Abstract: A high-resolution ocean model is used to characterize the local and remote driving mechanisms of the variability of the Patagonian shelf circulation. Local forcing includes the effects of tides, buoyancy fluxes and wind, while remote forcing represents the impact of the adjacent deep-ocean currents. There is an abrupt change of the dynamical characteristics of the shelf circulation at 40°S. South of 40°S, the seasonal variations of the shelf circulation are out of phase with the local wind stress and are driven by deep ocean inflows originated in the Drake Passage. The inter-annual variability of the shelf circulation is principally driven by the wind and shows a significant correlation with the time variations of the Southern Annular Mode index. The variability of the circulation and upwelling at the shelfbreak region are modulated by the variability of the Malvinas Current transport at low frequency (periods higher than two years), and by the local wind stress at higher frequencies. North of 40°S, the local wind forcing drives the seasonal variations of the shelf transport. The inter-annual variability of the flow is driven by the combined action of the Rio de la Plata discharge (significantly correlated with the El Niño Southern Oscillation), local wind stress and the Brazil-Malvinas Confluence in the outer shelf. In agreement with previous studies, we show that while the position of the confluence marks the location of the largest offshelf transports, it does not determine their magnitude. The offshelf transport variability is controlled by the local wind at high frequency (periods less than a year) and by the equatorward inflow of southern waters at longer periods. Our simulation indicates that the variability of the Subtropical Shelf Front is modulated by the local wind stress forcing, position of the Brazil/Malvinas Confluence and the equatorward inflow of Subantarctic waters.
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.