current conceptual framework for the spatial population structure of this species in the South Pacific off central Chile proposed by Arcos et al. (The jack mackerel fishery and El Niño 1997–98 effects off Chile. Progress in Oceanography 49: 597–617, 2001). Finally, we discuss the relevance of seamounts playing a double role (spawning and nursery grounds) from the standpoint of conservation and biodiversity.